Rubberduck Style Guide

As Rubberduck started to beef up its static code analysis capabilities in late 2015, it became evident that writing VBA (or VB6) code with Rubberduck loaded up in the Visual Basic Editor (VBE) would inevitably change not only how we work in VBA, but also how we write our VBA code in the first place.

Rubberduck is essentially providing a bridge between VBA land where people just get in and have a go and the VS land where if you don’t know a great deal about software development, you just waste your time and burn. Rubberduck will put a lot of people on a big learning curve and this will result in a lot of questions.” – AndrewM- commented on Oct 9, 2015

There’s an old issue (#823, still opened as of this writing) about having a coding style guide somewhere, that would enshrine the philosophy behind what Rubberduck is, in a way, trying to make your code-writing be/become; I think that was a great idea and I’m hoping this post captures the essence of it, at least as far as thinking code goes.

About Code Inspections

If you fire up Rubberduck on any legacy VBA project with any significant amount of code, there’s a very high probability that static code analysis generates tons of inspection results, for various mundane little things. Should your goal be to quick-fix all the things and have code that doesn’t spawn any Rubberduck inspection results?

Perhaps surprisingly, the answer is a resounding “no”.

Severity Levels

In Rubberduck each inspection has a configurable “severity level” that defaults to Warning for most inspections (it’s the default-unless-specified-otherwise for all Rubberduck inspections):

  • Error level indicates a potential problem you likely want to pay immediate attention to, because it could be (or cause) a bug. If inspection results rendered in the code pane, these would be red squiggly underlines.
  • Warning level indicates a potential issue you should be aware of.
  • Suggestion level is usually used for various opportunities, not necessarily problems.
  • Hint level is also for various non-problematic opportunities. If inspection results rendered in the code pane, these would be a subtle dotted underline with a hover text.
  • DoNotShow disables the inspection: not only its results won’t show, they won’t even be generated.

By default, Rubberduck is configured to run all (that’s currently over 110, counting the hidden/Easter egg ones) inspections, with a handful of cherry-picked exceptions for inspections that would be flagging the exact opposite situation that another enabled inspection is already flagging – for example we ship implicit ByRef modifier enabled (as a Hint), but redundant ByRef modifier is disabled unless you give it a severity level that’s anything other than DoNotShow. This avoids “fixing” one inspection result only to get a new one flagging the exact opposite, which would be understandably confusing for users that aren’t familiar with static code analysis tooling.

Are inspections somehow imbued with the knowledge of whether you should treat them as errors, warnings, or mere hints and suggestions? Sometimes, yes. Missing Option Explicit should make a clear consensus at Error level. On the flipside, whether an implicit default member call or the use of an empty string literal should be a Warning, a Hint, or shown at all probably depends more on how comfortable or experienced you are with VBA/VB6 as a language, or could be just a personal preference; what matters is that the static code analysis tooling is letting you know something about the code, that the code alone isn’t necessarily saying.


One of the very first inspection to be implemented in Rubberduck was the Option Explicit inspection. Okay, part of it was just because it was a trivial one to implement even before we had an actual parser… but the basic idea was (and still is) that nobody knows everything, and it’s with our combined knowledge that we make a mighty bunch, and that is why static code analysis in Rubberduck explains the reasoning behind each inspection result: there are quite many things Rubberduck warns of, that I had no idea about 10 or 15 years ago. That never stopped me (and won’t stop you either) from writing VBA code that worked perfectly fine (except when it didn’t), but whether we realize and accept it or not… a macro written in VBA code is a set of executable instructions, which makes it a program, which makes the act of writing it programming, which makes us programmers.

Being programmers that write and maintain VBA code does set us apart, mostly because the language isn’t going anywhere and the IDE is becoming more and more severely outdated and under-featured as years pass. Yet if the volume of VBA questions on Stack Overflow means anything, VBA is still very much alive, still very much being learned, and this is where Rubberduck and static code analysis comes in.

When I started learning about .NET and C# over a decade ago, there was this exciting new language feature they called LINQ for Language-INtegrated-Query where you could start querying object collections pretty much literally like you would a database, and it was awesome (still is!). In order to make this possible, the C# compiler and the .NET framework and runtime itself had to undergo some very interesting changes Jon Skeet covers in details, but the point is… the new syntax was a bit off-putting at first, and came with new and important implications (closures, deferred execution), and the company I worked for gave us all a ReSharper license, and that is how and when I discovered that thorough & accurate static code analysis tooling could be a formidable educational tool.

And I want Rubberduck to be like that, to be the companion tool that looks at your code and reveals bits of trivia, hints like “hey did you know this conditional assignment could be simplified?“, or “if that condition was inverted you wouldn’t need this empty block here“.

Maybe we don’t agree about Hungarian Notation, and that’s fine: Rubberduck wants you to be able to find it and rename it if that’s what you want to do, but you can mute that particular inspection anytime. But I believe the tool should tell you what Systems Hungarian notation is when it calls it out, and perhaps it should even explain what Apps Hungarian is and give examples, because Apps Hungarian notation absolutely is useful and meaningful (think o-for-OneBased, or src-for-Source and dst-for-Destination prefixes). But str-for-String, lng-for-Long, o-for-Object is different, in a bad kind of way.

Rubberduck flags obsolete code constructs and keywords, too. Global declarations, On Local Error statements, explicit Call statements, While...Wend loops, all have no reason to exist in brand new, freshly-written VBA code, and quick-fixes can easily turn them into Public declarations, On Error statements, implicit Call statements (without the Call keyword!), and Do While...Loop structures.

Rubberduck wants to push your programming towards objectively, quantitatively better code.

About Code Metrics

Rubberduck could count the number of lines in a procedure, and issue an inspection result when it’s above a certain configurable threshold. In fact, things are slowly falling into place for it to eventually happen. But we wouldn’t want you to just arbitrarily cut a procedure scope at 20 lines because an inspection said so! Rubberduck can measure line count, nesting levels, and cyclomatic complexity. These metrics can be used to identify problematic areas in a code base and methodically split up large complex problems into measurably much smaller and simpler ones.

Line Count simply counts the number of lines. Eventually this would expand into Statements and Comments counts, perhaps with percentages; 10% comments is probably considered a good sign, for example. But no tool is going to tell you that ' increments i is a useless comment, and even the best tools would probably not tell the difference between a huge ' the following chunk of code does XYZ banner comment and an actually valuable comment. Common wisdom is to keep this line count metric down as much as possible, but one should not do this at the expense of readability.

Nesting Levels counts the number of… well, nesting levels. While nesting two For...Next loops to iterate a 2D array (or a Range of cells) down and across is probably reasonable, further nesting is probably better off made implicit through a procedure call. Rule of thumb, it’s always good idea to pull the body of a loop into its own parameterized procedure scope. Arrow-shaped code gets flattened, line count gets lower, and procedures become more specialized and have fewer reasons to fail that way.

Cyclomatic Complexity essentially calculates the number of independent execution paths in a given procedure (wikipedia). A procedure with a cyclomatic complexity above 5 is harder to follow than one with a complexity of 1 or 2, but it’s not uncommon for a “God procedure” with nested loops and conditionals to measure in the high 40s or above.

The code metrics feature will eventually get all the attention it deserves, but as with inspections the general idea is to highlight procedures that could be harder to maintain than necessary, and nudge our users towards:

  • Writing more, smaller, more specialized procedure scopes.
  • Passing parameters between procedures instead of using global variables.
  • Having more, smaller, more cohesive modules.

Navigating the VBE

You may or may not have noticed, but the Visual Basic Editor is nudging you in the exact opposite direction, because…

  • Having fewer, larger, more general-purpose procedures puts you in a scripting mindset.
  • Using globals instead of passing parameters around is perhaps a simpler thing to do.
  • Having fewer, larger, more general-purpose modules makes it simpler to share the code between projects, and arguably easier to find things in the Project Explorer.

If you’re actually writing a small script, you can and probably should absolutely do that.

But if you’re like me then you’ve been pushing VBA to do things it wasn’t really meant to do, and you’re maintaining actual applications that could just as well be written in any other language out there, but you’re doing it in VBA because [your reasons are valid, whatever they are].

And that’s kind of a problem, because the VBE seems to actively not want you to write proper object-oriented code: its navigation tooling indeed makes it very hard to work in a project with many small modules, let alone an OOP project involving explicit interfaces and high abstraction levels.

Rubberduck lifts pretty much all the IDE limitations that hinder treating a VBA project as more than just an automation script. Now you can have a project with 135 class modules, all neatly organized by functionality into folders that can contain any module type, so a UserForm can appear right next to the classes that use it, without needing to resort to any kind of ugly prefixing schemes. You can right-click on an abstract interface (or one of its members) and quickly find all classes that implement it. You get a Find symbol command that lets you quickly navigate to literally anything that has a name, anywhere in the project. Curious about the definition of a procedure, but don’t want to break your flow by navigating to it? Peek definition (currently only in pre-release builds) takes you there without leaving where you’re at.

The Peek Definition command pops a floating panel conveniently showing the source code for the user-defined module or member you’ve selected.
Find all References shows all the places a given identifier is being used, and shows it in context so you can easily locate the specific usage you’re looking for – and then a double-click takes you there.
The Find all Implementations command is incredibly useful in object-oriented projects that leverage polymorphism through abstract interfaces: quickly locate and navigate to any implementation of any interface (class or member).

The VBE’s Project Explorer aims to give you a bird’s eye view of your project, regrouping modules by module type which is great for a small script that can get away with a small number of components, but that makes it very hard to manage larger projects. With Rubberduck’s Code Explorer you get to drill down to member level, and regroup modules by functionality using an entirely customizable folder hierarchy:

The Code Explorer leaves the VBE’s Project Explorer in the dust, fair & square.

These navigational enhancements greatly simplify moving around a project of any size, although some of them might feel a bit overkill in a smaller project, and some of them are only useful in more advanced OOP scenarios. Still, having more than just a text-based search to look for things is very useful.


If there’s one single over-arching principle guiding everything else, it would have to be write code that does what it says, and says what it does. Everything else seems to stem from this. These are warmly recommended guidelines, not dogma.


  • Use PascalCase if you like. Use camelCase if you like. Consistency is what you want to shoot for, and in a case-insensitive language that only stores a single version of any identifier name it’s much easier and simpler to just use PascalCase everywhere and move on to more interesting things, like tabs vs spaces.
  • Avoid _ underscores in identifier names, especially in procedure/member names.
    • Causes compile errors with Implements.
  • Use meaningful names that can be pronounced.
  • Avoid disemvoweling (arbitrarily stripping vowels) and Systems Hungarian prefixing schemes.
  • A series of variables with a numeric suffix is a missed opportunity to use an array.
  • A good identifier name is descriptive enough that it doesn’t need an explainer comment.
  • Use a descriptive name that begins with a verb for Sub and Function procedures.
  • Use a descriptive name (a noun) for Property procedures and modules.
  • For object properties, consider naming them after the object type they’re returning, like Excel.Worksheet.Range returns a Range object, or like ADODB.Recordset.Fields returns a Fields object.
  • Appropriately name everything the code must interact with: if a rounded rectangle shape is attached to a DoSomething macro, the default “Rounded Rectangle 1” name should be changed to “DoSomethingButton” or something that tells us about its purpose. This includes all controls on a UserForm designer, too. CommandButton12 is useless; SearchButton is much better. Consider also naming the controls that don’t necessarily interact with code, too: future code might, and the author of that future code will appreciate that the bottom panel is named BottomPanel and not Label34.


Naming is hard enough, renaming things should be easy. With Rubberduck’s Rename refactoring (Ctrl+Shift+R) you can safely rename any identifier once, and all references to that identifier automatically get updated. Without a refactoring tool, renaming a form control can only be done from the Properties toolwindow (F4), and doing this instantly breaks any event handlers for it; renaming a variable by hand can be tedious, and renaming a single-letter variable like a or i with a local-scope find/replace (Ctrl+H) can get funny if the scope has any comments. Rubberduck knows the exact location of every reference to every identifier in your project, so if you have a module with two procedures that each declare a localThing, when you rename the local variable localThing in the first procedure, you’re not going to be affecting the localThing in the other procedure. But if you rename CommandButton1 to OkButton, then CommandButton1_Click() becomes OkButton_Click().

Parameters & Arguments

  • Prefer passing values as parameters instead of bumping the scope of a variable to module-level, or instead of declaring global variables.
  • Pass parameters ByVal whenever possible.
    • Arrays and User-Defined Type structures cannot and should not be passed by value.
    • Objects are never passed anywhere no matter the modifier: it’s only ever (ByVal: a copy of) a pointer that gets passed around – and most of the time the intent of the author is to pass that pointer by value. A pointer is simply a 32-bit or 64-bit integer value, depending on the bitness of the process; passing that pointer ByRef (explicitly or not) leaves more opportunities for programming errors.
  • Use an explicit ByRef modifier whenever passing parameters by reference.
  • Consider specifying an out prefix to name ByRef return parameters.
    • Consider using named arguments for out-prefixed ByRef return parameters.


  • Use the single quote ' character to denote a comment.
  • Avoid line-continuing comments; use single quotes at position 1 of each line instead.
  • Consider having a @ModuleDescription annotation at the top of each module.
  • Consider having a @Description annotation for each Public member of a module.
  • Remove comments that describe what an instruction does, replace with comments that explain why an instruction needs to do what it does.
  • Remove comments that summarize what a block of code does; replace with a call to a new procedure with a nice descriptive name.
  • Avoid cluttering a module with banner comments that state the obvious. We know they’re variables, or properties, or public methods: no need for a huge green comment banner to tell us.
  • Avoid cluttering a procedure scope with banner comments that split up the different responsibilities of a procedure: the procedure is doing too many things, split it up and appropriately name the new procedure instead.


  • Declare all variables, always. Option Explicit should be enabled at all times.
  • Declare an explicit data type, always. If you mean As Variant, make it say As Variant.
  • Consider using a Variant to pass arrays between scopes, instead of typed arrays (e.g. String()).
    • Pluralize these identifier names: it signals a plurality of elements/items much more elegantly than Pirate Notation (arr*) does.
  • Avoid Public fields in class modules; encapsulate them with a Property instead.
  • Consider using a backing user-defined Private Type structure for the backing fields of class properties; doing so eliminates the need for a prefixing scheme, lets a property be named exactly as per its corresponding backing field, and cleans up the locals toolwindow by grouping the fields under a single module variable.
  • Limit the scope of variables as much as possible. Prefer passing parameters and keeping the value in local scope over promoting the variable to a larger scope.
  • Declare variables where you’re using them, as you need them. You should never need to scroll anywhere to see the declaration of a variable you’re looking at.

Late Binding

Late binding has precious little to do with CreateObject and whether or not a library is referenced. In fact, late binding happens implicitly rather easily, and way too often. Strive to remain in early-bound realm all the time: when the compiler / IntelliSense doesn’t know what you’re doing, you’re on your own, and even Option Explicit can’t save you from a typo (and error 438).

  • Avoid making a member call against Object or Variant. If a compile-time type exists that’s usable with that object, a local variable of that data type should be assigned (Set) the Object reference and the member call made early-bound against this local variable.
    • Taking an object presenting one interface and assigning it to another data type is called “casting”.
  • Of course explicit late binding is OK (As Object, no library reference, create objects with CreateObject instead of the New operator). Late binding is very useful and has many legitimate uses, but generally not when the object type is accessible at compile-time through a library reference.
  • Avoid the dictionary-access (aka “bang”) operator !, it is late-bound by definition, and makes what’s actually a string literal read like a member name, and any member call chained to it is inevitably late-bound too. Rubberduck can parse and resolve these, but they’re much harder to process than standard method calls.


  • Use explicit modifiers everywhere (Public/Private, ByRef/ByVal).
  • Declare an explicit data type, even (especially!) if it’s Variant.
  • Avoid implicit qualifiers for all member calls: in Excel watch for implicit ActiveSheet references, implicit ActiveWorkbook references, implicit containing worksheet references, and implicit containing workbook references, as they are an extremely frequent source of bugs.
  • Invoke parameterless default members explicitly.
    • Note: some object models define a hidden default member (e.g. Range.[_Default]) that redirects to another member depending on its parameterization. In such cases it’s best to invoke that member directly; for example use Range.Value as appropriate, but the hidden [_Default] member is better off not being invoked at all, for both readability and performance reasons.
  • Invoke parameterized default members implicitly when they are indexers that get a particular item in an object collection, for example the Item property of a Collection. Invoking them explicitly doesn’t hurt, but could be considered rather verbose.
  • Call is not a keyword that needs to be in your program’s vocabulary when you use expressive, descriptive procedure names that imply an action taking place.
  • Consider explicitly qualifying standard module member calls with the project (and module) name, including for standard and referenced libraries, especially in VBA projects that reference multiple object models.

Structured Programming (Procedural)

  • One macro/script per module. Do have it in a module rather than a worksheet’s code-behind.
  • Public procedure first, followed by parameterized Private procedures, in decreasing abstraction level order such that the top reads like a summary and the bottom like boring, small but specific operations.
    • You know it’s done right when you introduce a second macro/module and get to pull the small, low-abstraction, specific operations into Public members of a utility module, and reuse them.
  • Don’t Repeat Yourself (DRY).
  • Consider passing the relevant state to another procedure when entering a block of code. Code is simpler and easier to follow when the body of a loop or a conditional block is pulled into its own scope.
  • Avoid using error handling to control the flow of execution: the best error handling is no error handling at all, because assumptions are checked and things are validated. For example instead of opening a file from a parameter value, first verify that the file exists instead of handling a file not found error… but still handle errors, for any exceptional situations that might occur while accessing the file.
  • When it’s not possible to avoid error handling, consider extracting a Boolean function that swallows the expected error and returns False on failure, to simplify the logic.
  • Handle errors around all file and network I/O.
  • Never trust user inputs to be valid or formatted as expected.

Object Oriented Programming

In VBA/VB6 we get to go further than mere scripting and apply Object-Oriented Programming principles, probably more relevantly so in VB6 and larger VBA projects. For many years it has been drilled into our heads that VBA/VB6 cannot do “real” OOP because it doesn’t support inheritance. The truth is that there is much, much more to OOP than inheritance, and if you want to learn and apply OOP principles in your VBA/VB6 code, you absolutely can, and you absolutely should, and Rubberduck will absolutely help you do that.

  • Adhere to standard OOP best practices, they are general, language-agnostic concepts that couldn’t care less about the capabilities of VBA/VB6:
    • Single Responsibility Principle – each abstraction should be responsible for one thing.
    • Open/Closed Principle – write code that doesn’t need to change unless the purpose of the abstraction itself needs to change.
    • Liskov Substitution Principle – code should run the exact same execution paths regardless of the concrete implementation of a given abstraction.
    • Interface Segregation Principle – keep interfaces small and specialized, avoid a design that constantly needs new members to be added to an interface.
    • Dependency Inversion Principle – depend on abstractions, not concrete implementations.
  • Leverage composition where inheritance would be needed.
  • You cannot have parameterized constructors, but you still can leverage property injection in factory methods to inject instance-level dependencies.
  • Leverage method injection to inject method-level dependencies.
  • Avoid New-ing dependencies in-place, it couples a class with another, which hinders testability; inject the dependencies instead.
    • Use the New keyword in your composition root, as close as possible to an entry point.
    • The Workbook_Open event handler (Excel) is a possible entry point.
    • Macros (Sub procedures invoked from outside the code) are also valid entry points.
    • Let go of the idea that a module must control every last one of its dependencies: let something else deal with creating or dereferencing these objects.
  • Inject an abstract factory when a dependency cannot or should not be created at the composition root, for example if you needed to connect to a database and wish to keep the connection object as short-lived and tightly-scoped as possible.
  • Keep the default instance of a class stateless as much as possible. Actively protect/guard against accidental misuse by throwing a run-time error as necessary.
  • Use standard modules instead of a utility class with a @PredeclaredId, that never gets explicitly instantiated or used as an actual object.

User Interfaces

UI code is inherently object-oriented, and thus a UserForm should be treated as the object it wants to be. The responsibilities of a user interface are simple: display and collect data to/from the user, and/or offer a way to execute commands (which typically consume or otherwise manipulate the data).

  • Avoid working directly with the form’s default instance. New it up instead.
  • Form module / code-behind should be strictly concerned with presentation concerns.
    • Do implement UI logic in form’s code-behind, e.g. enable this control when this command says it can be executed, or show this label when the model isn’t valid, etc.
  • Consider creating a model class to encapsulate the form’s state/data.
    • Expose a read/write property for each editable field on the form.
    • Expose a read-only property for data needed by the controls (e.g. the items of a ListBox).
    • Controls’ Change handlers manipulate the model properties.
    • Expose additional methods and properties as needed for data/input validation.
      • Consider having an IsValid property that returns True when all required values are supplied and valid, False otherwise; use this property to enable or disable the form’s Accept button.
  • Avoid implementing any kind of side-effecting logic in a CommandButton‘s Click handler. A CommandButton should invoke a command, right?
    • In procedural code the command might be a Public Sub procedure in a standard module named after the form, e.g. a SomeDialogCommands module for a SomeDialog form.
    • In OOP the command might be a property-injected instance of a DoSomethingCommand class; the Click handler invokes the command’s Execute method and could pass the model as a parameter.
  • Consider implementing a presenter object that is responsible for owning and displaying the form instance; the Model-View-Presenter UI pattern is well documented, and like everything OOP, its concepts aren’t specific to any language or platform.

Caveat: Microsoft Access Data-Bound UI

VBA projects hosted in Microsoft Access can absolutely use UserForm modules too, but without Rubberduck you need to hunt down the IDE command for it because it’s hidden. Instead, in Access you mostly create Access Forms, which (being document modules owned by the host application) have much more in common with a Worksheet module in Excel than with a UserForm.

The paradigm is different in an Access form, because of data bindings: a data-bound form is inherently coupled with the underlying database storage, and any effort to decouple the UI from the database is working directly against everything Access is trying to make easier for you.

Treating an Access form the way one would treat a worksheet UI in Excel puts you in a bit of a different mindset. Imagine the Battleship worksheet UI implemented as an Access form: the game would be updating game state records in the underlying database, and instead of having code to pull the game state into the UI there would only need to be code to re-query the game state, and the data bindings would take care of updating the actual UI – and then the game could easily become multi-player, with two clients connecting to the database and sharing the same game state.

This is very fundamentally different than how one would go about getting the data into the controls without such data bindings. Binding the UI directly to a data source is perfectly fine when that data source happens to be running in the very same process your VBA code is hosted in: Access’ Rapid Application Development (RAD) approach is perfectly valid in this context, and its global objects and global state make a nice beginner-friendly API to accomplish quite a lot, even with only a minimal understanding of the programming language (and probably a bit of Access-SQL).

