Generic Tlist<IMyInterface> in delphi

Question

I have a TList of an interface

IImage = interface
// another methods...
  procedure Draw;
end;

that draw different stuff in a Canvas.

and a parent class

TCustomImage = class(TInterfacedObject, IImage)
   procedure Draw; virtual; abstract;
end;

and a couple of child classes

TSomeShape = class(TCustomImage, IImage)
   procedure Draw(aCanvas:TCanvas); reintroduce; overload;
end;

TSomeAnotherShape = class(TCustomImage, IImage)
   procedure Draw(aCanvas:TCanvas); reintroduce; overload;
end;

I would like to code as below:

var
  list : TList<IImage>;
  shape : IImage;
begin
  try
    list := TList<IImage>.Create;

    list.Add(TSomeShape.Create(atSomePosition));
    list.Add(TSomeAnotherShape.Create(atSomePosition));

    // or better :)
    for shape in list do
      shape.Draw(Canvas); // TSomeShape and TSomeAnotherShape respectively.

  finally
    FreeAndNil(list);
  end;  
end;

UPD
I want to use the list.Items[I].draw() with the correct classes (TSomeShape or TSomeAnotherShape). But now, I see that it is impossible with this IImage interface. I had a method Draw(Canvas:TCanvas) in the IImage but thought that it is acceptably to reintroduce method in classes.

Thanks, I`ll work on my code. :)

Answer 1

The problem is that your interface is defined incorrectly. Your concrete classes implement methods with this signature:

procedure Draw(Canvas: TCanvas);

But your interface defines a method with this signature:

procedure Draw;

You do need your interface and the concrete implementations to match. You'll want code like this:

type
  IImage = interface
    procedure Draw(Canvas: TCanvas);
  end;

  TCustomImage = class(TInterfacedObject, IImage)
    procedure Draw(Canvas: TCanvas); virtual; abstract;
  end;

  TSomeShape = class(TCustomImage)
    procedure Draw(Canvas: TCanvas); override;
  end;

  TSomeOtherShape = class(TCustomImage)
    procedure Draw(Canvas: TCanvas); override;
  end;

Now everything matches and your code should compile fine, and maybe even do what you want it to.

---

As an aside, your try is in the wrong place. You must place it after you instantiate the object. Write

obj := TSomeClass.Create;
try
  // use obj
finally
  obj.Free;
end;

As you have it in your code, an exception in the constructor will lead to the finally block running and attempting to call free on an uninitialized reference.