Additional methods in a class module implementing an interface in VBA

Question

I write a VBA macro for Microsoft Excel 2010.

Some of the modules in my project would really cry out for subclasses, but I already understood that inheritance is not supported.

Knowing that, I went the hard way and implemented all the methods again in each module using an interface.

However, now I experience that a method that only occures in one of the modules (but not in the interface) throws an exception:

Method or data member not found!

I am not entirely sure but I have the feeling that this is somehow related to the interface.

Therefore I'd like to know the following:

When implementing an interface in a module in VBA, is it still possible to write additional public subs for this module?
If it's not possible, what would be the correct pattern to append certain functionality to a module that implements an interface?

If you declare a variable as the interface class, you can only call methods/properties of that interface. — Rory
– Rory, Commented Jul 4, 2016 at 14:08
you mean static binding, right? However, I declared the variable as the "real class" not the interface class... Should it work then? — speendo
– speendo, Commented Jul 4, 2016 at 14:14
Yes it should. Unless you post the code, we can't tell you why it doesn't. — Rory
– Rory, Commented Jul 4, 2016 at 14:15
of course. But I guess nobody would like to read the whole code. When you tell me it should work, I will try to make a minimal example. — speendo
– speendo, Commented Jul 4, 2016 at 14:20
@Rory thank you for your comment! Knowing that it should work I found the source of the problem myself (and you were right, it was a problem with the variable declaration). As your comment the question comprehensively, I would like to accept it. Would you like to formulate it into an answer? — speendo
– speendo, Commented Jul 5, 2016 at 7:09

Rory · Accepted Answer · 2016-07-05 07:32:58Z

3

If you declare a variable as the interface class, you can only call methods/properties of that interface, regardless of any other methods/properties exposed by the actual object type itself.

answered Jul 5, 2016 at 7:32

Rory

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