If we’re talking about unbound MS-Access forms, then it’s probably worth exploring Model-View-Presenter and Model-View-Controller architectures regardless: in such exploratory OOP scenarios the above recommendations can all hold.

UI Design

I’m not going to pretend to be a guru of UI design, but over the years I’ve come to find myself consistently incorporating the same elements in my modal forms, and it has worked very well for me so here we go turning that into general guidelines.

  • TopPanel is a Label control with a white background that is docked at the top and tall enough to comfortably fit short instructions.
  • BottomPanel is also a Label control, with a dark gray background, docked at the bottom and no more than 32 pixels in height.
  • DialogTitle is another Label control with a bold font, overlapping the TopPanel control.
  • DialogInstructions is another Label control overlapping the TopPanel control.
  • DialogIcon is an Image control for a 16×16 or 24×24 .bmp icon aligned left, at the same Top coordinate as the DialogTitle control.
  • OkButton, CancelButton, CloseButton, ApplyButton would be CommandButton controls overlapping the BottomPanel control, right-aligned.

The actual client area content layout isn’t exactly free-for-all, and I doubt it’s possible to come up with a set of “rules” that can apply universally, but we can try, yeah?

  • Identify each field with a label; align all fields to make it look like an implicit grid.
  • Seek visual balance; ensure a relatively constant margin on all sides of the client area, space things out but not too much. Use Frame controls to group ComboBox options.
  • Avoid making a complex form with too many responsibilities and, inevitably, too many controls. Beyond a certain complexity level, consider making separate forms instead of tabs.
  • Use Segoe UI for a more modern font than MS Sans Serif.
  • Do not bold all the labels.
  • Have a ToolTip string for the label of every field the user must interact with. If a field is required or demands a particular format/pattern, indicate it.
  • Consider toggling the visibility of a 16×16 icon next to (or even inside, right-aligned) input fields, to clearly indicate any data validation errors (have a tooltip string on the image control with the validation error message, e.g. “this field is required”, or “value cannot be greater than 100”).
  • Name. All. The. Things.
  • Use background colors in input controls only to strongly signal something to the user, like a validation error that must be corrected in order to move on. Dark red text over a light pink background makes a very strong statement.
  • Keep a consistent color scheme/palette and style across all of your application’s UI components.

This pretty much concludes the “guidelines” section (although I’ll quite probably be adding more to it), but since discussing unit testing and testability lines up with everything above…

Unit Testing

A unit test is a small, simple procedure that is responsible for 3 things:

  1. Arrange dependencies and set expectations.
  2. Act, by invoking the method or function under test.
  3. Assert that the expected result matches the actual one.

When a unit test runs, Rubberduck tracks Assert.Xxxx method calls and their outcome; if a single Assert call fails, the test fails. Such automated tests are very useful to document the requirements of a particular model class, or the behavior of a given utility function with multiple optional parameters. With enough coverage, tests can actively prevent regression bugs from being inadvertently introduced as the code is maintained and modified: if a tweak breaks a test, you know exactly what functionality you broke, and if all tests are green you know the code is still going to behave as intended.

Have a test module per unit/class you’re testing, and consider naming the test methods following a MethodUnderTest_GivenAbcThenXyz, where MethodUnderTest is the name of the method you’re testing, Abc is a particular condition, and Xyz is the outcome. For tests that expect an error, consider following a MethodUnderTest_GivenAbc_Throws naming pattern. Rubberduck will not warn about underscores in test method names, and these underscores are safe because Rubberduck test modules are standard modules, and unit test naming recommendations usually heavily favor being descriptive over being concise.

What to test?

You want to test each object’s public interface, and treat an object’s private members as implementation details. You do NOT want to test implementation details. For example if a class’ default interface only exposes a handful of Property Get members and a Create factory method that performs property-injection and a handful of properties, then there should be tests that validate that each of the parameters of the Create method correspond to an injected property. If one of the parameters isn’t allowed to be Nothing, then there should be a guard clause in the Create method for it, and a unit test that ensures a specific error is being raised when the Create method is invoked with Nothing for that parameter.

Below is one such simple example, where we have 2 properties and a method; note how tests for the private InjectDependencies function would be redundant if the public Create function is already covered – the InjectDependencies function is an implementation detail of the Create function:

Option Explicit
Implements IClass1
Private Type TState
    SomeValue As String
    SomeDependency As Object
End Type
Private This As TState
Public Function Create(ByVal SomeValue As String, ByVal SomeDependency As Object) As IClass1
    If SomeValue = vbNullString Then Err.Raise 5
    If SomeDependency Is Nothing Then Err.Raise 5
    Dim Result As Class1
    Set Result = New Class1
    InjectProperties Result, SomeValue, SomeDependency
    Set Create = Result
End Function
Private Sub InjectProperties(ByVal Instance As Class1, ByVal SomeValue As String, ByVal SomeDependency As Object)
    Instance.SomeValue = SomeValue
    Set Instance.SomeDependency = SomeDependency
End Sub
Public Property Get SomeValue() As String
    SomeValue = This.SomeValue
End Property
Public Property Let SomeValue(ByVal RHS As String)
    This.SomeValue = RHS
End Property
Public Property Get SomeDependency() As Object
    SomeDependency = This.SomeDependency
End Property
Public Property Set SomeDependency(ByVal RHS As Object)
    Set This.SomeDependency = RHS
End Property
Private Property Get IClass1_SomeValue() As String
    IClass1_SomeValue = This.SomeValue
End Property
Private Property Get IClass1_SomeDependency() As Object
    IClass1_SomeDependency = This.SomeDependency
End Property

Note: the property injection mechanism doesn’t need a Property Get member on the Class1 interface, however not exposing a Property Get member for a property that has a Property Let (and/or Property Set) procedure, would leave the property as write-only on the Class1 interface. Write-only properties would be flagged as a design smell, so there’s a conundrum here: either we expose a Property Get that nothing is calling (except unit tests, perhaps), or we expose a write-only property (with a comment that explains its property injection purpose). There is no right or wrong, only a consistent design matters.

If we were to write unit tests for this class, we would need at least:

  • One test that invokes Class1.Create with an "" empty string for the first argument and fails if error 5 isn’t raised by the procedure call.
  • One test that invokes Class1.Create with Nothing for the second argument and fails if error 5 isn’t raised by the procedure call.
  • One test that invokes Class1.Create with valid arguments and fails if the returned object is Nothing.
  • One test that invokes Class1.Create with valid arguments and fails if the Class1.SomeValue property doesn’t return the value of the first argument.
  • One test that invokes Class1.Create with valid arguments and fails if the Class1.SomeDependency property doesn’t return the very same object reference as was passed for the second argument.
  • One test that invokes Class1.Create with valid arguments and fails if the IClass1.SomeValue property doesn’t return the same value as Class1.SomeValue does.
  • One test that invokes Class1.Create with valid arguments and fails if the IClass1.SomeDependency property doesn’t return the same object reference as Class1.SomeDependency does.

Obviously that’s just a simplified example, but it does perfectly illustrate the notion that the answer to “what to test?” is simply “every single execution path”… of every public member (private members are implementation details that are invoked from the public members; if they specifically need tests, then they deserve to be their own concern-addressing class module).

What is testable?

Without the Property Get members of Class1 and/or IClass1, we wouldn’t be able to test that the Create method is property-injecting SomeValue and SomeDependency, because the object’s internal state is encapsulated (as it should be). Therefore, there’s an implicit assumption that a Property Get member for a property-injected dependency is returning the encapsulated value or reference, and nothing more: by writing tests that rely on that assumption, we are documenting it.

Now SomeDependency might be an instance of another class, and that class might have its own encapsulated state, dependencies, and testable logic. A more meaty Class1 module might have a method that invokes SomeDependency.DoSomething, and the tests for that method would have to be able to assert that SomeDependency.DoSomething has been invoked once.

If Class1 wasn’t property-injecting SomeDependency (for example if SomeDependency was being New‘d it up instead), we would not be able to write such a test, because the outcome of the test might be dependent on a method being called against that dependency.

A simple example would be Class1 newing up a FileSystemObject to iterate the files of a given folder. In such a case, FileSystemObject is a dependency, and if Class1.DoSomething is newing it up directly then every time Class1.DoSomething is called, it’s going to try and iterate the files of a given folder, because that’s what a FileSystemObject does, it hits the file system. And that’s slow. I/O (file, network, …and user) is dependent on so many things that can go wrong for so many reasons, having it interfere with tests is something you want to avoid.

The way to avoid having user, network, and file inputs and outputs interfere with the tests of any method, is to completely let go of the “need” for a method to control any of its dependencies. The method doesn’t need to create a new instance of a FileSystemObject; what it really needs is actually a much simpler any object that’s capable of returning a list of files or file names in a given folder.

So instead of this:

Public Sub DoSomething(ByVal Path As String)
    With CreateObjet("Scripting.FileSystemObject")
        ' gets the Path folder...
        ' iterates all files...
        ' ...
    End With
End Sub

We would do this:

Public Sub DoSomething(ByVal Path As String, ByVal FileProvider As IFileProvider)
    Dim Files As Variant
    Files = FileProvider.GetFiles(Path)
    ' iterates all files...
    ' ...
End Sub

Where IFileProvider would be an interface/class module that might look like this:

Option Explicit
'@Description "Returns an array containing the file names under the specified folder."
Public Function GetFiles(ByVal Path As String) As Variant
End Function

That interface might very well be implemented in a class module named FileProvider that uses a FileSystemObject to return the promised array.

It could also be implemented in another class module, named TestFileProvider, that uses a ParamArray parameter so that unit tests can take control of the IFileProvider dependency and inject (here by method injection) a TestFileProvider instance. The DoSomething method doesn’t need to know where the file names came from, only that it can expect an array of existing, valid file names from IFileProvider.GetFiles(String). If the DoSomething method indeed doesn’t care where the files came from, then it’s adhering to pretty much all OOP design principles, and now a test can be written that fails if DoSomething is doing something wrong – as opposed to a test that might fail if some network drive happens to be dismounted, or works locally when working from home but only with a VPN.

The hard part is obviously identifying the dependencies in the first place. If you’re refactoring a procedural VBA macro, you must determine what the inputs and outputs are, what objects hold the state that’s being altered, and devise a way to abstract them away and inject these dependencies from the calling code – whether that caller is the original entry point macro procedure, or a new unit test.


In the above example, the TestFileProvider implementation of the IFileProvider dependency is essentially a test stub: you actually write a separate implementation for the sole purpose of being able to run the code with fake dependencies that don’t incur any file, network, or user I/O. Reusing these stubs in “test” macros that wire up the UI by injecting the test stubs instead of the actual implementations, should result in the application running normally… without hitting any file system or network.

With mocks, you don’t need to write a “test” implementation. Instead, you configure an object provided by a mocking framework to behave as the method/test needs, and the framework implements the mocked interface with an object that can be injected, that verifiably behaves as configured.

Sounds like magic? A lot of it actually is, from a VBA/VB6 standpoint. Many tests in Rubberduck leverage a very popular mocking framework called Moq. What we’re going to be releasing as an experimental feature is not only a COM-visible wrapper around Moq. The fun part is that the Moq methods we need to use are generic methods that take lambda expressions as parameters, so our wrapper needs to expose an API VBA code can use, and then “translate” it into member calls into the Moq API, but because they’re generic methods and the mocked interface is a COM object, we essentially build a .NET type on the fly to match the mocked VBA/COM interface, so that’s what Moq actually mocks: a .NET interface type Rubberduck makes up at run-time from any COM object. Moq uses Castle Windsor under the hood to spawn instances of proxy types – made-up actual objects that actually implement one or more interfaces. Castle Windsor is excellent at what it does; we use CW to automate dependency injection in Rubberduck (a technique dubbed Inversion of Control, where a single container object is responsible for creating all instances of all objects in the application in the composition root; that’s what’s going on while Rubberduck’s splash screen is being displayed).

There is a problem though: CW seems to be caching types with the reasonable but still implicit assumption that the type isn’t going to change at run-time. In our case however, this means mocking a VBA interface once and then modifying that interface (e.g. adding, removing, or reordering members, or changing a member signature in any way) and re-running the test would still be mocking the old interface, as long as the host process lives. This isn’t a problem for mocking a Range or a Worksheet dependency, but VBA user code is being punished here.

Verifiable Invocations

Going back to the IFileProvider example, the GetFiles method could be configured to return a hard-coded array of bogus test strings, and a test could be made to turn green when IFileProvider.GetFiles is invoked with the same specific Path parameter value that was given to Class1.DoSomething. If you were stubbing IFileProvider, you would perhaps increment a counter every time IFileProvider_GetFiles is invoked, and expose that counter with a property that the test could Assert is equal to an expected value. With Moq, you can make a test fail by invoking a Verify method on the mock itself, that verifies whether the specified method was invoked as configured.

A best practice with mocking would be to only setup the minimal amount of members to make the test work, because of the performance overhead: if a mocked interface has 5 methods and 3 properties but the method under test only needs 2 of these methods and 1 of these properties, then it should only setup these. Verification makes mocking a very valuable tool to test behavior that relies on side-effects and state changes.

The best part is that because VBA is COM, then everything is an interface, so if you don’t have an IFileProvider interface but you’re still passing a FileProvider object as a dependency, then you can mock the FileProvider directly and don’t need to introduce any extra “just-for-testing” IFileProvider interface if you don’t already have one.

I’m going to stop here and just publish, otherwise I’ll be editing this post forever. So much is missing…

Introducing Rubberduck 2.5.2

Version 2.5.1 was released August 22, 2020. Since then, the installer was downloaded over 11,600 times; we are now 420 commits and 650 modified files later, and the time has come to deliver all that work into a convenient little installer package and move on to the next dev/release cycle.

What’s New?

If you’ve kept up with latest pre-release builds (especially in the last few weeks), nothing much. If you’ve been patiently waiting for the next release, you’re in for a treat!

The first thing you’ll probably notice is the shiny new splash screen design:

It’s the same old yellow splash made with Paint.NET, with a tiled reflection distortion effect against the background, a semi-transparent white bottom panel, and a finer font. Do you like it?

Fixed Bugs

50-some issues labelled “bug” were closed between 2020-08-22 and mid-April 2021, many of them thanks to flicking the switch on leveraging our internal ITypeLib API for user code – thanks to earlier invaluable contributions from the amazing Wayne Phillips (vbWatchdog, twinBASIC), Rubberduck is now able to tap into the actual in-memory COM type library compiled from the VBA code and, eventually, fill the remaining the gaps in Rubberduck’s understanding of the code: Rubberduck now understands enough to be able to tell that ThisWorkbook has a _Workbook subtype, and that Sheet1 has a _Worksheet subtype, …and that’s enough to identify the ThisWorkbook module at long last, and as a result Rubberduck’s ImplicitActiveSheetReference and ImplicitActiveWorkbookReference inspections get to work exactly as intended, and the door is now opened for so many interesting things…

New Inspections

A Rubberduck release wouldn’t be a Rubberduck release without at least a handful of new inspections. The IllegalAnnotation inspection is being replaced by InvalidAnnotation, UnrecognizedAnnotation, and together with the new AnnotationInIncompatibleComponentType inspection they allow Rubberduck to better convey exactly what’s wrong with a given “illegal” annotation comment.

Annotation in Incompatible Component Type

Some annotations cannot be used in certain types of modules. For example, attribute-related annotations cannot be used in document modules (because Rubberduck cannot import back the modified modules), and a @TestModule annotation is only meaningful in a standard module.

Note that the @Description, @ModuleDescription and @VariableDescription annotations do work in document modules now, because Rubberduck is now reading docstrings off annotations rather than hidden attributes.

Implicit Containing Workbook Reference

Code in the ThisWorkbook module (Excel) referring to members of the Workbook class, have an implicit Me qualifier. This makes an unqualified Worksheets(1) retrieval in ThisWorkbook refer to ThisWorkbook.Worksheets(1), but an identical statement in any other module would be (implicitly) referring to ActiveWorkbook. By qualifying such member calls with Me, the intent is clarified.

Implicit Containing Worksheet Reference

Code in a worksheet module (Excel) referring to members of the Worksheet class, have an implicit Me qualifier. This makes an unqualified Range member call in the Sheet2 module refer to Sheet2, but an identical statement in any other module would be (implicitly) referring to ActiveSheet. By qualifying such member calls with Me, the intent is clarified.

Invalid Annotation

Flags unbound annotations; that is, annotation comments that were correctly parsed as Rubberduck annotations but that could not be associated with a target element. This would happen when a module annotation is used in local scope, or a member annotation at module level. This inspection only flags annotation comments that parsed as Rubberduck annotations.

Misleading ByRef Parameter

The RHS/Value parameter of a Property Let procedure is always passed by value. As such, an explicit ByRef modifier on such a parameter definition is misleading. From MS-VBAL (VBA language specifications) section Property Declarations:

§ If the <value-param> of a <property-LHS-declaration> does not have a <parameter-mechanism> element or has a <parameter-mechanism> consisting of the keyword ByRef, it has the same meaning as if it instead had a <parameter-mechanism> element consisting of the keyword ByVal.
§ The <value-param> of a <property-LHS-declaration> always has the runtime semantics of a ByVal parameter.

Unrecognized Annotation

This inspection flags comments that parsed like a Rubberduck annotation, but aren’t recognized or supported. It picks up typos in Rubberduck annotations, and annotation-like comments that aren’t Rubberduck annotations but parse as such. Splicing this specific scenario from other invalid annotations is particularly useful when you want to mute inspection results for non-Rubberduck annotations while still validating the supported ones.

New Quick Fixes

This release also introduces a handful of new quick-fixes:


This fix is now available for ProcedureNotUsed inspection results in standard and document modules; it simply annotates a member with the new @EntryPoint annotation which specifically instructs ProcedureNotUsed to ignore that member. Use this quick-fix for UDFs and macro procedures that are attached to document objects and don’t need an Excel hotkey. If your project is hosted in an Excel workbook, macros annotated with @ExcelHotkey are also considered as entry points now.


VariableTypeNotDeclared inspection results could always be “fixed” by making the declared type an explicit Variant; this new quick-fix makes Rubberduck infer the declared type from usage where possible, which is objectively awesome.


This new quick-fix is available for the new implicit containing workbook/worksheet reference inspections, making the reference to the containing module explicit.

Introduce Get Accessor

The Write-Only Property inspection gets a new quick-fix with this release; this iteration does not try to infer the backing field, so further manual edits are needed, but it’s a start.

New UI Language: Italian

Thanks to a timely contribution by @PhilCattivocaratere, we are thrilled to announce that this release introduces Italian as a UI language:

Every single UI string in Rubberduck comes from a localized resource file. Translating all the resources for a new language can take 3-5 hours, and then it’s only a matter of keeping the translations up-to-date by creating a small pull request when new resource strings are added for new features.

In a nutshell

Here’s a quick summary of the most significant pull requests and commits merged this cycle:

  • Encapsulate Field enhancements
  • We are now leveraging our internal ITypeLib API
  • We are now building Rubberduck with the latest version of Visual Studio 2019
  • Precompiler directives now parse correctly with line continuations
  • Internal CodeBuilder API honors indenter settings when generating code
  • Fixed a number of issues with name conflict validation
  • Test methods now support a @TestIgnore annotation to ignore a test
  • Specific projects can now be ignored by the parser
  • Users no longer need to accept the GPLv3 as if it were an End User License Agreement (EULA)
  • Custom templates extensions is changing from .rdt to .template
  • Implicit Variant inspection quick-fix will now infer the best type from usage instead of just making the variable an explicit Variant
  • For...Next loop variables no longer trigger a variable not used inspection
  • Implicit Public Member inspection will now flag Enum types and Type structures
  • Branch “master” was renamed to “main”
  • New Property Group indenter settings
  • Arrays declared with ReDim now correctly resolve the declared type
  • @Description, @VariableDescription, and @ModuleDescription can now be used in document modules (cannot be synchronized)
    • Documentation strings are now read from annotations when missing from attributes
  • Start menu link to website now uses https
  • Fixed context menu positioning
  • New @EntryPoint annotation marks a standard or document module member as invoked from outside the code; as such the Procedure Not Used inspection will no longer flags members annotated with @EntryPoint or @ExcelHotkey (Excel only).
  • Several other opportunistic fixes left & right, improved overall stability.
  • Shiny new splash screen; debug builds now indicate “debug” instead of a meaningless local build number (build version# is controlled by the AppVeyor CI build server; local builds are all .0).
  • Expand/collapse all in Code Explorer
  • Rubberduck CommandBar label will now show the corresponding parameter declaration for a selected argument, and Find all References will now include arguments at call sites for parameter declarations (previous versions would only count named arguments).
  • Find Symbol navigation tool works again.
  • Find all References search results will now highlight the target reference in its context.
  • Added Italian UI resources.

Possible (Silent) Crash on Exit

I haven’t personally experienced it in a long time in Excel, but Rubberduck may run into issues tearing down, sometimes causing an AccessViolationException when it unloads, which can either crash the host process or leave it hanging as a ghosted process that will interfere with reloading: verify that the host process (e.g. ACCESS.EXE) has shut down completely using Task Manager when you close everything, and make sure to kill any such ghosted processes before loading Rubberduck in a new process.

Sounds familiar? If you’ve been following the project all along, you probably remember similar behavior in earlier releases – at one point during this development cycle we thought the problem was finally under control, but the cure was worse than the disease and there was a chance that the host document / project gets completely corrupted and impossible to open in the VBE: because we think it’s much better to sometimes crash on teardown than to corrupt our users’ host documents forever, we have reverted that “fix” and will have to come up with something else.

What’s Next?

Lots of good stuff, including a new peek definition command to the code pane, Code Explorer, and the VBE’s own Project Explorer‘s context menus – the feature was developed too late to make the cut for this release, but will be available in 2.5.2.x pre-release builds very soon:

Peek Definition commands pop a panel that shows you the syntax-highlighted source code for a type or member. The pop-up panel can then be dragged around to keep it in sight while editing.

In the Unit Testing department, a mocking framework is about to debut as an experimental feature with a number of technical limitations.

I’m going to be turning my attention towards code path analysis this cycle; this internal API is needed to implement the more advanced inspection ideas, and an Extract Method refactoring needs it too.

To be continued…

Synchronizing your VBA project with files in a folder

Sync Project commands in the Code Explorer context menu.

VBA code being embedded in a host document might be very practical for certain aspects of both development and deployment, but let’s face it, it also makes using source control (e.g. git, SVN, mercurial, etc.) with VBA projects rather frustrating. As a developer, committing source code to a repository is usually a very simple task, because the code files live in the file system, and git can track changes and additions. With VBA, we commit the code that’s exported on the file system, the host document may contain different code, and merging remote changes implies exporting your code again, working out any merge conflicts with the exported code, then re-importing the merged changes into the host document.

