Interfaces polymorphism in Delphi

Question

I have two interfaces one deriving from antoher:

type
  ISomeInterface = interface
    ['{5A46CC3C-353A-495A-BA89-48646C4E5A75}']
  end;

  ISomeInterfaceChild = interface(ISomeInterface)
    ['{F64B7E32-B182-4C70-A5B5-72BAA92AAADE}']
  end;

Now I have a procedure which parameter is ISomeInterface like:

procedure DoSomething(SomeInterface: ISomeInterface);

I want to check if SomeInterface is ISomeInterfaceChild. Is operator is not supported in interfaces in Delphi 7 and I can't use Supports here neither. What can I do?

Answer 1

You can indeed use Supports. All you need is:

Supports(SomeInterface, ISomeInterfaceChild)

This program illustrates:

program SupportsDemo;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
  ISomeInterface = interface
    ['{5A46CC3C-353A-495A-BA89-48646C4E5A75}']
  end;

  ISomeInterfaceChild = interface(ISomeInterface)
    ['{F64B7E32-B182-4C70-A5B5-72BAA92AAADE}']
  end;

procedure Test(Intf: ISomeInterface);
begin
  Writeln(BoolToStr(Supports(Intf, ISomeInterfaceChild), True));
end;

type
  TSomeInterfaceImpl = class(TInterfacedObject, ISomeInterface);
  TSomeInterfaceChildImpl = class(TInterfacedObject, ISomeInterface, ISomeInterfaceChild);

begin
  Test(TSomeInterfaceImpl.Create);
  Test(TSomeInterfaceChildImpl.Create);
  Readln;
end.

Output

False
True

Answer 2

Why do you say you can't use the Supports function? It seems to be the solution, it has an overloaded version which takes IInterface as the first parameter so

procedure DoSomething(SomeInterface: ISomeInterface);
var tmp: ISomeInterfaceChild;
begin
  if(Supports(SomeInterface, ISomeInterfaceChild, tmp))then begin
     // argument is ISomeInterfaceChild
  end;

should do what you want.