Casting a delphi interface to its implementation class without modifying the interface [duplicate]

Answer 1

• The classic take on this is by Hallvard Vassbotn: Hack #7: Interface to Object
• More recently Barry Kelly, a Delphi compiler engineer, also provided an implementation: An ugly alternative to interface to object casting

I think both approaches should work.

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Incidentally, does anyone know if Hallvard is still active? I've not come across him in the past few years.

Answer 2

i do what the "possible duplicate" question's accepted answer does:

Have the object implement the IObject interface:

IObject = interface(IUnknown)
    ['{39B4F98D-5CAC-42C5-AF8D-0237C8EFBE4C}']
    function GetSelf: TObject;
end;

So it would be:

var
   thingy: IThingy;
   o: TOriginalThingy;

begin
   o := (thingy as IObject).GetSelf as TOriginalThingy;

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Update: To drive the point home, you can add a new interface to an existing object.

Existing object:

type
    TOriginalThingy = class(TInterfacedObject, IThingy)
    public
       //IThingy
       procedure DrinkCokeZero; safecall;
       procedure ExcreteCokeZero; cafecall;
    end;

Add IObject as one of the interfaces it exposes:

type
    TOriginalThingy = class(TInterfacedObject, IThingy, IObject)
    public
       //IThingy
       procedure DrinkCokeZero; safecall;
       procedure ExcreteCokeZero; cafecall;

       //IObject - provides a sneaky way to get the object implementing the interface
       function GetSelf: TObject;
    end;

    function TOriginalThingy.GetSelf: TObject;
    begin
       Result := Self;
    end;

Typical usage:

    procedure DiddleMyThingy(Thingy: IThingy);
    var
       o: TThingy;
    begin
       o := (Thingy as IObject).GetSelf as TThingy;

       o.Diddle;
    end;

Answer 3

H/t to my boss, the answer is: use the incredibly useful JEDI library, specifically the GetImplementorOfInterface method.