Which wouldn’t be so bad… if the VBE had a nice way of exporting more than one single file at a time, and if importing files had an option to replace modules when they already exist… instead of importing the module with a “1” suffix as if that were something anyone ever needed to do!

Did You Know?
The VBE’s “Import File…” command doesn’t make it very obvious, but it does support importing multiple files at once. Simply select multiple files when prompted for what file to import!
Another little known feature of the VBE (one of the few Rubberduck hasn’t enhanced yet) is that its Project Explorer toolwindow is a drag-and-drop destination that can accept files you dragged from the Windows Explorer (⊞+E).

Rubberduck’s Export Project… command prompts for a folder, and then proceeds to export all modules there – overwriting any existing files in that folder. By default, the hotkey for that command is Ctrl+Shift+E, but it can be reconfigured to any key combination you like.

The context menu of Rubberduck’s Code Explorer toolwindow has a Sync Project sub-menu that offers two commands:

  • Update Components from Files…
  • Replace Contents from Files…

Rubberduck in general needs more documentation, but exactly what these “Sync Project” commands do is something that goes well beyond just using Rubberduck and they really deserve all the attention they can get, since they exist to facilitate an actual development workflow that looks something like this:

Import source files; make code changes; export source files; commit, push, pull, merge; rinse & repeat!

Update Components

This command prompts for source code files to import into your project.

  • If the project already contains a module with the same name as one of the imported files, the module is considered the same, and replaced with the imported version.
  • If the project does not already contain the imported modules, they’re simply added to the project.
  • If the project contains modules with different names than the imported files, these modules remain in the project.

Replace Contents

This command also prompts for source code files to import into your project, but the selected files will replace everything in the current project. Because this command is potentially destructive, a confirmation is required.

  • The entire project becomes the selected files.
  • If the project contained (non-document) modules before, they are removed before the import is performed.

We have a number of open issues (here, here, and here) about getting the “export project” command to take the @Folder annotations into account, and transpose the virtual folder hierarchy into an actual folder hierarchy on the file system, which would play nicely with version control and would help better organize a VBA repository.

Making MVVM Work in VBA Part 1 – Testing

I have recently written (100% VBA) a proof-of-concept for a Model-View-ViewModel (MVVM) framework, and since the prototype works exactly as needed (with some rough edges of course)… I’ve decided to explore what Rubberduck can do to make MVVM fully supported, but going down that path poses a serious problem that needs a very good and well thought-out solution.

A Vision of a Framework

When you start a new project in Visual Studio (including 6.0 /VB6), the IDE prompts for a project type, essentially asking “what are we building today?

In VBA the assumption is that you just want to write a bit of script to automate some document manipulation. And then the framework so to speak, is the VBA Standard Library: functions, methods, constants, and actual objects too; all globally-scoped for convenience and quick-and-easy access: a fully spelled-out VBA.Interaction.MsgBox function call is a rare sight! Combined with the nonexistence of namespaces, the flip side is that the global scope is easily polluted, and name collisions are inevitable since anything exposed by any library becomes globally accessible. This makes fully-qualified global function calls appear sporadically sprinkled in the code, which can be confusing. I digress, but what I mean to get at is that this is part of what made Microsoft make the shift to the .NET platform in the early 2000’s, and eventually abandon the Visual Basic Editor to its fate. The COM platform and Win32 API was the framework, and Win32 programming languages built on top of that.

This leaves two approaches for a vision of a “framework” for VBA:

  • Package a type library and ship it.
    • Pros: any COM-visible library will work, can be written in .NET
    • Cons: projects now have a hard dependency on a specific type library; updating is a mess, etc.
  • Embed the framework into VBA projects, pretty much like JavaScript does.
    • Pros: devs are in charge of everything, framework is 100% VBA and inherently open-source, updating is essentially seamless for any non-breaking change, no early-bound dependencies, graceful late-bound degradation, etc.
    • Cons: VBA devs and maintainers that aren’t using Rubberduck will be massively lost in the source code (framework would cleanly leverage @Folder annotations), but then when the host application allows it this could be mitigated by embedding the code into its own separate VBA project and reference it from other projects (e.g. ship an Excel add-in with the framework code your VBA project depends on).

I think I’m slightly biased here, but I think this rules out the type library approach regardless. So we need a way to make this work in VBA, with VBA source code that lives in a GitHub repository with vetted, trusted content.

Where Rubberduck fits in

Like Visual Studio, Rubberduck could prompt VBA devs with “what are we building today?” and offer to pull various “bundles” of modules from this GitHub repository into the active project. Rubberduck would request the available “bundles” from, which would return with “bundle metadata” describing each “package” (is “nugget” forbidden to use as a name for these / play on “nuget” (the package manager for .NET)?), and then list them in a nice little dialog.

The “nugget” metadata would include a name, a description, and the path to each file to download for it. Every package would be the same “version”, but the tool could easily request any particular “tag” or “release” version, and/or pull from “main” or from “next” branches, and the source code / framework itself could then easily be a collaborative effort, with its own features and projects and milestones and collaborators, completely separate from the C# Rubberduck code base.

This complete decoupling from Rubberduck means you don’t need to use Rubberduck to leverage this VBA code in your VBA projects, and new tags / “releases” would be entirely independent of Rubberduck’s own release cycles. That means you’re using, say, future-Rubberduck 2.7.4 and the “nuggets” feature offers “v1.0 [main]” and “v1.1 [next]”; one day you’re still using Rubberduck 2.7.4 but now you get “v1.1 [main]”, “v1.0”, and “v1.2 [next]” to chose from, and if you updated the “nuggets” in your project from v1.0 to v1.1 then Rubberduck inspections would flag uses of any obsolete members that would now be decorated with @Obsolete annotations… it’s almost like this annotation was presciently made for this.

But before we can even think of implementing something like this and make MVVM infrastructure the very first “nugget”, we need a rock-solid framework in the first place.

Unit Tests

I had already written the prototype in a highly decoupled manner, mindful of dependencies and how things could later be tested from the outside. I’m very much not-a-zealot when it comes to things like Test-Driven Development (TDD), but I do firmly believe unit tests provide a solid safety net and documentation for everything that matters – especially if the project is to make any kind of framework, where things need to provably work.

And then it makes a wonderful opportunity to blog about writing unit tests with Rubberduck, something I really haven’t written nearly enough about.

Tests? Why?!

Just by writing these tests, I’ve found and fixed edge-case bugs and improved decoupling and cohesion by extracting (and naming!) smaller chunks of functionality into their own separate class module. The result is quite objectively better, simpler code.

Last but not least, writing testable code (let alone the tests!) in VBA makes a great way to learn these more advanced notions and concepts in a language you’re already familiar with.

If you’re new to VBA and programming in general, or if you’re not a programmer and you’re only interested in making macros, then reading any further may make your head spin a bit (if that’s already under way… I’m sorry!), so don’t hesitate to ask here or on the examples repository on GitHub if you have any questions! This article is covering a rather advanced topic, beyond classes and interfaces, but keep in mind that unit testing does not require OOP! It just so happens that object-oriented code adhering to SOLID principles tends to be easily testable.

This is an ongoing project and I’m still working on the test suite and refactoring things; I wouldn’t want to upload the code to GitHub in its current shape, so I’ll come back here with a link once I have something that’s relatively complete.

Where to Start?

There’s a relatively small but very critical piece of functionality that makes a good place to begin in the MVVM infrastructure code (see previous article): the BindingPath class, which I’ve pulled out of PropertyBinding this week. The (still too large for its own good) PropertyBinding class is no longer concerned with the intricacies of resolving property names and values: both this.Source and this.Target are declared As IBindingPath in a PropertyBinding now, which feels exactly right.

The purpose of a BindingPath is to take a “binding context” object and a “binding path” string (the binding path is always relative to the binding context), and to resolve the member call represented there. For example, this would be a valid use of the class:

Dim Path As IBindingPath
Set Path = BindingPath.Create(Sheet1.Shapes("Shape1").TextFrame.Characters, "Text")

This Path object implements TryReadPropertyValue and TryWritePropertyValue methods that the BindingManager can invoke as needed.

'@Folder MVVM.Infrastructure.Bindings
'@ModuleDescription "An object that can resolve a string property path to a value."
Option Explicit
Implements IBindingPath
Private Type TState
    Context As Object
    Path As String
    Object As Object
    PropertyName As String
End Type
Private This As TState
'@Description "Creates a new binding path from the specified property path string and binding context."
Public Function Create(ByVal Context As Object, ByVal Path As String) As IBindingPath
    GuardClauses.GuardNonDefaultInstance Me, BindingPath, TypeName(Me)
    GuardClauses.GuardNullReference Context, TypeName(Me)
    GuardClauses.GuardEmptyString Path, TypeName(Me)
    Dim Result As BindingPath
    Set Result = New BindingPath
    Set Result.Context = Context
    Result.Path = Path
    Set Create = Result
End Function
'@Description "Gets/Sets the binding context."
Public Property Get Context() As Object
    Set Context = This.Context
End Property
Public Property Set Context(ByVal RHS As Object)
    GuardClauses.GuardDefaultInstance Me, BindingPath, TypeName(Me)
    GuardClauses.GuardNullReference RHS, TypeName(Me)
    GuardClauses.GuardDoubleInitialization This.Context, TypeName(Me)
    Set This.Context = RHS
End Property
'@Description "Gets/Sets a string representing a property path against the binding context."
Public Property Get Path() As String
    Path = This.Path
End Property
Public Property Let Path(ByVal RHS As String)
    GuardClauses.GuardDefaultInstance Me, BindingPath, TypeName(Me)
    GuardClauses.GuardEmptyString RHS, TypeName(Me)
    GuardClauses.GuardDoubleInitialization This.Path, TypeName(Me)
    This.Path = RHS
End Property
'@Description "Gets the bound object reference."
Public Property Get Object() As Object
    Set Object = This.Object
End Property
'@Description "Gets the name of the bound property."
Public Property Get PropertyName() As String
    PropertyName = This.PropertyName
End Property
'@Description "Resolves the Path to a bound object and property."
Public Sub Resolve()
    This.PropertyName = ResolvePropertyName(This.Path)
    Set This.Object = ResolvePropertyPath(This.Context, This.Path)
End Sub
Private Function ResolvePropertyName(ByVal PropertyPath As String) As String
    Dim Parts As Variant
    Parts = Strings.Split(PropertyPath, ".")
    ResolvePropertyName = Parts(UBound(Parts))
End Function
Private Function ResolvePropertyPath(ByVal Context As Object, ByVal PropertyPath As String) As Object
    Dim Parts As Variant
    Parts = Strings.Split(PropertyPath, ".")
    If UBound(Parts) = LBound(Parts) Then
        Set ResolvePropertyPath = Context
        Dim RecursiveProperty As Object
        Set RecursiveProperty = CallByName(Context, Parts(0), VbGet)
        If RecursiveProperty Is Nothing Then Exit Function
        Set ResolvePropertyPath = ResolvePropertyPath(RecursiveProperty, Right$(PropertyPath, Len(PropertyPath) - Len(Parts(0)) - 1))
    End If
End Function
Private Property Get IBindingPath_Context() As Object
    Set IBindingPath_Context = This.Context
End Property
Private Property Get IBindingPath_Path() As String
    IBindingPath_Path = This.Path
End Property
Private Property Get IBindingPath_Object() As Object
    Set IBindingPath_Object = This.Object
End Property
Private Property Get IBindingPath_PropertyName() As String
    IBindingPath_PropertyName = This.PropertyName
End Property
Private Sub IBindingPath_Resolve()
End Sub
Private Function IBindingPath_ToString() As String
    IBindingPath_ToString = StringBuilder _
        .AppendFormat("Context: {0}; Path: {1}", TypeName(This.Context), This.Path) _
End Function
Private Function IBindingPath_TryReadPropertyValue(ByRef outValue As Variant) As Boolean
    If This.Object Is Nothing Then Resolve
    On Error Resume Next
    outValue = VBA.Interaction.CallByName(This.Object, This.PropertyName, VbGet)
    IBindingPath_TryReadPropertyValue = (Err.Number = 0)
    On Error GoTo 0
End Function
Private Function IBindingPath_TryWritePropertyValue(ByVal Value As Variant) As Boolean
    If This.Object Is Nothing Then Resolve
    On Error Resume Next
    VBA.Interaction.CallByName This.Object, This.PropertyName, VbLet, Value
    IBindingPath_TryWritePropertyValue = (Err.Number = 0)
    On Error GoTo 0
End Function

Here’s our complete “system under test” (SUT) as far as the BindingPathTests module goes. We have a Create factory method, Context and Path properties, just like the class we’re testing.

The path object is itself read-only once initialized, but the binding source may resolve to Nothing or to a different object reference over the course of the object’s lifetime: say we want a binding path to SomeViewModel.SomeObjectProperty; when we first create the binding, SomeObjectProperty might very well be Nothing, and then it’s later Set-assigned to a valid object reference. This is why the IBindingPath interface needs to expose a Resolve method, so that IPropertyBinding can invoke it as needed, as the binding is being applied.

We’ll want a test for every guard clause, and each method needs at least one test as well.

So, I’m going to add a new test module and call it BindingPathTests. Rubberduck’s templates are good-enough to depict the mechanics and how things work at a high level, but if you stick to the templates you’ll quickly find your unit tests rather boring, wordy, and repetitive: we must break out of the mold, there isn’t one true way to do this!

Rubberduck discovers unit tests in standard modules annotated with @TestModule. Test methods are any [parameterless, for now] method annotated with a @TestMethod annotation that can have a category string – the Test Explorer can group your tests using these categories. The declarations section of a test module must include a declaration (early or late bound) for an Rubberduck.AssertClass or Rubberduck.PermissiveAssertClass (both implement the same internal interface; the “permissive” one has VBA-like equality semantics, and the default one has stricter type equality requirements (a Long can’t be equal to a Double, for example). The default test template also defines a FakesProvider object, but we’re not going to need it now (if we needed to test logic that involved e.g. branching on the result of a MsgBox function call, we could hook into the MsgBox function and configure it to return what the test needs it to return, which is honestly wicked awesome). So our test module might look something like this at first:

'@Folder Tests.Bindings
Option Explicit
Option Private Module
#Const LateBind = LateBindTests
#If LateBind Then
Private Assert As Object
Private Assert As Rubberduck.AssertClass
#End If

With this conditionally-compiled setup, all we need to toggle between late and early binding is to define a project-scoped conditional compilation argument: bring up the project properties and type LateBindTests=0 or LateBindTests=1 in that box, and just like that you can control conditional compilation project-wide without modifying a single module.

The first thing to do is to get the test state defined, and implement TestInitialize and TestCleanup methods that configure this state – in the case of BindingManagerTests, I’m going to add a private type and a private field to define and hold the current test state:

Private Type TState
    ExpectedErrNumber As Long
    ExpectedErrSource As String
    ExpectedErrorCaught As Boolean
    ConcreteSUT As BindingManager
    AbstractSUT As IBindingManager
    HandlePropertyChangedSUT As IHandlePropertyChanged
    BindingSource As TestBindingObject
    BindingTarget As TestBindingObject
    SourcePropertyPath As String
    TargetPropertyPath As String
    Command As TestCommand
End Type
Private Test As TState

Unit Testing Paradigm

Test modules are special, in the sense that they aren’t (absolutely shouldn’t be anyway) accessible from any code path in the project. Rubberduck invokes them one by one when you run a command like “run all tests” or “repeat last run”. But there’s a little more to it than that, worthy of mention.

VBA being single-threaded, tests are invoked by Rubberduck on the UI/main thread, and uses a bit of trickery to keep its own UI somewhat responsive. Each module runs sequentially, and each test inside each module runs sequentially as well – but the test execution order still shouldn’t be considered deterministic, and each test should be completely independent of every other test, such that executing all tests in any given order always produces the same outcomes.

A test that makes no assertions will be green/successful. When writing unit tests, the first thing you want to see is a test that’s failing (you can’t trust a test you have never seen fail!), and with Rubberduck in order to give a test a reason to fail, you use Assert methods (wiki).

When Rubberduck begins processing a test module, it invokes the methods (again, sequentially but not in an order that should matter) marked @ModuleInitialize in the module – ideally that would be only one method.

This is where the Assert object should be assigned (the default test templates do this):

Private Sub ModuleInitialize()
#If LateBind Then
    'requires HKCU registration of the Rubberduck COM library.
    Set Assert = CreateObject("Rubberduck.PermissiveAssertClass")
    'requires project reference to the Rubberduck COM library.
    Set Assert = New Rubberduck.PermissiveAssertClass
#End If
End Sub

Rubberduck’s test engine will then execute all methods (usually cleaner with only one though) annotated with @TestInitialize before executing each test in the module; that is the best place to put the wordy setup code that would otherwise need to be in pretty much every single test of the module:

Private Sub TestInitialize()
    Dim Context As TestBindingObject
    Set Context = New TestBindingObject
    Set Context.TestBindingObjectProperty = New TestBindingObject
    Test.Path = "TestBindingObjectProperty.TestStringProperty"
    Test.PropertyName = "TestStringProperty"
    Set Test.BindingSource = Context.TestBindingObjectProperty
    Set Test.BindingContext = Context
    Set Test.ConcreteSUT = BindingPath.Create(Test.BindingContext, Test.Path)
    Set Test.AbstractSUT = Test.ConcreteSUT
End Sub

By moving the test state to module level rather than having it local to each test, we already eliminate a lot of code duplication, and the Test module variable makes a rather nifty way to access the current test state, too!

Methods annotated with @TestCleanup are automatically invoked after each test in the module; in order to avoid accidentally sharing state between tests, every object reference should be explicitly set to Nothing, and values of intrinsic data types should be explicitly reset to their respective default value:

Private Sub TestCleanup()
    Set Test.ConcreteSUT = Nothing
    Set Test.AbstractSUT = Nothing
    Set Test.BindingSource = Nothing
    Set Test.BindingContext = Nothing
    Test.Path = vbNullString
    Test.PropertyName = vbNullString
    Test.ExpectedErrNumber = 0
    Test.ExpectedErrSource = vbNullString
    Test.ExpectedErrorCaught = False
End Sub

What Goes Into the Test State?

A number of members should always be in the Test state structure:

  • ConcreteSUT (or just SUT) and AbstractSUT both point to the same object, through the default interface (BindingPath) and the explicit one (IBindingPath), respectively.
  • If the system under test class implements additional interfaces, having a pointer to the SUT object with these interfaces is also useful. For example the TState type for the BindingManager class has a HandlePropertyChangedSUT As IHandlePropertyChanged member, because the class implements this interface.
  • Default property values and dependency setup: we want a basic default SUT configured and ready to be tested (or fine-tuned and then tested).
  • ExpectedErrNumber, ExpectedErrSource, and ExpectedErrorCaught are useful when a test is expecting a given input to produce a particular specific error.

Expecting Errors

The “expected error” test method template works for its purpose, but having this on-error-assert logic duplicated everywhere is rather ugly. Consider pulling that logic into a private method instead (I’m considering adding this into Rubberduck’s test module templates):

Private Sub ExpectError()
    Dim Message As String
    If Err.Number = Test.ExpectedErrNumber Then
        If (Test.ExpectedErrSource = vbNullString) Or (Err.Source = Test.ExpectedErrSource) Then
            Test.ExpectedErrorCaught = True
            Message = "An error was raised, but not from the expected source. " & _
                      "Expected: '" & TypeName(Test.ConcreteSUT) & "'; Actual: '" & Err.Source & "'."
        End If
    ElseIf Err.Number <> 0 Then
        Message = "An error was raised, but not with the expected number. Expected: '" & Test.ExpectedErrNumber & "'; Actual: '" & Err.Number & "'."
        Message = "No error was raised."
    End If
    If Not Test.ExpectedErrorCaught Then Assert.Fail Message
End Sub

With this infrastructure in place, the unit tests for all guard clauses in the module can look like this – it’s still effectively doing Arrange-Act-Assert like the test method templates strongly suggest, only implicitly so (each “A” is essentially its own statement, see comments in the tests below):

Private Sub Create_GuardsNullBindingContext()
    Test.ExpectedErrNumber = GuardClauseErrors.ObjectCannotBeNothing '<~ Arrange
    On Error Resume Next
        BindingPath.Create Nothing, Test.Path '<~ Act
        ExpectError '<~ Assert
    On Error GoTo 0
End Sub
Private Sub Create_GuardsEmptyPath()
    Test.ExpectedErrNumber = GuardClauseErrors.StringCannotBeEmpty '<~ Arrange
    On Error Resume Next
        BindingPath.Create Test.BindingContext, vbNullString '<~ Act
        ExpectError '<~ Assert
    On Error GoTo 0
End Sub
Private Sub Create_GuardsNonDefaultInstance()
    Test.ExpectedErrNumber = GuardClauseErrors.InvalidFromNonDefaultInstance '<~ Arrange
    On Error Resume Next
        With New BindingPath
            .Create Test.BindingContext, Test.Path '<~ Act
            ExpectError '<~ Assert
        End With
    On Error GoTo 0
End Sub

And then similar tests exist for the respective guard clauses of Context and Path members. Having tests that validate that guard clauses are doing their job is great: it tells us exactly how not to use the class… and that doesn’t tell us much about what a BindingPath object actually does.

Testing the Actual Functionality

The methods we’re testing need to be written in a way that makes it possible for a test to determine whether it’s doing its job correctly or not. For functions and properties, the return value is the perfect thing to Assert on. For Sub procedures, you have to Assert on the side-effects, and have verifiable and useful, reliable ways to verify them.

These two tests validate that the BindingPath returned by the Create factory method has resolved the PropertyName and Object properties, respectively.

Private Sub Create_ResolvesPropertyName()
    Dim SUT As BindingPath
    Set SUT = BindingPath.Create(Test.BindingContext, Test.Path)
    Assert.IsFalse SUT.PropertyName = vbNullString
End Sub
Private Sub Create_ResolvesBindingSource()
    Dim SUT As BindingPath
    Set SUT = BindingPath.Create(Test.BindingContext, Test.Path)
    Assert.IsNotNothing SUT.Object
End Sub

I could have made multiple assertions in a test, like this…

Private Sub Create_ResolvesBindingSource()
    Dim SUT As BindingPath
    Set SUT = BindingPath.Create(Test.BindingContext, Test.Path)
    Assert.IsFalse SUT.PropertyName = vbNullString
    Assert.IsNotNothing SUT.Object
End Sub

The Test Explorer would say “IsFalse assert failed” or “IsNotNothing assert failed”, so it’s arguably (perhaps pragmatically so) still useful and clear enough why that test would fail (and if you had multiple Assert.IsFalse calls in a test you could provide a different message for each)… but really as a rule of thumb, tests want to have one reason to fail. If the conditions to meaningfully pass or fail a test aren’t present, use Assert.Inconclusive to report the test as such:

Private Sub Resolve_SetsBindingSource()
    With New BindingPath
        .Path = Test.Path
        Set .Context = Test.BindingContext
        If Not .Object Is Nothing Then Assert.Inconclusive "Object reference is unexpectedly set."
        Assert.AreSame Test.BindingSource, .Object
    End With
End Sub
Private Sub Resolve_SetsBindingPropertyName()
    With New BindingPath
        .Path = Test.Path
        Set .Context = Test.BindingContext
        If .PropertyName <> vbNullString Then Assert.Inconclusive "PropertyName is unexpectedly non-empty."
        Assert.AreEqual Test.PropertyName, .PropertyName
    End With
End Sub

This mechanism is especially useful when the test state isn’t in local scope and there’s a real possibility that the TestInitialize method is eventually modified and inadvertently breaks a test. Such conditional Assert.Inconclusive calls are definitely a form of defensive programming, just like having guard clauses throwing custom meaningful errors.

