Is VBA an OOP language, and does it support polymorphism?

OOP is sitting on 4 "pillars":

• Abstraction - Abstracting logic and concepts can easily be done by defining objects in class modules. Strictly speaking, abstraction is also achieved by using meaningful identifiers and extracting procedural code into methods (class members).

  Here's an example of a procedure written in VBA that demonstrates abstraction:

   Public Sub Test(ByVal checkin As Date, ByVal checkout As Date, ByVal custType As CustomerType)
       Dim finder As New HotelFinder
       InitializeHotels finder
       Debug.Print finder.FindCheapestHotel(checkin, checkout, custType)
   End Sub
  

  It's easy to tell what this Test procedure does at a glance, because the abstraction level is very high: the implementation details are abstracted away into more specialized objects and methods.

• Encapsulation - Classes can have private fields exposed by properties; classes can be made PublicNotCreatable, effectively exposing types to other VBA projects - and with a little bit of effort (by exporting the class module, opening it in your favorite text editor, manually editing class attributes, and re-importing the module), you can achieve actual read-only types. The fact that there are no parameterized constructors is irrelevant - just write a factory method that takes all the parameters you like and return an instance. This is COM, and COM likes factories anyway.

  Here's an example of how the HotelFinder class from the above snippet encapsulates a Collection object and only exposes it through a Property Get accessor - code outside this class simply cannot Set this reference, it's encapsulated:

   Private Type TFinder
       Hotels As Collection
   End Type
   Private this As TFinder
  
   Public Property Get Hotels() As Collection
       Set Hotels = this.Hotels
   End Property
  
   Private Sub Class_Initialize()
       Set this.Hotels = New Collection
   End Sub
  
   Private Sub Class_Terminate()
       Set this.Hotels = Nothing
   End Sub
  

• Polymorphism - Implements lets you implement abstract interfaces (and concrete classes, too), and then you can write code against an ISomething abstraction that can just as well be a Foo or a Bar (given Foo and Bar both implement ISomething) - and all the code ever needs to see is ISomething. Method overloading is a language feature that VBA lacks, but overloading has nothing to do with polymorphism, which is the ability to present the same interface for differing underlying forms (data types).

  Here's an example of applied polymorphism - the LogManager.Register method is happy to work with any object that implements the ILogger interface; here a DebugLogger and a FileLogger - two wildly different implementations of that interface, are being registered; when LogManager.Log(ErrorLevel, Err.Description) is invoked later, the two implementations will each do their own thing; DebugLogger will output to the immediate toolwindow, and FileLogger will write an entry into a specified log file:

   LogManager.Register DebugLogger.Create("MyLogger", DebugLevel)
   LogManager.Register Filelogger.Create("TestLogger", ErrorLevel, "C:\Dev\VBA\log.txt")
  

• Inheritance - VBA does not let you derive a type from another: inheritance is not supported.

Now the question is, can a language that doesn't support inheritance be qualified as "object-oriented"? It turns out composition is very often preferable to inheritance, which has a number of caveats. And VBA will let you compose objects to your heart's content.

Is VBA an OOP language?

Given all that's missing is inheritance, and that composition is preferable to inheritance, I'm tempted to answer "Yes". I've written full-blown OOP VBA code before (Model-View-Presenter with Unit-of-Work and Repository, anyone?), that I wouldn't have written any differently in a "real OOP" language that supports inheritance.

Here are a few examples, all 100% VBA:

• Model-View-ViewModel infrastructure & example (proof-of-concept)
• Full-blown OOP Battleship game with Model-View-Controller (MVC) architecture
• A reusable progress indicator
• Model-View-Presenter pattern
• UnitOfWork with Repository pattern
• Polymorphic logger
• Automagic Unit Testing framework

The code in this last link was eventually ported to C#, and quickly evolved into a COM add-in for the VBA IDE that gives you refactorings, better navigation, code inspections, and other tools.

VBA is only as limiting as you make it.

The short answers are no and no.

VBA is object based, allowing you to define classes and create instances of objects but it lacks the features that would normally be associated with a fully fledged OOP language, for example:

• Encapsulation and abstraction: VBA provides this to an extent. Classes can be kept private with public interfaces defined, however there is no provision for constructors within classes. Classes have a Class_Inititalize event which can do some construction but cannot take arguments. Passing arguments would require a public factory function workarounds are still required to create a constructor-style design pattern.
• Inheritance: Doesn't really exist in VBA but can be almost replicated
• Polymorphism: Can be achieved to an extent through interfaces (using Implements) although the ability to overload functions (for example) doesn't exist and each "overload" would technically require a unique function name. You can work around this by passing in an object as the only parameter to a function or sub and vary the procedure depending on the values of the properties.

So while you can work with objects to an extent and MS Office applications are based around an object model, VBA is not truely an Object Oriented language. Polymorphism cannot be achieved to the extent that you would be familiar with in C++.