Delphi interface inheritance: Why can't I access ancestor interface's members?

Question

Assume you have the following:

//Note the original example I posted didn\'t reproduce the problem so
//I created an clean example  
  type
    IParent = inte         


        

Answer 1

If an implementing class does not declare that it supports an inherited interface, then the class will not be assignment compatible with variables of the inherited interface. The code sample you posted should work fine (using the IChild interface), but if you try to assign from an instance of TMyClass to a variable of IParent, then you'll run into trouble.

The reason is because COM and ActiveX allow an implementation to implement a descendent interface (your IChild) but deny the ancestor of that interface (IParent). Since Delphi interfaces are intended to be COM compatible, that's where this goofy artifact comes from.

I'm pretty sure I wrote an article about this about 10 or 12 years ago, but my Borland blog did not survive the transition to the Embarcadero server.

There may be a compiler directive to change this behavior, I don't recall.