Note that while we know that the BindingPath.Create function invokes the Resolve method, the tests for Resolve don’t involve Create: the Path and Context are being explicitly spelled out, and the .Resolve method is invoked from a New instance.

And that’s pretty much everything there is to test in the BindingPath class.

There’s one thing I haven’t mentioned yet, that you might have caught in the TState type:

BindingSource As TestBindingObject
BindingTarget As TestBindingObject

This TestBindingObject is a test stub: it’s a dependency of the class (it’s the “binding context” of the test path) and it’s a real object, but it is implemented in a bit of a special way that the BindingPath tests don’t do justice to.

Test Stubs

Eventually Rubberduck’s unit testing framework will feature a COM-visible wrapper around Moq, a popular mocking framework for .NET that Rubberduck already uses for its own unit test requirements. When this happens Rubberduck unit tests will no longer need such “test stubs”. Instead, the framework will generate them at run-time and make them work exactly as specified/configured by a unit test, and “just like that” VBA/VB6 suddenly becomes surprisingly close to being pretty much on par with professional, current-day IDE tooling.

The ITestStub interface simply formalizes the concept:

'@Folder Tests.Stubs
'@ModuleDescription "An object that stubs an interface for testing purposes."
Option Explicit
'@Description "Gets the number of times the specified member was invoked in the lifetime of the object."
Public Property Get MemberInvokes(ByVal MemberName As String) As Long
End Property
'@Description "Gets a string representation of the object's internal state, for debugging purposes (not intended for asserts!)."
Public Function ToString() As String
End Function

A TestStubBase “base class” provides the common implementation mechanics that every class implementing ITestStub will want to use – the idea is to use a keyed data structure to track the number of times each member is invoked during the lifetime of the object:

'@Folder Tests.Stubs
Option Explicit
Private Type TState
    MemberInvokes As Dictionary
End Type
Private This As TState
'@Description "Tracks a new invoke of the specified member."
Public Sub OnInvoke(ByVal MemberName As String)
    Dim newValue As Long
    If This.MemberInvokes.Exists(MemberName) Then
        newValue = This.MemberInvokes.Item(MemberName) + 1
        This.MemberInvokes.Remove MemberName
        newValue = 1
    End If
    This.MemberInvokes.Add MemberName, newValue
End Sub
'@Description "Gets the number of invokes made against the specified member in the lifetime of this object."
Public Property Get MemberInvokes(ByVal MemberName As String) As Long
    If This.MemberInvokes.Exists(MemberName) Then
        MemberInvokes = This.MemberInvokes.Item(MemberName)
        MemberInvokes = 0
    End If
End Property
'@Description "Gets a string listing the MemberInvokes cache content."
Public Function ToString() As String
    Dim MemberNames As Variant
    MemberNames = This.MemberInvokes.Keys
    With New StringBuilder
        Dim i As Long
        For i = LBound(MemberNames) To UBound(MemberNames)
            Dim Name As String
            Name = MemberNames(i)
            .AppendFormat "{0} was invoked {1} time(s)", Name, This.MemberInvokes.Item(Name)
        ToString = .ToString
    End With
End Function
Private Sub Class_Initialize()
    Set This.MemberInvokes = New Dictionary
End Sub

With this small bit of infrastructure, the TestBindingObject class is a full-fledged mock object that can increment a counter whenever a member is invoked, and that can be injected as a dependency for anything that needs an IViewModel:

'@Folder Tests.Stubs
'@ModuleDescription "An object that can stub a binding source or target for unit tests."
Option Explicit
Implements ITestStub
Implements IViewModel
Implements INotifyPropertyChanged
Private Type TState
    Stub As TestStubBase
    Handlers As Collection
    TestStringProperty As String
    TestNumericProperty As Long
    TestBindingObjectProperty As TestBindingObject
    Validation As IHandleValidationError
End Type
Private This As TState
Public Property Get TestStringProperty() As String
    This.Stub.OnInvoke "TestStringProperty.Get"
    TestStringProperty = This.TestStringProperty
End Property
Public Property Let TestStringProperty(ByVal RHS As String)
    This.Stub.OnInvoke "TestStringProperty.Let"
    If This.TestStringProperty <> RHS Then
        This.TestStringProperty = RHS
        OnPropertyChanged Me, "TestStringProperty"
    End If
End Property
Public Property Get TestNumericProperty() As Long
    This.Stub.OnInvoke "TestNumericProperty.Get"
    TestNumericProperty = This.TestNumericProperty
End Property
Public Property Let TestNumericProperty(ByVal RHS As Long)
    This.Stub.OnInvoke "TestNumericProperty.Let"
    If This.TestNumericProperty <> RHS Then
        This.TestNumericProperty = RHS
        OnPropertyChanged Me, "TestNumericProperty"
    End If
End Property
Public Property Get TestBindingObjectProperty() As TestBindingObject
    This.Stub.OnInvoke "TestBindingObjectProperty.Get"
    Set TestBindingObjectProperty = This.TestBindingObjectProperty
End Property
Public Property Set TestBindingObjectProperty(ByVal RHS As TestBindingObject)
    This.Stub.OnInvoke "TestBindingObjectProperty.Set"
    If Not This.TestBindingObjectProperty Is RHS Then
        Set This.TestBindingObjectProperty = RHS
        OnPropertyChanged Me, "TestBindingObjectProperty"
    End If
End Property
Private Sub OnPropertyChanged(ByVal Source As Object, ByVal PropertyName As String)
    Dim Handler As IHandlePropertyChanged
    For Each Handler In This.Handlers
        Handler.OnPropertyChanged Source, PropertyName
End Sub
Private Sub Class_Initialize()
    Set This.Stub = New TestStubBase
    Set This.Handlers = New Collection
    Set This.Validation = ValidationManager.Create
End Sub
Private Sub INotifyPropertyChanged_OnPropertyChanged(ByVal Source As Object, ByVal PropertyName As String)
    OnPropertyChanged Source, PropertyName
End Sub
Private Sub INotifyPropertyChanged_RegisterHandler(ByVal Handler As IHandlePropertyChanged)
    This.Handlers.Add Handler
End Sub
Private Property Get ITestStub_MemberInvokes(ByVal MemberName As String) As Long
    ITestStub_MemberInvokes = This.Stub.MemberInvokes(MemberName)
End Property
Private Function ITestStub_ToString() As String
    ITestStub_ToString = This.Stub.ToString
End Function
Private Property Get IViewModel_Validation() As IHandleValidationError
    Set IViewModel_Validation = This.Validation
End Property

This functionality will be extremely useful when testing the actual property bindings: for example we can assert that a method was invoked exactly once, and fail a test if the method was invoked twice (and/or if it never was).

There’s a lot more to discuss about unit testing in VBA with Rubberduck! I hope this article gives a good idea of how to get the best out of Rubberduck’s unit testing feature.

Model, View, ViewModel

100% VBA, 100% OOP

We’ve seen in UserForm1.Show what makes a Smart UI solution brittle, and how to separate the UI concerns from rest of the logic with the Model-View-Presenter (MVP) UI pattern. MVP works nicely with the MSForms library (UserForms in VBA), just like it does with its .NET Windows Forms successor. While the pattern does a good job of enhancing the testability of application logic, it also comes with its drawbacks: the View’s code-behind (that is, the code module “behind” the form designer) is still littered with noisy event handlers and boilerplate code, and the back-and-forth communication between the View and the Presenter feels somewhat clunky with events and event handlers.

Rubberduck’s UI elements are made with the Windows Presentation Foundation (WPF) UI framework, which completely redefines how everything about UI programming works, starting with the XML/markup-based (XAML) design, but the single most compelling element is just how awesome its data binding capabilities are.

We can leverage in VBA what makes Model-View-ViewModel (MVVM) awesome in C# without going nuts and writing a whole UI framework from scratch, but we’re still going to need a bit of an abstract infrastructure to work with. It took the will to do it and only costed a hair or two, but as far as I can tell this works perfectly fine, at least at the proof-of-concept stage.

This article is the first in a series that revolves around MVVM in VBA as I work (very much part-time) on the rubberduckdb content admin tool. There’s quite a bit of code to make this magic happen, so let’s kick this off with what it does and how to use it – subsequent articles will dive into how the MVVM infrastructure internals work. As usual the accompanying code can be found in the examples repository on GitHub (give it a star, and fork it, then make pull requests with your contributions during Hacktoberfest next month and you can get a t-shirt, stickers, and other free stuff, courtesy of Digital Ocean!).


The code in the examples repository isn’t the reason I wrote this: I mentioned in the previous post that I was working on an application to maintain the website content, and decided to explore the Model-View-ViewModel pattern for that one. Truth be told, MVVM is hands-down my favorite UI pattern, by far. This is simply the cleanest UI code I’ve ever written in VBA, and I love it!

A screenshot of a carefully-crafted dialog form for managing content served by A modal prompts the user for SQL Server credentials, all commands but the "reload" button are disabled.
The app is work in progress, but the property and command bindings work!

The result is an extremely decoupled, very extensible, completely testable architecture where every user action (“command”) is formally defined, can be programmatically simulated/tested with real, stubbed, or faked dependencies, and can be bound to multiple UI elements and programmatically executed as needed.

MVVM Quick Checklist

These would be the rules to follow as far a relationships go between the components of the MVVM pattern:

  • View (i.e. the UserForm) knows about the ViewModel, but not the Model;
  • ViewModel knows about commands, but nothing about a View;
  • Exactly what the Model actually is/isn’t/should/shouldn’t be, is honestly not a debate I’m interested in – I’ll just call whatever set of classes is responsible for hydrating my ViewModel with data my “model” and sleep at night. What matters is that whatever you call the Model knows nothing of a View or ViewModel, it exists on its own.

Before we dive into bindings and the infrastructure code, we need to talk about the command pattern.


A command is an object that implements an ICommand interface that might look like this:

'@Folder MVVM.Infrastructure
'@ModuleDescription "An object that represents an executable command."
Option Explicit

'@Description "Returns True if the command is enabled given the provided binding context (ViewModel)."
Public Function CanExecute(ByVal Context As Object) As Boolean
End Function

'@Description "Executes the command given the provided binding context (ViewModel)."
Public Sub Execute(ByVal Context As Object)
End Sub

'@Description "Gets a user-friendly description of the command."
Public Property Get Description() As String
End Property

In the case of a CommandBinding the Context parameter is always the DataContext / ViewModel (for now anyway), but manual invokes could supply other kinds of parameters. Not all implementations need to account for the ViewModel, a CanExecute function that simply returns True is often perfectly fine. The Description is used to set a tooltip on the target UI element of the command binding.

The implementation of a command can be very simple or very complex, depending on the needs. A command might have one or more dependencies, for example a ReloadCommand might want to be injected with some IDbContext object that exposes a SelectAllTheThings function and the implementation might pull them from a database, or make them up from hard-coded strings: the command has no business knowing where the data comes from and how it’s acquired.

Each command is its own class, and encapsulates the logic for enabling/disabling its associated control and executing the command. This leaves the UserForm module completely devoid of any logic that isn’t purely a presentation concern – although a lot can be achieved solely with property bindings and validation error formatters.

The infrastructure code comes with AcceptCommand and CancelCommand implementations, both useful to wire up [Ok], [Cancel], or [Close] dialog buttons.


The AcceptCommand can be used as-is for any View that can be closed with a command involving similar semantics. It is implemented as follows:

'@Folder MVVM.Infrastructure.Commands
'@ModuleDescription "A command that closes (hides) a View."
Option Explicit
Implements ICommand

Private Type TState
    View As IView
End Type

Private this As TState

'@Description "Creates a new instance of this command."
Public Function Create(ByVal View As IView) As ICommand
    Dim result As AcceptCommand
    Set result = New AcceptCommand
    Set result.View = View
    Set Create = result
End Function

Public Property Get View() As IView
    Set View = this.View
End Property

Public Property Set View(ByVal RHS As IView)
    GuardClauses.GuardDoubleInitialization this.View, TypeName(Me)
    Set this.View = RHS
End Property

Private Function ICommand_CanExecute(ByVal Context As Object) As Boolean
    Dim ViewModel As IViewModel
    If TypeOf Context Is IViewModel Then
        Set ViewModel = Context
        If Not ViewModel.Validation Is Nothing Then
            ICommand_CanExecute = ViewModel.Validation.IsValid
            Exit Function
        End If
    End If
    ICommand_CanExecute = True
End Function

Private Property Get ICommand_Description() As String
    ICommand_Description = "Accept changes and close."
End Property

Private Sub ICommand_Execute(ByVal Context As Object)
End Sub


This command is similar to the AcceptCommand in that it simply invokes a method in the View. This implementation could easily be enhanced by making the ViewModel track “dirty” (modified) state and prompting the user when they are about to discard unsaved changes.

'@Folder MVVM.Infrastructure.Commands
'@ModuleDescription "A command that closes (hides) a cancellable View in a cancelled state."
Option Explicit
Implements ICommand

Private Type TState
    View As ICancellable
End Type

Private this As TState

'@Description "Creates a new instance of this command."
Public Function Create(ByVal View As ICancellable) As ICommand
    Dim result As CancelCommand
    Set result = New CancelCommand
    Set result.View = View
    Set Create = result
End Function

Public Property Get View() As ICancellable
    Set View = this.View
End Property

Public Property Set View(ByVal RHS As ICancellable)
    GuardClauses.GuardDoubleInitialization this.View, TypeName(Me)
    Set this.View = RHS
End Property

Private Function ICommand_CanExecute(ByVal Context As Object) As Boolean
    ICommand_CanExecute = True
End Function

Private Property Get ICommand_Description() As String
    ICommand_Description = "Cancel pending changes and close."
End Property

Private Sub ICommand_Execute(ByVal Context As Object)
End Sub

This gives us very good indications about how the pattern wants user actions to be implemented:

  • Class can have a @PredeclaredId annotation and expose a factory method to property-inject any dependencies; here a IView object, but a custom SaveChangesCommand would likely get injected with some DbContext service class.
  • All commands need a description; that description is user-facing as a tooltip on the binding target (usually a CommandButton).
  • CanExecute can be as simple as an unconditional ICommand_CanExecute = True, or as complex as needed (it has access to the ViewModel context); keep in mind that this method can be invoked relatively often, and should perform well and return quickly.

It’s a simple interface with a simple purpose: attach a command to a button. The EvaluateCanExecute method invokes the command’s CanExecute function and accordingly enables or disables the Target control.

By implementing all UI commands as ICommand objects, we keep both the View and the ViewModel free of command logic and Click handlers. By adopting the command pattern, we give ourselves all the opportunities to achieve low coupling and high cohesion. That is, small and specialized modules that depend on abstractions that can be injected from the outside.

Property Bindings

In XAML we use a special string syntax (“markup extensions”) to bind the value of, say, a ViewModel property, to that of a UI element property:

<TextBox Text="{Binding SomeProperty, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

As long as the ViewModel implements INotifyPropertyChanged and the property fires the PropertyChanged event when its value changes, WPF can automatically keep the UI in sync with the ViewModel and the ViewModel in sync with the UI. WPF data bindings are extremely flexible and can also bind to static and dynamic resources, or other UI elements, and they are actually slightly more complex than that, but this captures the essence.

Obviously MVVM with MSForms in VBA isn’t going to involve any kind of special string syntax, but the concept of a PropertyBinding can very much be encapsulated into an object (and XAML compiles down to objects and methods, too). At its core, a binding is a pretty simple thing: a source, a target, and a method to update them.

Technically nothing prevents binding a target to any object type (although with limitations, since non-user code won’t be implementing INotifyPropertyChanged), but for the sake of clarity:

  • The binding Source is the ViewModel
  • The SourcePropertyPath is the name of a property of the ViewModel
  • The binding Target is the MSForms control
  • The binding TargetProperty is the name of a property of the MSForms control

Note that the SourcePropertyPath resolves recursively and can be a property of a propertyof a property – as long as the string ultimately resolves to a non-object member.

.BindPropertyPath ViewModel, "SourcePath", Me.PathBox, _
    Validator:=New RequiredStringValidator, _
    ErrorFormat:=AggregateErrorFormatter.Create(ViewModel, _
        ValidationErrorFormatter.Create(Me.PathBox) _ 
            .WithErrorBackgroundColor _
            .WithErrorBorderColor, _
        ValidationErrorFormatter.Create(Me.InvalidPathIcon) _
            .WithTargetOnlyVisibleOnError("SourcePath"), _                
        ValidationErrorFormatter.Create(Me.ValidationMessage1) _

The IBindingManager.BindPropertyPath method is pretty flexible and accepts a number of optional parameters while implementing sensible defaults for common MSForms controls’ “default property binding”. For example, you don’t need to specify a TargetProperty when binding a ViewModel property to a MSForms.TextBox: it will automatically binds to the Text property, but will accept to bind any other property.

The optional arguments are especially useful for custom data validation, but some of them also control various knobs that determine what and how the binding updates.

TwoWayBindingBinding will update the source when the target changes, and will update the target when the source changes.
OneWayBindingBinding will update the target when the source changes.
OneWayToSourceBinding will update the source when the target changes.
OneTimeBindingBinding will only update the target once.
The BindingMode enum values
OnPropertyChangedBinding will update when the bound property value changes.
OnKeyPressBinding will update the source at each keypress. Only available for TextBox controls. Data validation may prevent the keypress from reaching the UI element.
OnExitBinding will update the source just before target loses focus. Data validation may cancel the exit and leave the caret inside. This update source trigger is the most efficient since it only updates bindings when the user has finished providing a value.
The UpdateSourceTrigger enum values

Property Paths

The binding manager is able to recursively resolve a member path, so if your ViewModel has a ThingSection property that is itself a ViewModel with its own bindings and commands, that itself has a Thing property, know that the binding path can legally be “ThingSection.Thing“, and as long as the Source is the ViewModel object where a ThingSection property exists, and that the ThingSection porperty yields an object that has a Thing property, then all is good and the binding works. If ThingSection were to be Nothing when the binding is updated, then the target is assigned with a default value depending on the type. For example if ThingSection.Thing was bound to some TextBox1 control and the ThingSection property of the ViewModel was Nothing, then the Text property would end up being an empty string – note that this default value may be illegal, depending on what data validation is in place.

Data Validation

Every property binding can attach any IValueValidator implementation that encapsulates specialized, bespoke validation rules. The infrastructure code doesn’t include any custom validator, but the example show how one can be implemented. The interface mandates an IsValid function that returns a Boolean (True when valid), and a user-friendly Message property that the ValidationManager uses to create tooltips.

'@Folder MVVM.Example
Option Explicit
Implements IValueValidator

Private Function IValueValidator_IsValid(ByVal Value As Variant, ByVal Source As Object, ByVal Target As Object) As Boolean
    IValueValidator_IsValid = Len(Trim$(Value)) > 0
End Function

Private Property Get IValueValidator_Message() As String
    IValueValidator_Message = "Value cannot be empty."
End Property

The IsValid method provides you with the Value being validated, the binding Source, and the binding Target objects, which means every validator has access to everything exposed by the ViewModel; note that the method being a Function strongly suggests that it should not have side-effects. Avoid mutating ViewModel properties in a validator, but the message can be constructed dynamically if the validator is made to hold module-level state… although I would really strive to avoid making custom validators stateful.

While the underlying data validation mechanics are relatively complex, believe it or not there is no other step needed to implement custom validation for your property bindings: IBindingManager.BindPropertyPath is happy to take in any validator object, as long as it implements the IValueValidator interface.

Presenting Validation Errors

Without taking any steps to format validation errors, commands that can only execute against a valid ViewModel will automatically get disabled, but the input field with the invalid value won’t give the user any clue. By providing an IValidationErrorFormatter implementation when registering the binding, you get to control whether hidden UI elements should be displayed when there’s a validation error.

The ValidationErrorFormatter class meets most simple scenarios. Use the factory method to create an instance with a specific target UI element, then chain builder method calls to configure the formatting inline with a nice, fluent syntax:

Set Formatter = ValidationErrorFormatter.Create(Me.PathBox) _
                                        .WithErrorBackgroundColor(vbYellow) _
CreateFactory method, ensures every instance is created with a target UI element.
WithErrorBackgroundColorMakes the target have a different background color given a validation error. If no color is specified, a default “error background color” (light red) is used.
WithErrorBorderColorMakes the target have a different border color given a validation error. If no color is specified, a default “error border color” (dark red) is used. Method has no effect if the UI control isn’t “flat style” or if the border style isn’t “fixed single”.
WithErrorForeColorMakes the target have a different fore (text) color given a validation error. If no color is specified, a default “error border color” (dark red) is used.
WithErrorFontBoldMakes the target use a bold font weight given a validation error. Method has no effect if the UI element uses a bolded font face without a validation error.
WithTargetOnlyVisibleOnErrorMakes the target UI element normally hidden, only to be made visible given a validation error. Particularly useful with aggregated formatters, to bind the visibility of a label and/or an icon control to the presence of a validation error.
The factory and builder methods of the ValidationErrorFormatter class.

The example code uses an AggregateErrorFormatter to tie multiple ValidationErrorFormatter instances (and thus possibly multiple different target UI controls) to the the same binding.

Value Converters

IBindingManager.BindPropertyPath can take an optional IValueConverter parameter when a conversion is needed between the source and the target, or between the target and the source. One useful value converter can be one like the InverseBooleanConverter implementation, which can be used in a binding where True in the source needs to bind to False in the target.

The interface mandates the presence of Convert and ConvertBack functions, respectively invoked when the binding value is going to the target and the source. Again, pure functions and performance-sensitive implementations should be preferred over side-effecting code.

