Question

I have an Excel add-in with a class module. I want to instantiate the class module in C# and call a method on it. How do I do that?

Answer 1

If you really need access to an instance of the class, you could do the following:

• Generate a type library for a COM interface that you want to expose from your VBA class (e.g. IMyComInterface)

• Add a reference to this type library in your VBA project

• Implement the interface in your VBA class module – e.g. MyVbaClass (use the Implements keyword):

  Option Explicit
  Implements IMyComInterface
  
  Private Sub IMyComInterface_SomeMethod(...)
      ...
  End Sub
  ...
  

• Reference the same type library in your C# project

• Create a ComVisible C# class with a method that accepts a reference to the VBA interface instance. Something like:

  public class MyVbaLoader
  {
      public IMyComInterface MyComInterface
      {
          get { return myComInterface; }
          set { myComInterface = value; }
      }
  }
  

• Write a “factory” method in a VBA standard module, that takes an object as a ByRef parameter. This object should assume the object passed as an argument has a property “MyComInterface” and should set this property to a new instance of the VBA class MyClass.

  Public Sub MyFactoryMethod(MyVbaLoader As Object) 
      Dim objClass As MyVbaClass
      Set objClass = New MyVbaClass
      ... any initialization of objClass here ...
  
      ' Pass a reference to the VBA class to the C# object MyVbaLoader
      MyVbaLoader.MyComInterface = objClass
  End Sub
  

• Call the factory method from your C# code. Assuming you have opened the workbook and have a refence “workbook” in your VBA code, the code will look something like:

  MyVbaLoader loader = new MyVbaLoader();
  workbook.Application.Run("MyModule.MyFactoryMethod", loader, Type.Missing, ... , Type.Missing);
  // we now have a reference to the VBA class module in loader.MyComInterface
  // ...
  

As you can see, it’s rather complex. Without more detail of the problem you’re trying to solve it’s difficult to say whether this complexity is justified, or whether there’s a simpler solution.

If the above isn’t clear, let me know and I’ll try to clarify.

Basically you can’t return a value from a VBA macro called from your C# code using Application.Run, so you have to resort to passing an object by value that has a method or property that can be called from VBA to set the instance.