Question

Why I cannot assign a class to a variable of its base interface type.

Consider we have the following:

IInt1 = interface
  procedure Test1;
end;

IInt2 = interface(IInt1)
  procedure Test2;
end;

TCustomClass = class(TInterfacedObject)
end;

TMyClass = class(TCustomClass, IInt2)
  procedure Test1;
  procedure Test2;
end;

var
  Obj: IInt1;
begin
  Obj := TMyClass.Create; // <!-- E2010 Incompatible types: 'IInt1' and 'TMyClass'
  ...
end;


E2010 Incompatible types: 'IInt1' and 'TMyClass'

Is this a bug in the compiler or simply not an allowed action. Can this be worked around?

Answer 1

Include IInt1 in the list of implemented interfaces in your declaration:

TMyClass = class(TCustomClass, IInt1, IInt2)
end;