'@Folder MVVM.Infrastructure.Bindings.Converters
'@ModuleDescription "A value converter that inverts a Boolean value."
Option Explicit
Implements IValueConverter

Public Function Default() As IValueConverter
    GuardClauses.GuardNonDefaultInstance Me, InverseBooleanConverter
    Set Default = InverseBooleanConverter
End Function

Private Function IValueConverter_Convert(ByVal Value As Variant) As Variant
    IValueConverter_Convert = Not CBool(Value)
End Function

Private Function IValueConverter_ConvertBack(ByVal Value As Variant) As Variant
    IValueConverter_ConvertBack = Not CBool(Value)
End Function

Converters used in single-directional bindings don’t need to necessarily make both functions return a value that makes sense: sometimes a value can be converted to another but cannot round-trip back to the original, and that’s fine.

String Formatting

One aspect of property bindings I haven’t tackled yet, is the whole StringFormat deal. Once that is implemented and working, the string representation of the target control will be better separated from its actual value. And a sensible default format for some data types (Date, Currency) can even be inferred from the type of the source property!

Another thing string formatting would enable, is the ability to interpolate the value within a string. For example there could be a property binding defined like this:

.BindPropertyPath ViewModel, "NetAmount", Me.NetAmountBox, StringFormat:="USD$ {0:C2}"

And the NetAmountBox would read “USD$ 1,386.77” given the value 1386.77, and the binding would never get confused and would always know that the underlying value is a numeric value of 1386.77 and not a formatted string. Now, until that is done, string formatting probably needs to involve custom value converters. When string formatting works in property bindings, any converter will get invoked before: it’s always going to be the converted value that gets formatted.


Every ViewModel class is inherently application-specific and will look different, but there will be recurring themes:

  • Every field in the View wants to bind to a ViewModel property, and then you’ll want extra properties for various other things, so the ViewModel quickly grows more properties than comfort allows. Make smaller “ViewModel” classes by regrouping related properties, and bind with a property path rather than a plain property name.
  • Property changes need to propagate to the “main” ViewModel (the “data context”) somehow, so making all ViewModel classes fire a PropertyChanged event as appropriate is a good idea. Hold a WithEvents reference to the “child” ViewModel, and handle propagation by raising the “parent” ViewModel’s own PropertyChanged event, all the way up to the “main” ViewModel, where the handler nudges command bindings to evaluate whether commands can execute. One solution could be to register all command bindings with some CommandManager object that would have to implement IHandlePropertyChanged and would relieve the ViewModel of needing to do this.

Each ViewModel should implement at least two interfaces:

  • IViewModel, because we need a way to access the validation error handler and this interface makes a good spot for it.
  • INotifyPropertyChanged, to notify data bindings when a ViewModel property changes.

Here is the IViewModel implementation for the example code – the idea is really to expose properties for the view to bind, and we must not forget to notify handlers when a property value changes – notice the RHS-checking logic in the Property Let member:

'@Folder MVVM.Example
'@ModuleDescription "An example ViewModel implementation for some dialog."
Implements IViewModel
Implements INotifyPropertyChanged
Option Explicit

Public Event PropertyChanged(ByVal Source As Object, ByVal PropertyName As String)

Private Type TViewModel
    'INotifyPropertyChanged state:
    Handlers As Collection
    SomeCommand As ICommand
    'Read/Write PropertyBindings:
    SourcePath As String
    SomeOption As Boolean
    SomeOtherOption As Boolean
End Type

Private this As TViewModel
Private WithEvents ValidationHandler As ValidationManager

Public Function Create() As IViewModel
    GuardClauses.GuardNonDefaultInstance Me, ExampleViewModel, TypeName(Me)
    Dim result As ExampleViewModel
    Set result = New ExampleViewModel
    Set Create = result
End Function

Public Property Get Validation() As IHandleValidationError
    Set Validation = ValidationHandler
End Property

Public Property Get SourcePath() As String
    SourcePath = this.SourcePath
End Property

Public Property Let SourcePath(ByVal RHS As String)
    If this.SourcePath <> RHS Then
        this.SourcePath = RHS
        OnPropertyChanged "SourcePath"
    End If
End Property

Public Property Get SomeOption() As Boolean
    SomeOption = this.SomeOption
End Property

Public Property Let SomeOption(ByVal RHS As Boolean)
    If this.SomeOption <> RHS Then
        this.SomeOption = RHS
        OnPropertyChanged "SomeOption"
    End If
End Property

Public Property Get SomeOtherOption() As Boolean
    SomeOtherOption = this.SomeOtherOption
End Property

Public Property Let SomeOtherOption(ByVal RHS As Boolean)
    If this.SomeOtherOption <> RHS Then
        this.SomeOtherOption = RHS
        OnPropertyChanged "SomeOtherOption"
    End If
End Property

Public Property Get SomeCommand() As ICommand
    Set SomeCommand = this.SomeCommand
End Property

Public Property Set SomeCommand(ByVal RHS As ICommand)
    Set this.SomeCommand = RHS
End Property

Public Property Get SomeOptionName() As String
    SomeOptionName = "Auto"
End Property

Public Property Get SomeOtherOptionName() As String
    SomeOtherOptionName = "Manual/Browse"
End Property

Public Property Get Instructions() As String
    Instructions = "Lorem ipsum dolor sit amet, consectetur adipiscing elit."
End Property

Private Sub OnPropertyChanged(ByVal PropertyName As String)
    RaiseEvent PropertyChanged(Me, PropertyName)
    Dim Handler As IHandlePropertyChanged
    For Each Handler In this.Handlers
        Handler.OnPropertyChanged Me, PropertyName
End Sub

Private Sub Class_Initialize()
    Set this.Handlers = New Collection
    Set ValidationHandler = ValidationManager.Create
End Sub

Private Sub INotifyPropertyChanged_OnPropertyChanged(ByVal Source As Object, ByVal PropertyName As String)
    OnPropertyChanged PropertyName
End Sub

Private Sub INotifyPropertyChanged_RegisterHandler(ByVal Handler As IHandlePropertyChanged)
    this.Handlers.Add Handler
End Sub

Private Property Get IViewModel_Validation() As IHandleValidationError
    Set IViewModel_Validation = ValidationHandler
End Property

Private Sub ValidationHandler_PropertyChanged(ByVal Source As Object, ByVal PropertyName As String)
    OnPropertyChanged PropertyName
End Sub

Nothing much of interest here, other than the INotifyPropertyChanged implementation and the fact that a ViewModel is really just a fancy word for a class that exposes a bunch of properties that magically keep in sync with UI controls!


In a Smart UI, that module is, more often than not, a complete wreck. In Model-View-Presenter it quickly gets cluttered with many one-liner event handlers, and something just feels clunky about the MVP pattern. Now, I’m trying really hard, but I can’t think of a single reason to not want UserForm code-behind to look like this all the time… this is absolutely all of it, there’s no cheating going on:

'@Folder MVVM.Example
'@ModuleDescription "An example implementation of a View."
Implements IView
Implements ICancellable
Option Explicit

Private Type TView
    'IView state:
    ViewModel As ExampleViewModel
    'ICancellable state:
    IsCancelled As Boolean
    'Data binding helper dependency:
    Bindings As IBindingManager
End Type

Private this As TView

'@Description "A factory method to create new instances of this View, already wired-up to a ViewModel."
Public Function Create(ByVal ViewModel As ExampleViewModel, ByVal Bindings As IBindingManager) As IView
    GuardClauses.GuardNonDefaultInstance Me, ExampleView, TypeName(Me)
    GuardClauses.GuardNullReference ViewModel, TypeName(Me)
    GuardClauses.GuardNullReference Bindings, TypeName(Me)
    Dim result As ExampleView
    Set result = New ExampleView
    Set result.Bindings = Bindings
    Set result.ViewModel = ViewModel
    Set Create = result
End Function

Private Property Get IsDefaultInstance() As Boolean
    IsDefaultInstance = Me Is ExampleView
End Property

'@Description "Gets/sets the ViewModel to use as a context for property and command bindings."
Public Property Get ViewModel() As ExampleViewModel
    Set ViewModel = this.ViewModel
End Property

Public Property Set ViewModel(ByVal RHS As ExampleViewModel)
    GuardClauses.GuardExpression IsDefaultInstance, TypeName(Me)
    GuardClauses.GuardNullReference RHS
    Set this.ViewModel = RHS

End Property

'@Description "Gets/sets the binding manager implementation."
Public Property Get Bindings() As IBindingManager
    Set Bindings = this.Bindings
End Property

Public Property Set Bindings(ByVal RHS As IBindingManager)
    GuardClauses.GuardExpression IsDefaultInstance, TypeName(Me)
    GuardClauses.GuardDoubleInitialization this.Bindings, TypeName(Me)
    GuardClauses.GuardNullReference RHS
    Set this.Bindings = RHS

End Property

Private Sub BindViewModelCommands()
    With Bindings
        .BindCommand ViewModel, Me.OkButton, AcceptCommand.Create(Me)
        .BindCommand ViewModel, Me.CancelButton, CancelCommand.Create(Me)
        .BindCommand ViewModel, Me.BrowseButton, ViewModel.SomeCommand
    End With
End Sub

Private Sub BindViewModelProperties()
    With Bindings
        .BindPropertyPath ViewModel, "SourcePath", Me.PathBox, _
            Validator:=New RequiredStringValidator, _
            ErrorFormat:=AggregateErrorFormatter.Create(ViewModel, _
                ValidationErrorFormatter.Create(Me.PathBox).WithErrorBackgroundColor.WithErrorBorderColor, _
                ValidationErrorFormatter.Create(Me.InvalidPathIcon).WithTargetOnlyVisibleOnError("SourcePath"), _
        .BindPropertyPath ViewModel, "Instructions", Me.InstructionsLabel
        .BindPropertyPath ViewModel, "SomeOption", Me.OptionButton1
        .BindPropertyPath ViewModel, "SomeOtherOption", Me.OptionButton2
        .BindPropertyPath ViewModel, "SomeOptionName", Me.OptionButton1, "Caption", OneTimeBinding
        .BindPropertyPath ViewModel, "SomeOtherOptionName", Me.OptionButton2, "Caption", OneTimeBinding
    End With
End Sub

Private Sub InitializeBindings()
    If ViewModel Is Nothing Then Exit Sub
    Bindings.ApplyBindings ViewModel
End Sub

Private Sub OnCancel()
    this.IsCancelled = True
End Sub

Private Property Get ICancellable_IsCancelled() As Boolean
    ICancellable_IsCancelled = this.IsCancelled
End Property

Private Sub ICancellable_OnCancel()
End Sub

Private Sub IView_Hide()
End Sub

Private Sub IView_Show()
    Me.Show vbModal
End Sub

Private Function IView_ShowDialog() As Boolean
    Me.Show vbModal
    IView_ShowDialog = Not this.IsCancelled
End Function

Private Property Get IView_ViewModel() As Object
    Set IView_ViewModel = this.ViewModel
End Property

Surely some tweaks will be made over the next couple of weeks as I put the UI design pattern to a more extensive workout with the Rubberduck website content maintenance app – but having used MVVM in C#/WPF for many years, I already know that this is how I want to be coding VBA user interfaces going forward.

I really love how the language has had the ability to make this pattern work, all along.

To be continued…

Rubberduck 2.5.1

Here we are again, some 580+ commits and 1000+ modified files later, with 10 contributors involved (with particular thanks to @MDoerner and @BZngr, and honorable mentions to @IvenBach and @testingoutgith1) in over 60 pull requests since the last release: time to look back at what was done and call it version 2.5.1! If you’ve been keeping up with pre-release builds, none of this is going to be news to you, but with over 9.1K downloads of v2.5.0 a lot of you seem to prefer to upgrade less often but more significantly, so here’s a timely recap.

But first, let’s get the known problems out of the way.

Known Issues

Making a VBIDE add-in means we can’t know or assume what our host application is going to be, and different hosts sometimes wire things up differently – and this can spell trouble under certain circumstances. Making a VBIDE add-in in .NET has even further implications: while it’s how we can extend a 64-bit VBE, it’s also causing various type cast errors/exceptions when other add-ins are loaded.

Possible Crash

The Visual Basic Editor has a peculiar way of loading its add-ins: Rubberduck’s (and any other VBIDE add-in’s) entry point is invoked by the VBE before the VBE has completely finished constructing itself – accessing the object model too early can throw COM exceptions that take down Rubberduck as it initializes.

Normally Rubberduck initializes itself, then proceeds to parse the project (if it’s an empty project then the bulk of that is Rubberduck loading everything defined in VBA7.DLL and the type library for the host application’s object model) – normally if the VBE isn’t ready for this yet, we bail out and don’t access any objects and the “Refresh” button says “Pending” instead of “Ready”, and by the time you manually run that command the VBE has finished initializing and the only annoyance is that the initial parse isn’t automatic.

But in certain host applications (Microsoft Access being a known one, but I’ve seen it happen in Excel as well, although not with a recent build), sometimes the VBE actually isn’t ready to take member calls against its own object model, and the result is a COM exception that is either caught and then Rubberduck says it can’t initialize, or thrown several layers deeper, uncaught, and then everything goes up in flames.

Loading Rubberduck manually from the VBE’s Add-Ins Manager is sure annoying, but is really the only 100% sure-shot way to load any VBIDE add-in with a properly initialized VBE, regardless of the host application. Note that the installer registers Rubberduck as a VBE add-in with the LoadBehavior flag set to load at startup. If Rubberduck blows up at startup or fails to initialize, consider editing this configuration to make it load manually (exception details should normally be logged for the first start-up).

Heavy on Memory (RAM)

Rubberduck has always used a lot of memory to keep all the code metadata handy and cache a lot of things to improve processing performance. Working on a large legacy project that generates lots of inspection results can grind the main thread of the host process to a halt as the toolwindow renders the many objects (whether the toolwindow is displayed or not).

Unless you are discovering Rubberduck with a new, empty VBA project, consider first reviewing the settings – can’t hurt to review them either way:

  • Disable “run inspections automatically on successful parse”, so that they only run if you explicitly refresh them from the Inspection Results toolwindow;
  • Set inspection severity to “Do not Show” for inspections that could produce thousands upon thousands of results, like “use meaningful names” if you’re into Hungarian Notation for example, or “use of bang operator” if that’s the only way you’re ever accessing recordset fields in Access;

Other general performance tips:

  • Rubberduck parses per-module, so when you leave a module after modifying it, trigger a parse – by the time you’re in the other module and have scrolled to where you want to be and are in that mindset, the modified module will have processed.
  • Reduce coupling: the more modules are inter-dependent, the more modifying a module requires re-resolving identifier references in the dependent modules.
  • Avoid complex grammar: bang operators, among other code constructs, are somewhat ambiguously defined and ultimately parse in two passes, with the first one failing. The standard member call syntax parses faster, in a single parser pass.

Undesirable Interactions

If you are using the free but rather old 32-bit MZ-Tools 3.x productivity add-in, this section shouldn’t be a concern. However MZ-Tools 8.x was rewritten from the ground up, ported from VB6 into .NET-land, and while its author Carlos Quintero took extraordinary steps to isolate MZ-Tools from other in-process .NET add-ins and has issued recommendations for Rubberduck to do the same, …there is still a chance the two add-ins bump into each other; if MZ wins, RD is essentially bricked.

MZ-Tools normally runs inside its own .NET AppDomain, except when hosted in AutoDesk products (Inventor, AutoCAD), which implement VBE initialization in a way that breaks MZ-Tools’ startup mechanics – up until recently it was assumed this collision only happens in AutoDesk hosts, but a recent support ticket involving Microsoft Access was filed and implicates interactions with MZ-Tools.

This issue manifests itself with InvalidCastException being thrown at various points, often during initialization, or later during parse: the exception message involves attempting to cast COM objects like Microsoft.Vbe.interop._VBProject into types such as VBClassicExtensibility.VBProjectClass, where VBClassicExtensibility is defined by MZ-Tools, not Rubberduck.

One thing that can be attempted to mitigate this problem, would be to set MZ-Tools to not load on start-up, and manually load it after Rubberduck has initialized… but sadly this cross-add-in confused COM marshaling is simply not supposed to happen given MZ-Tools’ AppDomain mechanics, and we don’t really have any solutions for this – same as we don’t really have any solution for cases where COM registrations are broken (e.g. when multiple Microsoft Office product versions are running side-by-side but were not installed in chronological order – that’s an officially unsupported scenario, per Microsoft).

As a result, using Rubberduck together with other .NET-based add-ins cannot be considered a completely fail-safe scenario, and we have to treat this as a “known issue” here, and the work-around sucks and boils down to “drop other add-ins, or drop Rubberduck”. This is actually probably true at various degrees of all .NET-based VBIDE add-ins.

On the bright side, we have taken several steps in this release cycle to prepare the ground not only to get Rubberduck to build correctly in the latest & greatest Visual Studio 2019, but also to get most of our build process ready for .NET Core – so when .NET Core 5 is released in a few weeks, we can try to get Rubberduck to run on the shiny new Core framework, which theoretically makes AppDomain completely moot, and so we have very little incentive to work on getting Rubberduck to load its own AppDomain the way MZ-Tools does: if we can make Rubberduck build and run on .NET Core 5, then this problem should simply disappear… in theory.

Enhancements & New Features

This release does not introduce any new top-level Rubberduck features, but makes a number of very useful user-facing additions nonetheless, on top of the many under-the-hood enhancements made this cycle.

Surfacing Annotations

One of the most useful and powerful features of Rubberduck, annotations are special comments that use a particular but relatively simple syntax – these are all grammatically valid:

'@AnnotationName "text"
'@AnnotationName("text", 123) : there can be comments here
'@AnnotationName "text", 123
'@AnnotationName Identifier1, Identifier2, ...IdentifierN

While the syntax itself is reasonably simple to use, the problem was that unless you knew every supported annotation, well then the @AnnotationName part kind of had to be a guess.

Rubberduck uses these annotations for various purposes, from identifying Rubberduck test modules to keeping hidden module/member attributes in sync with these comments (this includes the ability to document and literally map Excel hotkeys using VBA comments). You can read everything we’ve documented about them on the project’s website.

In Rubberduck 2.5.1.x builds, we finally get new commands in the code pane and Code Explorer context menus, that bring up a dialog that gives us all the options to easily and safely annotate everything that can be annotated, using the correct syntax and arguments every time:

a window describing the member to annotate and the annotation to add
Select the annotation to add, and supply the argument values. Magic!
Adding an @Ignore annotation to ignore a specific inspection, can now be done without needing to know the exact name we decided to call that inspection class in Rubberduck’s source code!

Encapsulate Field Enhancements

This particular refactoring has seen a terrific enhancement that makes it very easy to cleanly and quickly turn a set of public fields into Property Get/Let members, with a Private Type TClassName and a module-scope Private this As TClassName instance variable – and all properties automatically reading/writing from it. You can see this feature in action in the previous article.

The new Wrap fields in Private Type functionality leverages the very useful Private Type pattern.

Unit Testing

The Test Explorer now makes it easier to ignore one or more specific selected tests, or all tests under a given category/group, by exposing the context menu commands that add or remove the @TestIgnore annotation as appropriate; having this command in the Test Explorer makes it possible to annotate a test method while a completely different and unrelated module is maximized in the code editor.

Hmm, …these icons are out of control, aren’t they… expect that to change soon-ish…

Running tests while results are regrouped by outcome is still a known issue (tests run painfully slow because the UI thread is busy re-sorting and re-rendering the list every time a test finishes running), but everything works much more smoothly when the tests are regrouped any other way.

Code Inspections

Ok the logic for that is currently broken on the website (working on that… somewhat) so this is much harder than it will be in the future when I’ll just look at the [New] tab on the inspections page of the website and every inspection that is in [next] but not in [main] will be listed right there. From skimming through every pull request merged since the last release:

  • Function return value not used inspection now more clearly targets call sites, and now ignores non-user code.
  • Function return value is always discarded inspection is the old “return value not used” logic targeting the function itself, when none of the call sites capture the function’s return value.
  • Implicitly typed const inspection was added to flag Const declarations without an As clause to specify an explicit type.
  • Assignment not used inspection now correctly handles an assignment that is overridden in the next statement but first read in the RHS expression of the assignment.

Not user-facing but critically important nonetheless, is all the behind-the-scenes work done to simplify inspecting VBA code as much as possible. This cycle saw a tremendous amount of technical debt paid in the code inspections department, that pave the way for future enhancements like, say, having the ability to run inspections per-module; as the number of implemented inspections continues to grow, the ability to scope inspections in a more granular way is going to be very useful for our plan to eventually report inspection results in a custom code pane, with colored squiggly lines (that’s v3.x stuff, though).

Applying Quick-Fixes

The Code Inspections toolwindow has been updated with a context menu that makes it much simpler to apply a quick-fix to one or more inspection results; all available quick-fixes appear in the context menu, each with various options to apply to the selection. This menu is also shown by clicking the “Fix” drop-down menu from the toolwindow’s toolbar.

The weird “fix all occurrences in procedure/module/project” link buttons in the bottom panel are now gone, completely replaced with a more flexible menu system.

Code Explorer Enhancements

The Code Explorer toolwindow context menu now includes a move to folder command to easily organize your project components, and there’s a new setting to enable drag and drop in Code Explorer (disabled by default):

A new setting enables dragging & dropping capabilities in the Code Explorer toolwindow.

With that setting enabled, you can now move a code file to an existing folder, simply by dragging it from its location and dropping it onto a folder node! The setting was made to require being explicitly enabled, to avoid discovering that feature by accidentally dragging a code file somewhere (that did happen in beta/testing).

Website Integration

You may have noticed is “under reconstruction”. Before that, the site’s content was mostly static, with only the inspections list assembled from content parsed from the Rubberduck.CodeAnalysis.xml, downloaded periodically off GitHub through unauthenticated REST API requests. That worked relatively well until a spike in traffic occurred following the release of Excel Insights, a book collectively authored by 24 Microsoft MVP Award recipients, including myself: suddenly a bug in the caching mechanism became very apparent when the site’s home page started getting served as a wonderful HTTP 500 error page.

Since the website hosting came with a SQL Server database that I wasn’t using, I decided to start using it and make the site pull the content from there rather than directly off the GitHub API. I wrote a small console application, got myself a private API key to make properly authenticated REST API requests, and now there’s a scheduled task running on a virtual machine in my garage, that runs this application every 30 minutes to update the installer asset download counts and verify whether the XML documentation assets are up-to-date for the latest pre-release build, and then proceeds to parse the XML docs and generate/update the database records: the website simply pulls the data from the database at every request, and now the website couldn’t bust GitHub’s REST API limits even if it tried.

Documenting Rubberduck is challenging: there are a lot of features, and there isn’t really any user guide that’s constantly being kept up-to-date. The wiki on the repository is terribly outdated in several parts, and the feature announcements on this blog are nice when you’re following the project along its journey, but in a dream world using Rubberduck would be content found on the website, and contributing to Rubberduck would be content found in the repository’s wiki.

Parsing the xml-docs into website content is a step in that direction. Nobody wants to maintain documentation, but xml-doc comments are part of the source code, and we even put source code analyzers in place that will break the build if we try to introduce an inspection, quick-fix, or annotation, without properly documenting it with xml-docs.

Every single inspection, quick-fix, and annotation has thorough documentation, including code examples that may span multiple modules. But best of all, every single page generated from source code includes an “edit this page” link that points to a GitHub page where you can literally edit the xml documentation for the inspection you were looking at (and review its source code if you like – it’s the same file!) – and just like that, all you need to contribute to Rubberduck (yes, single-character typo fixes and additional useful code examples are welcome!) is a GitHub login!

Every page generated from xml documentation includes an “Edit this page” link at the bottom.

Because of how the request routing on the website was setup, it was easy to make Rubberduck link in-app inspection results to this website content – you can now click a URL at the bottom of the inspection results toolwindow (this will likely change one way or another in the future) to bring up the details page with the xml documentation and code examples:

The URL at the bottom of the adjustable panel (it’s possible you need to scroll the content or make the bottom panel tall enough to show it) opens that URL in a new tab in whatever your default browser is.

The in-app content exists as localized resources, lovingly translated by our international contributors; the website content however, is only available in English, because we’re absolutely not going to start translating XML comments in the source code. But the processed content actually resides in a database, so it wouldn’t be impossible to eventually localize it at that level, as well – we’re just not there yet at all at the moment.

The website content is often different than the in-app content, and over time it should be expected to grow more and more in-depth, thorough and descriptive.

The revamped domain will ultimately span 3 sites, including, which will eventually expose REST endpoints for various purposes, including Rubberduck’s “check for newer version at startup” feature; for example something like that might accept some VBA code in a JSON object in the request’s body, and respond with a JSON object in the response body containing the indented VBA code. Or that might also accept some VBA code (presumably along with some metadata about the module type) in a JSON object in the request’s body, but could respond with a JSON object representing all inspection results for it. It’s still all just brewing ideas at this stage. The other subdomain,, is going to host a web-based, GitHub-authenticated version of the VBA program I’m going to present in my next article: a tool for managing and editing most of the website’s content.

Moving Forward

Rubberduck is becoming a pretty mature code base and has an ever-increasingly better abstracted internal framework/API to understand and manipulate VBA code. The project now builds with the latest version of Microsoft Visual Studio 2019, and we’re hoping COM Automation support in .NET Core 5 will allow us to build an increasing number of the project’s components with it; I’m thinking the “main” type library is better off under the old tech, but I’ll be more than happy to be proven wrong here!

A rough roadmap for v2.5.1.x might include…

  • More resolver capabilities unlocked by fully leveraging our internal ITypeLib API
  • Syntax-highlighted preview of the changes for all refactorings (and quick-fixes?)
  • Some Code Path Analysis API, to help implement the more complex inspection ideas
  • The Moq wrapper mocking framework
  • Block Completion, maybe
  • Anything else anyone feels like contributing to the project!

The goal for the rest of the 2.x cycle is to prepare everything that needs to happen in order to implement our own custom code editor window – giving us full, complete control over every single token and everything that can possibly happen in that custom code pane. We’re talking code folding, custom theming, that kind of thing.

‘Main’ vs ‘Master’ – Why it Matters

You may have noticed (or not) that the website is now labeling “main” the branch formerly known as “master”. As a French native, “master/slave” terminology in any non-actual master/slave context has always sounded a bit weird to me, but I’m a white man in North America (although not in the US) and I get the luxury to read these words and decide that they don’t affect me, and reflecting on the events of this summer has taught me that this is part of what white privilege is.

I don’t do political & editorial commenting, I prefer to leave that space to others – but I warmly recommend watching 13th on Netflix and, if you can handle it, When They See Us. Black lives matter, it’s simple – and no, it doesn’t say “but white lives don’t” anywhere between the lines.

So yes, we’re going to be taking steps to alter the language in Rubberduck a bit in this cycle. The “master” branch will be renamed to “main”, yes, but we’ll also come up with a better term for “white-listing” identifier names. It won’t stop racism, no, indeed. But it won’t hurt anyone, either.


Builder Walkthrough

Note: this article was updated 2021-04-13 with screenshots from the latest v2.5.1.x pre-release build; the extract interface enhancements shown will green-release with v2.5.2.

We’ve seen how to leverage the default instance of a class module to define a stateless interface that’s perfect for a factory method. At the right abstraction level, most objects will not require more than just a few parameters. Often, parameters are related and can be abstracted/regrouped into their own object. Sometimes that makes things expressive enough. Other times, there’s just nothing we can do to work around the fact that we need to initialize a class with a dozen or more values.

The example code for this article can be found in our Examples repository.

A class with many properties

Such classes are actually pretty common; any entity object representing a database record would fit the bill. Let’s make a User class. We’re using Rubberduck, so this will be quick!

We start with a public field for each property we want:

Option Explicit
Public Id As String
Public UserName As String
Public FirstName As String
Public LastName As String
Public Email As String
Public EmailVerified As Boolean
Public TwoFactorEnabled As Boolean
Public PhoneNumber As String
Public PhoneNumberVerified As Boolean
Public AvatarUrl As String

Now we hit Ctrl+` to trigger a parse, right-click any of the variables and select Encapsulate Field from the Refactor menu (or Ctrl+Shift+F if you haven’t tweaked the default hotkeys):

The command will be disabled if the caret/selection isn’t on a public field, or if the module has been modified since the last time Rubberduck parsed it.

Check the wrap fields in private type box, then click the Select all button and hit OK.

The Encapsulate Field refactoring also lets you rename the properties and their respective backing field.

Now the module looks like this, and all you had to do was to declare a bunch of public fields:

Option Explicit
Private Type TUser
    Id As String
    UserName As String
    FirstName As String
    LastName As String
    Email As String
    EmailVerified As Boolean
    TwoFactorEnabled As Boolean
    PhoneNumber As String
    PhoneNumberVerified As Boolean
    AvatarUrl As String
End Type
Private this As TUser
Public Property Get Id() As String
    Id = this.Id
End Property
Public Property Let Id(ByVal value As String)
    this.Id = value
End Property
Public Property Get UserName() As String
    UserName = this.UserName
End Property
Public Property Let UserName(ByVal value As String)
    this.UserName = value
End Property
Public Property Get FirstName() As String
    FirstName = this.FirstName
End Property
Public Property Let FirstName(ByVal value As String)
    this.FirstName = value
End Property
Public Property Get LastName() As String
    LastName = this.LastName
End Property
Public Property Let LastName(ByVal value As String)
    this.LastName = value
End Property
Public Property Get Email() As String
    Email = this.Email
End Property
Public Property Let Email(ByVal value As String)
    this.Email = value
End Property
Public Property Get EmailVerified() As Boolean
    EmailVerified = this.EmailVerified
End Property
Public Property Let EmailVerified(ByVal value As Boolean)
    this.EmailVerified = value
End Property
Public Property Get TwoFactorEnabled() As Boolean
    TwoFactorEnabled = this.TwoFactorEnabled
End Property
Public Property Let TwoFactorEnabled(ByVal value As Boolean)
    this.TwoFactorEnabled = value
End Property
Public Property Get PhoneNumber() As String
    PhoneNumber = this.PhoneNumber
End Property
Public Property Let PhoneNumber(ByVal value As String)
    this.PhoneNumber = value
End Property
Public Property Get PhoneNumberVerified() As Boolean
    PhoneNumberVerified = this.PhoneNumberVerified
End Property
Public Property Let PhoneNumberVerified(ByVal value As Boolean)
    this.PhoneNumberVerified = value
End Property
Public Property Get AvatarUrl() As String
    AvatarUrl = this.AvatarUrl
End Property
Public Property Let AvatarUrl(ByVal value As String)
    this.AvatarUrl = value
End Property

I love this feature! Rubberduck has already re-parsed the module, so next we right-click anywhere in the module and select the Extract Interface refactoring, and check the box to select all Property Get accessors (skipping Property Let):

Extract Interface can automatically implement the extracted interface for you, and you can extract a public interface out of a private class.

Having a read-only interface for client code that doesn’t need the Property Let accessors makes an objectively cleaner API: assignments are recognized as invalid at compile time.

We get a read-only IUser interface for our efforts (!), and now the User class has an Implements IUser instruction at the top, …and these new members at the bottom:

Private Property Get IUser_ThingId() As String
    IUser_ThingId = ThingId
End Property

Private Property Get IUser_UserName() As String
    IUser_UserName = UserName
End Property

Private Property Get IUser_FirstName() As String
    IUser_FirstName = FirstName
End Property

Private Property Get IUser_LastName() As String
    IUser_LastName = LastName
End Property

Private Property Get IUser_Email() As String
    IUser_Email = Email
End Property

Private Property Get IUser_EmailVerified() As Boolean
    IUser_EmailVerified = EmailVerified
End Property

Private Property Get IUser_TwoFactorEnabled() As Boolean
    IUser_TwoFactorEnabled = TwoFactorEnabled
End Property

Private Property Get IUser_PhoneNumber() As String
    IUser_PhoneNumber = PhoneNumber
End Property

Private Property Get IUser_PhoneNumberVerified() As Boolean
    IUser_PhoneNumberVerified = PhoneNumberVerified
End Property

Private Property Get IUser_AvatarUrl() As String
    IUser_AvatarUrl = AvatarUrl
End Property

The scary part is that it feels as though if Extract Interface accounted for the presence of a Private Type in a similar way Encapsulate Field does, then even the TODO placeholder bits could be fully automated. Might be something to explore there… Update: automagic implementation completed!

Now we have our read-only interface worked out, if we go by previous posts’ teachings, , that is where we make our User class have a predeclared instance, and expose a factory method that I’d typically name Create:

'@Description "Creates and returns a new user instance with the specified property values."
Public Function Create(ByVal Id As String, ByVal UserName As String, ...) As IUser
End Function

Without Rubberduck, in order to have a predeclared instance of your class you would have to export+remove the class module, locate the exported .cls file, open it in Notepad++, edit the VB_PredeclaredId attribute value to True, save+close the file, then re-import it back into your VBA project.

With Rubberduck, there’s an annotation for that: simply add '@PredeclaredId at the top of the class module, parse, and there will be a result for the AttributeValueOutOfSync inspection informing you that the class’ VB_PredeclaredId attribute value disagrees with the @PredeclaredId annotation, and then you apply the quick-fix you want, and you just might have synchronized hidden attributes across the with a single click.

Option Explicit

When it’s a factory method for a service class that takes in dependencies, 2-3 parameters is great, 5+ is suspicious. But here we’re taking in values, pure data – not some IFileWriter or other abstraction. And we need quite a lot of them (here 10, but who knows how many that can be!), and that’s a problem, because this is very ugly:

Set identity = User.Create("01234", "Rubberduck", "", False, ...)

Using named parameters can help:

Set identity = User.Create( _
    Id:="01234", _
    UserName:="Rubberduck", _
    Email:="", _
    EmailVerified:=False, _
    Phone:="555-555-5555", _
    PhoneVerified:=False, _

But the resulting code still feels pretty loaded, and that’s with consistent line breaks. Problem is, that limits the number of factory method parameters to 20-ish (if we’re nice and stick to one per line), since that’s how many line continuations the compiler will handle for a single logical line of code.

Surely there’s a better way.

Building the Builder

I wrote about this pattern in OOP Design Patterns: The Builder, but in retrospect that article was really just a quick overview. Let’s explore the builder pattern.

I like to design objects from the point of view of the code that will be consuming them. In this case what we want to end up with, is something like this:

Set identity = UserBuilder.Create("01234", "Rubberduck") _
    .WithEmail("", Verified:=False) _
    .WithPhone("555-555-5555", Verified:=False) _

This solves a few problems that the factory method doesn’t:

  • Optional arguments become explicitly optional member calls; long argument lists are basically eliminated.
  • Say Id and UserName are required, i.e. a User object would be invalid without these values; the builder’s own Create factory method can take these required values as arguments, and that way any User instance that was built with a UserBuilder is guaranteed to at least have these values.
  • If we can provide a value for EmailVerified but not for Email, or for PhoneVerified but not for Phone, and neither are required… then with individual properties the best we can do is raise some validation error after the fact. With a UserBuilder, we can have WithEmail and WithPhone methods that take a Verified Boolean parameter along with the email/phone, and guarantee that if EmailVerified is supplied, then Email is supplied as well.

I like to start from abstractions, so let’s add a new class module – but don’t rename it just yet, otherwise Rubberduck will parse it right away. Instead, copy the IUser interface into the new Class1 module, select all, and Ctrl+H to replace “Property Get ” (with the trailing space) with “Function With” (without the trailing space). Still with the whole module selected, we replace “String” and “Boolean” with “IUserBuilder”. The result should look like this:

Option Explicit
Public Function WithId() As IUserBuilder
End Function
Public Function WithUserName() As IUserBuilder
End Function
Public Function WithFirstName() As IUserBuilder
End Function
Public Function WithLastName() As IUserBuilder
End Function
Public Function WithEmail() As IUserBuilder
End Function
Public Function WithEmailVerified() As IUserBuilder
End Function
Public Function WithTwoFactorEnabled() As IUserBuilder
End Function
Public Function WithPhoneNumber() As IUserBuilder
End Function
Public Function WithPhoneNumberVerified() As IUserBuilder
End Function
Public Function WithAvatarUrl() As IUserBuilder
End Function

We’re missing a Build method that returns the IUser we’re building:

Public Function Build() As IUser
End Function

Now we add the parameters and remove the members we don’t want, merge the related ones into single functions – this is where we define the shape of our builder API: if we want to make it hard to create a User with a LastName but without a FirstName, or one with TwoFactorEnabled and PhoneNumberVerified set to True but without a PhoneNumber value… then with a well-crafted builder interface we can make it do exactly that.

Once we’re done, we can rename the class module to IUserBuilder, and that should trigger a parse. The interface might look like this now:

'@ModuleDescription("Incrementally builds a User instance.")
Option Explicit
'@Description("Returns the current object.")
Public Function Build() As IUser
End Function
'@Description("Builds a user with a first and last name.")
Public Function WithName(ByVal FirstName As String, ByVal LastName As String) As IUserBuilder
End Function
'@Description("Builds a user with an email address.")
Public Function WithEmail(ByVal Email As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
End Function
'@Description("Builds a user with SMS-based 2FA enabled.")
Public Function WithTwoFactorAuthentication(ByVal PhoneNumber As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
End Function
'@Description("Builds a user with an avatar at the specified URL.")
Public Function WithAvatar(ByVal Url As String) As IUserBuilder
End Function

Then we can add another class module, and type Implements IUserBuilder under Option Explicit, then hit Ctrl+` to parse. Unless you disabled the “check if code compiles before parsing” setting (it’s enabled by default), you should be seeing this warning:

The project can’t compile, because the interface isn’t implemented.

Click Yes to parse anyway (normally we only want compilable code, but in this case we know what we’re doing, I promise), then right-click somewhere in the Implements IUserBuilder statement, and select the Implement Interface refactoring:

Creating all these method stubs manually, or… letting Rubberduck create them all at once in a split-second?

The result is as follows, and makes a good starting point:

Option Explicit
Implements IUserBuilder
Private Function IUserBuilder_Build() As IUser
    Err.Raise 5 'TODO implement interface member
End Function
Private Function IUserBuilder_WithName(ByVal FirstName As String, ByVal LastName As String) As IUserBuilder
    Err.Raise 5 'TODO implement interface member
End Function
Private Function IUserBuilder_WithEmail(ByVal Email As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
    Err.Raise 5 'TODO implement interface member
End Function
Private Function IUserBuilder_WithTwoFactorAuthentication(ByVal PhoneNumber As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
    Err.Raise 5 'TODO implement interface member
End Function
Private Function IUserBuilder_WithAvatar(ByVal Url As String) As IUserBuilder
    Err.Raise 5 'TODO implement interface member
End Function

We’re “building” an IUser object. So we have a module-level User object (we need the class’ default interface here, so that we can access the Property Let members), and each With method sets one property or more and then returns the current object (Me). That last part is critical, it’s what makes the builder methods chainable. We’ll need a Build method to return an encapsulated IUser object. So the next step will be to add a @PredeclaredId annotation and implement a Create factory method that takes the required values and injects the IUser object into the IUserBuilder instance we’re returning; then we can remove the members for these required values, leaving only builder methods for the optional ones. We will also add a value parameter of the correct type to each builder method, and make them all return the current object (Me). Once the class module looks like this, we can rename it to UserBuilder, and Rubberduck parses the code changes – note the @PredeclaredId annotation (needs to be synchronized to set the hidden VB_PredeclaredId attribute to True:

'@ModuleDescription("Builds a User object.")
Option Explicit
Implements IUserBuilder
Private internal As User
'@Description("Creates a new UserBuilder instance.")
Public Function Create(ByVal Id As String, ByVal UserName As String) As IUserBuilder
    Dim result As UserBuilder
    Set result = New UserBuilder
    '@Ignore UserMeaningfulName FIXME
    Dim obj As User
    Set obj = New User
    obj.Id = Id
    obj.UserName = UserName
    Set result.User = internal
    Set Create = result
End Function
'@Ignore WriteOnlyProperty
'@Description("For property injection of the internal IUser object; only the Create method should be invoking this member.")
Friend Property Set User(ByVal value As IUser)
    If Me Is UserBuilder Then Err.Raise 5, TypeName(Me), "Member call is illegal from default instance."
    If value Is Nothing Then Err.Raise 5, TypeName(Me), "'value' argument cannot be a null reference."
    Set internal = value
End Property
Private Function IUserBuilder_Build() As IUser
    If internal Is Nothing Then Err.Raise 91, TypeName(Me), "Builder initialization error: use UserBuilder.Create to create a UserBuilder."
    Set IUserBuilder_Build = internal
End Function
Private Function IUserBuilder_WithName(ByVal FirstName As String, ByVal LastName As String) As IUserBuilder
    internal.FirstName = FirstName
    internal.LastName = LastName
    Set IUserBuilder_WithName = Me
End Function
Private Function IUserBuilder_WithEmail(ByVal Email As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
    internal.Email = Email
    internal.EmailVerified = Verified
    Set IUserBuilder_WithEmail = Me
End Function
Private Function IUserBuilder_WithTwoFactorAuthentication(ByVal PhoneNumber As String, Optional ByVal Verified As Boolean = False) As IUserBuilder
    internal.TwoFactorEnabled = True
    internal.PhoneNumber = PhoneNumber
    internal.PhoneNumberVerified = Verified
    Set IUserBuilder_WithTwoFactorAuthentication = Me
End Function
Private Function IUserBuilder_WithAvatar(ByVal Url As String) As IUserBuilder
    internal.AvatarUrl = Url
    Set IUserBuilder_WithAvatar = Me
End Function

Now, when I said default instances and factory methods (here too) are some kind of fundamental building block, I mean we’re going to be building on top of that, starting with this builder pattern; the Create method is intended to be invoked off the class’ default instance, like this:

Set builder = UserBuilder.Create(internalId, uniqueName)

The advantages are numerous, starting with the possibility to initialize the builder with everything it needs (all the required values), so that the client code can call Build and consume a valid User object right away.

Side note about this FIXME comment – there’s more to it than it being a signpost for the reader/maintainer:

    '@Ignore UserMeaningfulName FIXME
    Dim obj As User

By default only TODO, BUG, and NOTE markers are picked up, but you can easily configure Rubberduck to find any marker you like in comments, and then the ToDo Explorer lets you easily navigate them all:

Rubberduck has a ToDo Explorer toolwindow that can be configured (click the cogwheel icon) to pick up “FIXME” anywhere in comments, anywhere in the project. Or “HERE BE DRAGONS”.

Another noteworthy observation:

'@Ignore WriteOnlyProperty
'@Description("For property injection of the internal IUser object; only the Create method should be invoking this member.")
Friend Property Set User(ByVal value As IUser)
    If Me Is UserBuilder Then Err.Raise 5, TypeName(Me), "Member call is illegal from default instance."
    If value Is Nothing Then Err.Raise 5, TypeName(Me), "'value' argument cannot be a null reference."
    Set internal = value
End Property

Me is always the current object, as in, an instance of this class module, presenting the default interface of this class module: the If Me Is UserBuilder condition evaluates whether Me is the object known as UserBuilder – and right now there’s no such thing and the code doesn’t compile.

Synchronizing Attributes & Annotations

Rubberduck knows we mean that class to have a VB_PredeclaredId attribute value of True because of the @PredeclaredId annotation, but it’s still just a comment at this point. Bring up the inspection results toolwindow, and find the results for the MissingAttribute inspection under Rubberduck Opportunities:

Clicking Fix all occurrences in project will automatically add all the missing attributes.

That didn’t fix the VB_PredeclaredId attributes! Why?! The reason is that the attribute isn’t missing, only its value is out of sync. We’ll have to change this (pull requests welcome!), but for now you’ll find the AttributeValueOutOfSync inspection results under the Code Quality Issues group. If you group results by inspection, its miscategorization doesn’t matter though:

When attributes and annotations contradict each other, the AttributeValueOutOfSync inspection starts issuing results.

Adjust the attribute value accordingly (right-click the inspection result, or select “adjust attribute value(s)” from the “Fix” dropdown menu), and now your UserBuilder is ready to use:

Dim identity As IUser
Set identity = UserBuilder.Create(uniqueId, uniqueName) _
                          .WithName(first, last) _
                          .WithEmail(emailAddress) _

…and misuse:

Set UserBuilder.User = New User '<~ runtime error, illegal from default instance
Debug.Print UserBuilder.User.AvatarUrl '<~ compile error, invalid use of property
Set builder = New UserBuilder
Set identity = builder.Build '<~ runtime error 91, builder state was not initialized
Set builder = New UserBuilder
Set builder = builder.WithEmail(emailAddress) '<~ runtime error 91


Model classes with many properties are annoying to write, and annoying to initialize. Sometimes properties are required, other times properties are optional, others are only valid if another property has such or such value. This article has shown how effortlessly such classes can be created with Rubberduck, and how temporal coupling and other state issues can be solved using the builder creational pattern.

Using this pattern as a building block in the same toolbox as factory methods and other creational patterns previously discussed, we can now craft lovely fluent APIs that can chain optional member calls to build complex objects with many properties without needing to take a gazillion parameters anywhere.

Password Authentication

Authenticating the user of our application is a common problem, with common pitfalls – some innocuous, some fatal. It’s also a solved problem, with a fairly standard solution. Unfortunately, it’s also a problem that’s too often solved with naive, “good-enough” solutions that make any security expert twitch.

The vast majority of scenarios don’t need any custom authentication. Accessing a SQL Server database? Use Windows Authentication! Windows Auth not possible? Use SQL Authentication over a secure network! App authentication isn’t for authenticating a user with a server. More like, the application itself needs a concept of users and privileges granted to certain groups of users, and so we need to prompt the user for a user name and a password. What could possibly go wrong?

Security First: Threat Model Assessment

The first question we need to ask ourselves, is literally “what could possibly go wrong?” — as in, what are we trying to do? If the answer is along the lines of:

  • Enhance user experience with tailored functionality
  • Grouping users into “roles” for easier management
  • Prevent accidental misuse of features

…then you’re on the right track. However if you’re thinking more in terms of…

  • Prevent intentional misuse of features
  • Securely prevent groups of users from accessing functionalities
  • Securely $(anything)

…then you’re going to need another kind of approach. VBA code is not secure, period. Even if the VBA project is password-protected, the VBE can be tricked into unlocking it with some clever Win32 API calls. So, the threat model should take into account that a power user that wants to see your code… will likely succeed …pretty easily, too.

That doesn’t mean VBA code gets a pass to do everything wrong! If you’re going to do password authentication, you might as well do it right.

Where to store users’ passwords?

We’ve all done this:

Private Const ADMIN_PWD As String = "@Dm!n"

…without realizing that the code of a VBA project – even locked – is compressed into a binary file that’s zipped with the rest of the Excel host document. But nothing prevents anyone from peeking at it, say, with Notepad++

Of course it had to be on line 42.

Obviously, hard-coding passwords is the worst possible idea: we need somewhere safe, right?

Truth is, not really. You could have everything you need in a hidden worksheet that anyone can see if they like; a database server is ideal, of course, but not necessary, if parts of your host document can be used as one (looking at you too, Microsoft Access).

The reason it doesn’t matter if the “passwords table” is compromised, is because you do not store passwords, period. Not even encrypted: the “passwords table” contains nothing that can be processed (decrypted) and then used as a password.

What you store is a hash of the users’ passwords, never the passwords themselves. For example, if a user’s password was password and we hashed it with the SHA256 hashing algorithm, we would be storing the following value:


Contrary to encryption and encoding, there is by definition no way to revert a hash value back to the original string password. It’s possible that some random string that’s not password might produce the same hash value (i.e. a hash collision) – but very (very very) unlikely, at least with SHA256 or higher.

There are many different hashing algorithms, producing values of varying length, at varying speeds: with cryptographically secure requirements, using slow algorithms that produce values with a low risk of collision will be preferred (harder/longer to brute-force). Other applications might use a faster MD5 hash that’s “good enough” if not very secure, for many things but a password.

Now obviously, if any two users have the same password, their SHA256 hash would be the same. If that’s a concern (it should be), then the solution is to use a salt: prepend a random string to the password, and hash the salted password string – assuming all users use a different salt value (it can be safely stored alongside the user record), then it becomes impossible to tell whether any two users have the same password just by looking at the table contents… and this is why a hidden worksheet is a perfectly fine place to store your user passwords if you can’t use a database for whatever reason.

Even if you know every user’s hash and salt value, you can’t know what password could possibly be prefixed with that particular salt value for a SHA256 hash algorithm to give exactly these specific bytes. These users all have the same password.

Storing a salted password hash prevents “translating” the hash values wholesale, using a lookup/”rainbow” table that contains common passwords and their corresponding hash representation. Even if one password is compromised, other users with the same password wouldn’t be, because their hash is different, thanks to the “salt” bytes.

Whether we code in C#, PHP, JavaScript, Python, Java, ..or VBA, there’s simply not a single valid reason to store user passwords in plain text. But how do we get that hash value out of a password string in the first place?

Hashing with VBA

There’s… no built-in support whatsoever for hashing in VBA… but nothing says we can’t make explicit late binding and the .NET Framework work for us! Note that we’re invoking the ComputeHash_2 method, because it’s an overload of the ComputeHash method that takes the byte array we want to give it. COM/VBA doesn’t support method overloading, so when .NET exposes overloads to COM, it appends _2 to the method name, _3, _4, and so on for each overload. The order depends on… the order they were written to the IDL, which means you could… just trust Stack Overflow on that one, and go with ComputeHash_2:

Public Function ComputeHash(ByVal value As String) As String
    Dim bytes() As Byte
    bytes = StrConv(value, vbFromUnicode)
    Dim algo As Object
    Set algo = CreateObject("System.Security.Cryptography.SHA256Managed")
    Dim buffer() As Byte
    buffer = algo.ComputeHash_2(bytes)
    ComputeHash = ToHexString(buffer)
End Function
Private Function ToHexString(ByRef buffer() As Byte) As String
    Dim result As String
    Dim i As Long
    For i = LBound(buffer) To UBound(buffer)
        result = result & Hex(buffer(i))
    ToHexString = result
End Function

This code would feel right at home in a SHA256Managed standard module, or it could be a class that implements some IHashAlgorithm interface with a ComputeHash method – and with it we have everything we need to start handling password-based authentication in VBA …by today’s best practices.

What follows is an object-oriented approach to leveraging this function in a VBA project that needs to authenticate a user. An online copy of this code can be downloaded from GitHub:


I like having functionality neatly abstracted, so instead of just having a public ComputeHash function that computes the SHA256 hash for a given string, I’ll have a class module formalizing what a hash algorithm does:

'@ModuleDescription("An interface representing a hashing algorithm.")
Option Explicit
'@Description("Computes a hash for the given string value.")
Public Function ComputeHash(ByVal value As String) As String
End Function

One implementation would be this SHA256Managed class module:

Option Explicit
Implements IHashAlgorithm
Private base As HashAlgorithmBase
'@Description("Factory method creates and returns a new instance of this class.")
Public Function Create() As IHashAlgorithm
    Set Create = New SHA256Managed
End Function
Private Sub Class_Initialize()
    Set base = New HashAlgorithmBase
End Sub
Private Function IHashAlgorithm_ComputeHash(ByVal value As String) As String
    Dim bytes() As Byte
    bytes = StrConv(value, vbFromUnicode)
    Dim algo As Object
    Set algo = CreateObject("System.Security.Cryptography.SHA256Managed")
    Dim buffer() As Byte
    buffer = algo.ComputeHash_2(bytes)
    IHashAlgorithm_ComputeHash = base.ToHexString(buffer)
End Function

By coding against an interface (i.e. by invoking ComputeHash off the IHashAlgorithm interface), we are making the code easier to modify later without breaking things: if a functionality needs a MD5 hash algorithm instead of SHA256, we can implement a MD5Managed class and inject that, and no client code needs to be modified, because the code doesn’t care what specific algorithm it’s working with, as long as it implements the IHashAlgorithm interface.

The HashAlgorithmBase class is intended to be used by all implementations of IHashAlgorithm, so we’re using composition to simulate inheritance here (the coupling is intended, there’s no need to inject that object as a dependency). The class simply exposes the ToHexString function, so that any hashing algorithm can get a hex string out of a byte array:

'@ModuleDescription("Provides common functionality used by IHashAlgorithm implementations.")
Option Explicit
'@Description("Converts a byte array to a string representation.")
Public Function ToHexString(ByRef buffer() As Byte) As String
    Dim result As String
    Dim i As Long
    For i = LBound(buffer) To UBound(buffer)
        result = result & Hex(buffer(i))
    ToHexString = result
End Function

At this point we can already test the hashing algorithm in the immediate pane:


The next step is to create an object that’s able to take user credentials, and tell its caller whether or not the credentials are good. This is much simpler than it sounds like.


The first thing we need to address, is the data we’re going to be dealing with – the model. In the case of a dialog that’s prompting for a user name and a password, our model is going to be a simple class exposing Name and Password read/write properties, and here an IsValid property returns True if the Name and Password values aren’t empty:

Option Explicit
Private Type TAuthModel
    Name As String
    Password As String
    IsValid As Boolean
End Type
Private this As TAuthModel
Public Property Get Name() As String
    Name = this.Name
End Property
Public Property Let Name(ByVal value As String)
    this.Name = value
End Property
Public Property Get Password() As String
    Password = this.Password
End Property
Public Property Let Password(ByVal value As String)
    this.Password = value
End Property
Public Property Get IsValid() As Boolean
    IsValid = this.IsValid
End Property
Private Sub Validate()
    this.IsValid = Len(this.Name) > 0 And Len(this.Password) > 0
End Sub

Since this isn’t a model for changing a password, the validation logic doesn’t need to worry about the password’s length and/or content – only that a non-empty value was provided; your mileage may vary!

If we wanted the UI to provide a ComboBox dropdown to pick a user name, then the model class would need to encapsulate an array or collection that contains the user names, and that array or collection would be provided by another component.


When my object-oriented brain thinks “authentication”, what shapes up in my mind is a simple interface that exposes a single Boolean-returning function that takes user credentials, and returns True when authentication succeeds with the provided credentials.

Something like this:

'@ModuleDescription("An interface representing an authentication mechanism.")
Option Explicit
'@Description("True if the supplied credentials are valid, False otherwise.")
Public Function Authenticate(ByVal model As UserAuthModel) As Boolean
End Function

If we have a hidden worksheet with a table containing the user names, salt values, and hashed passwords for all users, then we could implement this interface with some WorksheetAuthService class that might look like this:

'@ModuleDescription("A service responsible for authentication.")
Option Explicit
Implements IAuthService
Private Type TAuthService
    Algorithm As IHashAlgorithm
End Type
Private Type TUserAuthInfo
    Salt As String
    Hash As String
End Type
Private this As TAuthService
Public Function Create(ByVal hashAlgorithm As IHashAlgorithm)
    With New WorksheetAuthService
        Set .Algorithm = hashAlgorithm
        Set Create = .Self
    End With
End Function
Public Property Get Self() As IHashAlgorithm
    Set Self = Me
End Property
Public Property Get Algorithm() As IHashAlgorithm
    Set Algorithm = this.Algorithm
End Property
Public Property Set Algorithm(ByVal value As IHashAlgorithm)
    Set this.Algorithm = value
End Property
Private Function GetUserAuthInfo(ByVal user As String, ByRef outInfo As TUserAuthInfo) As Boolean
'gets the salt value & password hash for the specified user; returns false if user can't be retrieved.
    On Error GoTo CleanFail
    With PasswordsSheet.Table
        Dim nameColumnIndex As Long
        nameColumnIndex = .ListColumns("Name").Index
        Dim saltColumnIndex As Long
        saltColumnIndex = .ListColumns("Salt").Index
        Dim hashColumnIndex As Long
        hashColumnIndex = .ListColumns("PasswordHash").Index
        Dim userRowIndex As Long
        userRowIndex = Application.WorksheetFunction.Match(user, .ListColumns(nameColumnIndex).DataBodyRange, 0)
        outInfo.Salt = Application.WorksheetFunction.Index(.ListColumns(saltColumnIndex).DataBodyRange, userRowIndex)
        outInfo.Hash = Application.WorksheetFunction.Index(.ListColumns(hashColumnIndex).DataBodyRange, userRowIndex)
    End With
    GetUserAuthInfo = True
    Exit Function
    Debug.Print Err.Description
    Debug.Print "Unable to retrieve authentication info for user '" & user & "'."
    outInfo.Salt = vbNullString
    outInfo.Hash = vbNullString
    GetUserAuthInfo = False
    Resume CleanExit
End Function
Private Function IAuthService_Authenticate(ByVal model As UserAuthModel) As Boolean
    Dim info As TUserAuthInfo
    If Not model.IsValid Or Not GetUserAuthInfo(model.Name, outInfo:=info) Then Exit Function
    Dim pwdHash As String
    pwdHash = this.Algorithm.ComputeHash(info.Salt & model.Password)
    IAuthService_Authenticate = (pwdHash = info.Hash)
End Function

If we only look at the IAuthService_Authenticate implementation, we can easily tell what’s going on:

  • If for any reason we can’t identify the specified user / get its authentication info, we bail
  • Using the user’s Salt string, we use the hashing algorithm’s ComputeHash method to get a hash string for the specified password.
  • Authentication succeeds if the hashed salted password matches the stored hash string for that user.

Note how the provided model.Password string isn’t being copied anywhere, or compared against anything.

The GetUserAuthInfo function is being considered an implementation detail here, but could easily be promoted to its own IUserAuthInfoProvider interface+implementation: the role of that function is to get the Salt and PasswordHash values for a given user, and here we’re pulling that from a table on a worksheet, but other implementations could be pulling it from a database: this is a concern in its own right, and could very easily be argued to belong in its own class, abstracted behind its own interface.


If we’re going to have a dialog for the user to enter their credentials into, then everything we’ve seen about the Model-View-Presenter UI design pattern is applicable here – we already have our model, and now we need an abstraction for a view.

Option Explicit
'@Description("Shows the view as a modal dialog. Returns True unless the dialog is cancelled.")
Public Function ShowDialog() As Boolean
End Function
Public Property Get UserAuthModel() As UserAuthModel
End Property

From an abstract standpoint, the view is nothing more than a function that displays the dialog and returns False if the dialog was cancelled, True otherwise.

The concrete implementation will be a UserForm that includes two textboxes, two command buttons, and a few labels – like this:

The code-behind for the form is very simple:

  • Change handlers for the textboxes assign the corresponding model property
  • Click handlers for the command buttons simply Hide the form
  • A Create factory method takes a UserAuthModel object reference
  • Model is exposed for property injection (only the factory method uses this property)
Option Explicit
Implements IAuthView
Private Type TAuthDialog
    UserAuthModel As UserAuthModel
    IsCancelled As Boolean
End Type
Private this As TAuthDialog
Public Function Create(ByVal model As UserAuthModel) As IAuthView
    If model Is Nothing Then Err.Raise 5, TypeName(Me), "Model cannot be a null reference"
    Dim result As AuthDialogView
    Set result = New AuthDialogView
    Set result.UserAuthModel = model
    Set Create = result
End Function
Public Property Get UserAuthModel() As UserAuthModel
    Set UserAuthModel = this.UserAuthModel
End Property
Public Property Set UserAuthModel(ByVal value As UserAuthModel)
    Set this.UserAuthModel = value
End Property
Private Sub OnCancel()
    this.IsCancelled = True
End Sub
Private Sub Validate()
    OkButton.Enabled = this.UserAuthModel.IsValid
End Sub
Private Sub CancelButton_Click()
End Sub
Private Sub OkButton_Click()
End Sub
Private Sub NameBox_Change()
    this.UserAuthModel.Name = NameBox.Text
End Sub
Private Sub PasswordBox_Change()
    this.UserAuthModel.Password = PasswordBox.Text
End Sub
Private Sub UserForm_QueryClose(Cancel As Integer, CloseMode As Integer)
    If CloseMode = VbQueryClose.vbFormControlMenu Then
        Cancel = True
    End If
End Sub
Private Function IAuthView_ShowDialog() As Boolean
    Me.Show vbModal
    IAuthView_ShowDialog = Not this.IsCancelled
End Function
Private Property Get IAuthView_UserAuthModel() As UserAuthModel
    Set IAuthView_UserAuthModel = this.UserAuthModel
End Property

The important thing to note, is that the form itself doesn’t do anything: it’s just an I/O device your code uses to interface with the user – nothing more, nothing less. It collects user-provided data into a model, and ensures the dialog validates that model.

The form knows about the UserAuthModel and its properties (Name, Password, IsValid), and nothing else. It doesn’t know how to get a list of user names to populate a dropdown so that the user can pick a name from a list (that could be done, but then the model would need a UserNames property). It doesn’t know how to verify whether the provided password string is correct. It’s …just not its job to do anything other than relay messages to & from the user.


We have a UserAuthModel that holds the user-supplied credentials. We have a WorksheetAuthService that can take these credentials and tell us if they’re good, using any IHashAlgorithm implementation. We’re missing an object that pieces it all together, and that’s the job of a presenter.

What we want is for the code that needs an authenticated user, to be able to consume a simple interface, like this:

'@ModuleDescription("Represents an object that can authenticate the current user.")
Option Explicit
'@Description("True if user is authenticated")
Public Property Get IsAuthenticated() As Boolean
End Property
'@Description("Prompts for user credentials")
Public Sub Authenticate()
End Sub

Now, any class that encapsulates functionality that involves authenticating the current user can be injected with an IAuthPresenter interface, and when IsAuthenticated is True we know our user is who they say they are. And if we inject the same instance everywhere, then the user only needs to enter their credentials once for the authentication state to be propagated everywhere – without using any globals!

'@ModuleDescription("Represents an object responsible for authenticating the current user.")
Option Explicit
Implements IAuthPresenter
Private Type TPresenter
    View As IAuthView
    AuthService As IAuthService
    IsAuthenticated As Boolean
End Type
Private this As TPresenter
Public Function Create(ByVal service As IAuthService, ByVal dialogView As IAuthView) As IAuthPresenter
    Dim result As AuthPresenter
    Set result = New AuthPresenter
    Set result.AuthService = service
    Set result.View = dialogView
    Set Create = result
End Function
Public Property Get AuthService() As IAuthService
    Set AuthService = this.AuthService
End Property
Public Property Set AuthService(ByVal value As IAuthService)
    Set this.AuthService = value
End Property
Public Property Get View() As IAuthView
    Set View = this.View
End Property
Public Property Set View(ByVal value As IAuthView)
    Set this.View = value
End Property
Private Sub IAuthPresenter_Authenticate()
    If Not this.View.ShowDialog Then Exit Sub
    this.IsAuthenticated = this.AuthService.Authenticate(this.View.UserAuthModel)
End Sub
Private Property Get IAuthPresenter_IsAuthenticated() As Boolean
    IAuthPresenter_IsAuthenticated = this.IsAuthenticated
End Property

At this point any standard module macro (aka entry point) can create the presenter and its dependencies:

Public Sub DoSomething()
    Dim model As UserAuthModel
    Set model = New UserAuthModel
    Dim dialog As IAuthView
    Set dialog = AuthDialogView.Create(model)
    Dim algo As IHashAlgorithm
    Set algo = SHA256Managed.Create()
    Dim service As IAuthService
    Set service = WorksheetAuthService.Create(algo)
    Dim presenter As IAuthPresenter
    Set presenter = AuthPresenter.Create(service, dialog)
    If presenter.IsAuthenticated Then
        MsgBox "Welcome!", vbInformation
        MsgBox "Access denied", vbExclamation
    End If
End Sub 

If this were real application code, instead of consuming the presenter it would be injecting it into some class instance, and invoking a method on that class. This composition root (where we compose the application / instantiate and inject all the dependencies) would probably be in the Workbook_Open handler, so that the authentication state can be shared between components.


Up to this point, we only cared for authentication, i.e. identifying the current user. While very useful, it doesn’t tell us who’s authorized to do what. Without some pretty ugly code that special-cases specific users (e.g. “Admin”), we’re pretty limited here.

One proven solution, is to use role-based authorisations. Users belong to a “group” of users, and it’s the “group” of users that’s authorized to do things, not users themselves.

In order to do this, the WorksheetAuthService implementation needs to be modified to add a RoleId member to the TUserAuthInfo, and the IAuthService.Authenticate method could return a Long instead of a Boolean, where 0 would still mean a failed authentication, but any non-zero value would be the authenticated user’s RoleId.

Roles could be defined by an enum (note the default / 0 value):

Public Enum AuthRole
    Unauthorized = 0
End Enum

Or, role membership could be controlled in Active Directory (AD), using security groups – in that case you’ll want your IAuthService implementation to query AD instead of a worksheet, and the IAuthPresenter implementation to hold the current user’s role ID along with its authentication status.

There are many ways to go about implementing authentication, and many implementation-specific concerns. For example, if you’re querying a database for this, you’ll want to use commands and proper parameterization to avoid the problems associated with SQL Injection vulnerabilities: maybe a user named Robert');DROP TABLE USERS;-- isn’t part of your threat model, but can Tom O'Neil log onto your system without breaking anything?

Regardless of the approach, if you’re comparing the user’s plain-text password input with the plain-text password stored in $(storage_medium), you’re doing it wrong – whether that’s in VBA or not.

Rubberduck Annotations

I wrote about this unfortunately hard-to-discover feature in 2017, but a lot has happened since then, and there’s 5 times more of you now! The wiki is essentially up-to-date, but I’m not sure of its viewership. So here’s a recap of annotations in the late Rubberduck 2.4.1.x pre-release builds, that 2.5.0.x will launch with.

What we call “annotations” are special comments that look like these:

'@ModuleDescription("An interface that describes an object responsible for something.")

'@Description("Does something")
Public Sub DoSomething()
End Sub


Rubberduck’s parser includes a grammar rule that captures these special comments, such that we “see” them like any other language syntax element (tokens), and can analyze them as such, too.

The syntax is rather simple, and is made to look like a procedure call – note that string arguments must be surrounded with double quotes:

'@AnnotationName arg1, arg2, "string argument"

If desired, parentheses can be used, too:

'@AnnotationName(arg1, arg2)
'@AnnotationName("string argument")

Whether you use one notation or the other is entirely up to personal preference, both are completely equivalent. As with everything else, consistency should be what matters.

There’s an inspection that flags illegal/unsupported annotations that you, if you’re using this @PseudoSyntax for other purposes, will probably want to disable: that’s done by setting its severity level to DoNotShow in the inspection settings, or by simply clicking “disable this inspection” from the inspection results toolwindow.

Keep in mind that while they are syntactically comments as far as VBA is concerned, to Rubberduck parsing the argument list of an annotation needs to follow strict rules. This parses correctly:

'@Folder "Some.Sub.Folder" @ModuleDescription "Some description" : some comment

Without the : instruction separator token, the @ModuleDescription annotation parses as a regular comment. After : though, anything goes.

There are two distinct types of annotation comments: some annotations are only valid at module level, and others are only valid at member level.

Module Annotations

Module-level annotations apply to the entire module, and must appear in that module’s declarations section. Personally, I like having them at the very top, above Option Explicit. Note that if there’s no declaration under the last annotation, and no empty line, then the placement becomes visually ambiguous – even though Rubberduck correctly understands it, avoid this:

Option Explicit
'@Description("description here")
Public Sub DoSomething() '^^^ is this the module's or the procedure's annotation?
End Sub

Let it breathe – always have an empty line between the end of the module’s declarations section (there should always at least be Option Explicit there) and the module’s body:

Option Explicit
'@Folder("MyProject") : clearly belongs to the module

'@Description("description here")
Public Sub DoSomething() '^^^ clearly belongs to the procedure
End Sub

What follows is a list of every single module-level annotation currently supported (late v2.4.1.x pre-release builds), that v2.5.0 will launch with.


The Visual Basic Editor regroups modules in its Project Explorer toolwindow, by component type: you get a folder for your “Modules”, another folder for your “Class Modules”; if you have userforms they’re all under a “Forms” folder, and then the document modules are all lumped under some “Microsoft Excel Objects” folder (in an Excel host, anyway). While this grouping is certainly fine for tiny little automation scripts, it makes navigation wildly annoying as soon as a project starts having multiple features and responsibilities.

In a modern IDE like Visual Studio, code files can be regrouped by functionality into a completely custom folder hierarchy: you get to have a form in the same folder as the presenter class that uses it, for example. With Rubberduck’s Code Explorer toolwindow, you get to do exactly the same, and the way you do this is with @Folder annotations.

Option Explicit

The @Folder annotation takes a single string argument representing the “virtual folder” a module should appear under, where a dot (.) denotes a sub-folder – a bit like .NET namespaces. Somewhere deep in the history of this annotation, there’s a version that’s even named @Namespace. “Folder” was preferred though, because “Namespace” was deemed too misleading for VBA/VB6, given the language doesn’t support them: all module names under a given project must still be unique. The Code Explorer toolwindow uses these annotations to build the folder hierarchy to organize module nodes under, but the folders don’t actually exist: they’re just a representation of the annotation comments in existing modules – and that is why there is no way to create a new, empty folder to drag-and-drop modules into.

It is strongly recommended to adopt a standard and consistent PascalCase naming convention for folder names: future Rubberduck versions might very well support exporting modules accordingly with these folder annotations, so these “virtual folders” might not be “virtual” forever; by using a PascalCase naming convention, you not only adopt a style that can be seamlessly carried into the .NET world; you also make your folders future-proof. Avoid spaces and special characters that wouldn’t be legal in a folder name under Windows.

The ModuleWithoutFolder inspection (under “Rubberduck Opportunities”), if enabled, will warn you of modules where this annotation is absent. By default, Rubberduck’s Code Explorer will put all modules under a single root folder named after the VBA project. While this might seem rather underwhelming, it was a deliberate decision to specifically not re-create the “by component type” grouping of the VBE and encourage our users to instead regroup modules by functionality.


The @IgnoreModule annotation is automatically added by the “Ignore in Module” inspection quick-fix, which effectively disables a specific code inspection, but only in a specific module. This can be useful for inspections that have false positives, such as procedure not used firing results in a module that contains public parameterless procedures that are invoked from ActiveX controls on a worksheet, which Rubberduck isn’t seeing (hence the false positives), but that are otherwise useful, such that you don’t necessarily want to completely disable the inspection (i.e. set its severity level to DoNotShow).

If no arguments are specified, this annotation will make all inspections skip the module. To skip a specific inspection, you may provide its name (minus the Inspection suffix) as an argument. To ignore multiple inspections, you can separate them with commas like you would any other argument list:

'@IgnoreModule ProcedureNotUsed, ParameterNotUsed

Alternatively, this annotation may be supplied multiple times:

'@IgnoreModule ProcedureNotUsed
'@IgnoreModule ParameterNotUsed

Use the : instruction separator to terminate the argument list and add an explanatory comment as needed:

'@IgnoreModule ProcedureNotUsed : These are public macros attached to shapes on Sheet1

Note that the arguments (inspection names) are not strings: enclosing the inspection names in string literals will not work.


This was the very first annotation supported by Rubberduck. This annotation is only legal in standard/procedural modules, and marks a module for test discovery: the unit testing engine will only scan these modules for unit tests. This annotation does not support any parameters.


Given a string value, this annotation can be used to control the value of the module’s hidden VB_Description attribute, which determines the module’s “docstring” – a short description that appears in the VBE’s Object Browser, and that Rubberduck displays in its toolbar and in the Code Explorer.

Because Rubberduck can’t alter module attributes in document modules, this annotation is illegal in modules representing objects owned by the host application (i.e. “document” modules), such as Worksheet modules and ThisWorkbook.


This annotation does not support any parameters, and can be used to control the value of the hidden VB_PredeclaredId attribute, which determines whether a class has a default instance. When a class has a default instance, its members can be invoked without an instance variable (rather, using an implicit one named after the class itself), like you did every single time you’ve ever written UserForm1.Show – but now you get to have a default instance for your own classes, and this opens up a vast array of new possibilities, most notably the ability to now write factory methods in the same class module as the class being factory-created, effectively giving you the ability to initialize new object instances with parameters, just like you would if VBA classes had parameterized constructors:

Dim something As Class1
Set something = Class1.Create("test", 42)


VBA classes are private by default: this means if you make a VBA project that references another, then you can’t access that class from the referencing project. By setting the class’ instancing property to PublicNotCreatable, a referencing project is now able to consume the class (but the class can only be instantiated inside the project that defines it… and that’s where factory methods shine).

This annotation visibly documents that the class’ instancing property has a non-default value (this can easily be modified in the VBE’s properties toolwindow).


In VBA every class modules defines a public interface: every class can Implements any other class, but not all classes are created equal, and in the vast majority of the time what you want to follow the Implements keyword will be the name of an abstract interface. An abstract interface might look like this:

Option Explicit

Public Sub DoSomething()
End Sub

Adding this annotation to a module serves as metadata that Rubberduck uses when analyzing the code: the Code Explorer will display these modules with a dedicated “interface” icon, and an inspection will be able to flag procedures with a concrete implementation in these modules.


Rubberduck’s Smart Indenter port can indent your entire VBA project in a few milliseconds, but automatically indenting a module can have undesirable consequences, such as losing hidden member attributes. Use this annotation to avoid accidentally wiping hidden attributes in a module: the indenter will skip that module when bulk-indenting the project.

Member Annotations

Member-level annotations apply to the entire procedure they’re annotating, and must be located immediately over the procedure’s declaration:

'@Description("Does something")
Public Sub DoSomething()
End Sub

As with module annotations, multiple member annotations can be specified for the same procedure – either by stacking them, or enumerating them one after the other:

'@Description("Gets the item at the specified index")
Public Property Get Item(ByVal index As Long) As Object
End Property

Member annotations that aren’t immediately above the procedure declaration, will be flagged as illegal by the IllegalAnnotation inspection:

'@Description("Does something") : <~ annotation is illegal/misplaced

Public Sub DoSomething()
End Sub


This very useful annotation controls the value of the member’s hidden VB_Description attribute, which defines a docstring that appears in the bottom panel of the Object Browser when the member is selected – Rubberduck also displays this content in the context-sensitive (selection-dependent) label in the Rubberduck VBIDE toolbar.

Toolbar label content is dependent on the current selection in the editor and includes the value of the hidden attribute’s value.


Similar to @IgnoreModule, the purpose of the member-level @Ignore annotation is to get specific inspections to ignore the annotated procedure: it works identically.


Only one single member of a class can be the class’ default member. Default members should generally be avoided, but they are very useful for indexed Item properties of custom collection classes. This annotation takes no arguments.


Custom collections that need to support For Each enumeration are required to have a member that returns an IUnknown, and hidden flags and attributes: this annotation clearly identifies the special member, and gets the hidden flags and attributes right every time.

Public Property Get NewEnum() As IUnknown
    Set NewEnum = encapsulatedCollection.[_NewEnum]
End Property


This rather specific annotation works in Excel-hosted VBA projects (as of this writing its absence may cause inspection false positives in other host applications, like Microsoft Word).

When the VBA project is hosted in Microsoft Excel, you can use this annotation to assign hotkeys using the same mechanism Excel uses to map hotkeys to recorded macros.

'@ExcelHotkey "D" : Ctrl+Shift+D will invoke this procedure in Excel
Public Sub DoSomething()
End Sub

'@ExcelHotkey "d" : Ctrl+D will invoke this procedure in Excel
Public Sub DoSomethingElse()
End Sub

Note that the annotation will work regardless of whether the argument is treated as a string literal or not – only the first character of the annotation argument is used, and its case determines whether the Shift key is involved in the hotkey combination (all hotkeys involve the Ctrl key): use an uppercase letter for a Ctrl+Shift hotkey.


Code under continued maintenance is constantly evolving, and sometimes in order to avoid breaking existing call sites, a procedure might need to be replaced by a newer version, while keeping the old one around: this annotation can be used to mark the old version as obsolete with an explanatory comment, and inspections can flag all uses of the obsolete procedure:

'@Obsolete("Use DoSomethingElse instead.")
Public Sub DoSomething()
End Sub

Public Sub DoSomethingElse()
End Sub
The argument string appears in the inspection results for each use of the obsolete member.

Test Method Annotations

These annotations have been in Rubberduck for a very long time, and they are actually pretty easy to discover since they are automatically added by Rubberduck when adding test modules and test methods using the UI commands – but since Test Settings can be configured to not include setup & teardown stubs, it can be easy to forget they exist and what they do.


This annotation is used in test modules to identify test methods: every test must be marked with this annotation in order to be discoverable as a test method. It is automatically added by Rubberduck’s “add test method” commands, but needs to be added manually if a test method is typed manually in the editor rather than inserted by Rubberduck.

This annotation supports a string argument that determines the test’s category, which appears in the Test Explorer toolwindow and enables grouping by category. If no category argument is specified, “Uncategorized” is used as a default:

@TestMethod("Some Category")
Private Sub TestMethod1()
End Sub

The other @TestXxxxx member annotations are used for setup & teardown. If test settings have the “Test module initialization/cleanup” option selected, then @ModuleInitialize and @ModuleCleanup procedure stubs are automatically added to a new test module. If test settings have “Test method initialization/cleanup” selected, then @TestInitialize and @TestCleanup procedure stubs are automatically added a new test modules.


In test modules, this annotation marks procedures that are invoked before every single test in the module. Use that method to run setup/initialization code that needs to execute before each test. Each annotated procedure is invoked, but the order of invocation cannot be guaranteed… however there shouldn’t be a need to have more than one single such initialization method in the module.


Also used in test modules, this annotation marks methods that are invoked after every single test in that test module. Use these methods to run teardown/cleanup code that needs to run after each test. Again, each annotated procedure is invoked, but the order of invocation cannot be guaranteed – and there shouldn’t be a need to have more than one single such cleanup method in the module.


Similar to @TestInitialize, but for marking procedures that are invoked once for the test module, before the tests start running. Use these procedures to run setup code that needs to run before the module’s tests begin to run; each annotated procedure will be invoked, but the order of invocation cannot be guaranteed. Again, only one such initialization procedure should be needed, if any.


Similar to @TestCleanup, but for marking procedures that are invoked once for the test module, after all tests in the module have executed. Use these procedures to run teardown/cleanup code that needs to run after all module’s tests have completed; each annotated procedure will be invoked, but the order of invocation isn’t guaranteed. Only one such cleanup procedure should be needed, if any.

Annotations are one of Rubberduck’s most useful but unfortunately also one of its most obscure and hard-to-discover features. Fortunately, we have plans to surface them as right-click context menu commands in the 2.5.x release cycle.

Document Modules

The first time you discovered the Visual Basic Editor and visualized the VBA project in the Project Explorer, when you first met ThisWorkbook and Sheet1, document modules were the world: your baby steps in VBA were very likely in the code-behind of ThisWorkbook or Sheet1 – document modules are the gateway to VBA-land.

In the VBIDE Extensibility object model, the modules are accessed via the VBComponents property of a VBProject object, and the VBComponent.Type property has a value of vbext_ComponentType.vbext_ct_Document (an enum member value) for both ThisWorkbook and Sheet1 modules, but would be vbext_ct_StdModule for a standard module like Module1. Clearly if document modules are class modules, there’s enough “special” about them that they need to be treated differently by the VBA compiler.


VBA itself is host-agnostic: whether it’s hosted in Excel, PowerPoint, AutoCAD, Sage, Corel Draw, or any other one of over 200 licensed hosts, it’s the same VBA (there’s no such thing as “Excel VBA”, really), and it doesn’t have any intrinsic knowledge of what an Excel.Worksheet is – only the Excel library does; when your VBA project is hosted in Excel, then the Excel type library is automatically added to (and locked; you can’t remove it from) your project. Document modules are a special kind of module that the VBIDE treats as part of the VBA project, but it can’t add or remove them: If you want to add or remove a Worksheet module from a VBA project, you need to actually add or remove a worksheet from the host Excel workbook.

So, other than we can’t directly add such components to a VBA project without involving the host application’s object model, what’s special about them?

They’re in the VBA project, because the host application decided that’s what’s in a VBA project hosted in – here Excel, but each host application gets to decide whether a project includes a module to represent the host document, and under what circumstances to add or remove other types of modules, like Worksheet, or Chart.

Document modules can’t be instantiated or destroyed, so there’s no Initialize or Terminate event handler for them. However, you get to create a handler for any or every one of many convenient events that the host application fires at various times and that the authors of the object model deemed relevant to expose as programmatic extensibility points. For ThisWorkbook, this includes events like Workbook.Open and Workbook.NewSheet, for example. For Worksheet modules, that’s worksheet-specific events like Worksheet.SelectionChange or Worksheet.Change, that you can use to “hook” custom code when such or such thing happens in the host document.


Your VBA project is hosted inside an Excel document. In the VBA project, the host document is referred to with an identifier, and that identifier is (by default anyway, and renaming it is not something I’d recommend doing) ThisWorkbook.

The Excel workbook that contains the current VBA project: that’s ThisWorkbook. When you’re in the code-behind of that module, you’re extending a Workbook object: if you type Me., the VBE’s IntelliSense/autocomplete list will display all the public members you’d find on any other Workbook object, plus any Public (explicitly or not) member. That’s what’s special about a document module: it literally inherits members from another class, as in inheritance – something VBA user code cannot do. Isn’t it fascinating that, under the hood, Visual Basic for Applications apparently has no problem with class inheritance? Something similar happens with UserForm code: the UserForm1 class inherits the members of any other UserForm, “for free”. And of course every Sheet1 inherits the members of every other Worksheet in the world.

So, procedures you write in a document module, should logically be very closely related to that particular document. And because host-agnostic logic can’t add/remove these modules, you’ll want to have as little code as possible in them – and then as a bonus, your VBA project becomes easier to keep under source control, because the code is in modules that VBE add-ins (wink wink) are able to properly import back in and synchronize to & from the file system.

What about ActiveWorkbook?

ActiveWorkbook refers to the one and only workbook that is currently active in the Excel Application instance, which may or may not be ThisWorkbook / the host document. It’s easy to confuse the two, and even easier to write code that assumes one is the other: the macro recorder does it, many documentation examples and Stack Overflow answers do it too. But reliable code is code that makes as few assumptions as possible – sooner or later, built-in assumptions are broken, and you’re faced with an apparently intermittent error 1004 that sometimes happens when you’re debugging and stepping through the code, and it happened to a user once or twice but the problem always seemed to vaporize just because you showed up at the user’s cubicle and stood there watching as nothing blew up and everything went fine. *Shrug*, right?

Accessing Worksheets

You shouldn’t be dereferencing worksheets all the time. In fact, you rarely even need to. But when you do, it’s important to do it right, and for the right reasons. The first thing you need to think of, is whether the sheet exists in ThisWorkbook at compile-time. Meaning, it’s there in the host document, you can modify it in Excel and there’s a document module for it in the VBA project.

That’s the first thing you need to think about, because if the answer to that is “yep, it’s right there and it’s called Sheet3“, then you already have your Worksheet object and there’s no need to dereference it from any Sheets or Worksheets collection!

Dim sheet As Worksheet
Set sheet = ThisWorkbook.Worksheets("Sheet1") '<~ bad if Sheet1 exists at compile-time!

Set sheet = Sheet1 '<~ bad because redundant: the variable obfuscates the target!
sheet.Range("A1").Value = 42 '<~ bad if sheet is a local variable, but good if a parameter

Sheet1.Range("A1").Value = 42 '<~ most reliable way to refer to a worksheet

The magic Sheet1 identifier comes from the (Name) property of the Sheet1 document module under ThisWorkbook in the VBA project: set that property to a valid and meaningful name for that specific worksheet, and you have a user-proof way to refer to your ConfigurationSheet, the SummarySheet, and that DataSheet. If the user decides to rename the DataSheet to “Data (OLD)” for some reason, this code is now broken:

ThisWorkbook.Worksheets("Data").Range("A1").Value = 42

Meanwhile this code will survive any user-induced sheet-name tampering:

DataSheet.Range("A1").Value = 42

Wait, is it .Sheets() or .Worksheets()?

The first thing to note, is that they aren’t language-level keywords, but member calls. If you don’t qualify them, then in the ThisWorkbook module you’ll be referring to Me.Worksheets (i.e. ThisWorkbook.Worksheets), and anywhere else in the VBA project that same code be implicitly referring to ActiveWorkbook.Worksheets: that’s why it’s important to properly qualify member calls. Worksheets is a member of a Workbook object, so you explicitly qualify it with a Workbook object.

Now, Sheets and Worksheets both return an Excel.Sheets collection object, whose default member Item returns an Object. Both are happy to take a string with a sheet name, or an integer with a sheet index. Both will be unhappy (enough to raise runtime error 9 “subscript out of range”) with an argument that refers to a sheet that’s not in the (implicitly or not) qualifying workbook object. Both will return a Sheets collection object if you give it an array of sheet names: that’s one reason why the Item member returns an Object and not a Worksheet. Another reason is that sometimes a sheet is a Chart, not a Worksheet.

Use the Worksheets collection to retrieve Worksheet items; the Sheets collection contains all sheets in the qualifying workbook, regardless of the type, so use it e.g. to retrieve the Chart object for a chart sheet. Both are equivalent, but Worksheets is semantically more specific and should be preferred over Sheets for the common Worksheet-dereferencing scenarios.

Dereferencing Workbooks

If you only need to work with ThisWorkbook, then you don’t need to worry about any of this. But as soon as your code starts opening other workbooks and manipulating sheets that are in these other workbooks, you need to either go nuts over what workbook is currently the ActiveWorkbook as you Activate workbooks and repeatedly go Workbooks("foo.xlsm").Activate…. or properly keep a reference to the objects you’re dealing with.

When you open another workbook with Application.Workbooks.Open, that Open is a function, a member of the Excel.Workbooks class that returns a Workbook object reference if it successfully opens the file.

Workbooks.Open is also side-effecting: successfully opening a workbook makes that workbook the new ActiveWorkbook, and so global state is affected by its execution.

When you then go and work off ActiveWorkbook or unqualified Worksheets(...) member calls, you are writing code that is heavily reliant on the side effects of a function, and global state in general.

The right thing to do, is to capture the function’s return value, and store the object reference in a local variable:

Dim book As Workbook
Set book = Application.Workbooks.Open(path) '<~ global-scope side effects are irrelevant!

Dim dataSheet As Worksheet
Set dataSheet = book.Worksheets("DATA")

If a workbook was opened by your VBA code, then your VBA code has no reason to not have a Workbook reference to that object.

So when is ActiveWorkbook useful then?

As an argument to a procedure that takes a Workbook parameter because it doesn’t care what workbook it’s working with, or when you need to assign a Workbook object variable (presumably a WithEvents module-scope private variable in some class module) to whatever workbook is currently active. With few specific exceptions, that’s all.

ActiveWorkbook.Whatever is just not code that you normally want to see anywhere.

Cha-cha-cha-Chaining Calls

Strive to keep the VBA compiler aware of everything that’s going on. Moreover if all the code you write is early-bound, then Rubberduck understands it as completely as it can. But implicit late binding is alas too easy to introduce, and the primary cause for it is chained member calls:

book.Worksheets("Sheet1").Range("A1").Value = 42 '<~ late bound at .Range

Everything after the Worksheets("Sheet1") call is late-bound, because as described above, Excel.Sheets.Item returns an Object, and member calls against Object can only ever be resolved at run-time.

By introducing a Worksheet variable to collect the Object, we cast it to a usable compile-time interface (that’s Worksheet), and now the VBA compiler can resume validating the .Range member call:

Dim sheet As Worksheet
Set sheet = book.Worksheets("Sheet1")
sheet.Range("A1").Value = 42 '<~ all early bound

Chained early-bound member calls are fine: the compiler will be able to validate the Range.Value member call, because the Excel.Worksheet.Range property getter returns a Range reference. If it returned Object, we would have to declare a Range local variable to capture the Excel.Range object we want to work with, like this:

Dim sheet As Worksheet
Set sheet = book.Worksheets("Sheet1") '<~ good: casts Object into Worksheet.

Dim cell As Range
Set cell = sheet.Range("A1") '<~ redundant: sheet.Range("A1") returns an early-bound Range.
cell.Value = 42 '<~ early-bound, but sheet.Range("A1").Value = 42 would also be early-bound.

Avoid declaring extraneous variables, but never hesitate to use a local variable to turn an Object into a compile-time type that gives you IntelliSense, autocompletion, and parameter quick-info: you’ll avoid accidentally running into run-time error 438 for typos Option Explicit can’t save you from. Using the compiler to validate everything it can validate, is a very good idea.

If you need to repeatedly invoke members off an early-bound object, introducing a local variable reduces the dereferencing and helps make the code feel less repetitive:

sheet.Range("A1").Value = "Rubberduck"
sheet.Range("A1").Font.Bold = True
sheet.Range("A1").Font.Size = 72
sheet.Range("A1").Font.Name = "Showcard Gothic"

By introducing a local variable, we reduce the cognitive load and no longer repeatedly dereference the same identical Range object pointer every time:

Dim cell As Range
Set cell = sheet.Range("A1")

cell.Value = "Rubberduck"
cell.Font.Bold = True
cell.Font.Size = 72
cell.Font.Name = "Showcard Gothic"

Arguably, nested With blocks could hold all the object references involved, and reduces the dereferencing to a strict minimum (.Font is only invoked once, and the reference is witheld), but it’s very debatable whether it enhances or hurts readability:

With sheet.Range("A1")
    .Value = "Rubberduck"
    With .Font
        .Bold = True
        .Size = 72
        .Name = "Showcard Gothic"
    End With
End With

Avoiding nested With blocks seems a fair compromise:

Dim cell As Range
Set cell = sheet.Range("A1")

cell.Value = "Rubberduck"
With cell.Font
    .Bold = True
    .Size = 72
    .Name = "Showcard Gothic"
End With

All this is fairly subjective, of course, and really applies to everything you ever write in VBA (not just when coding against the Excel object model), but any of it is better than this (very cleaned-up) macro-recorder code:

    ActiveCell.Value = "Rubberduck"
    With Selection.Font
        .Name = "Showcard Gothic"
        .Bold = True
        .Size = 72
    End With

We note the implicit ActiveSheet reference with the implicitly-qualified Range member call; we note the use of Range.Select followed by a use of ActiveCell; we note a With block holding a late-bound reference to Range.Font through the Selection object, and the compiler gets to validate absolutely nothing inside that With block.

The macro recorder doesn’t declare local variables: instead, it Selects things and then works late-bound against the Selection. That’s why it’s a bad teacher: while it’s very useful to show us what members to use to accomplish something, it does everything without leveraging any compile-time checks, and teaches to Activate the sheet you want to work with so that your unqualified Range and Cells member calls can work off the correct sheet… but now you know why, how, and when to dereference a Worksheet object into a local variable, you don’t need any Select and Activate!