🚀 go-pugleaf

I have a series of frames, each implementing a common interface

type
  ImyInterf = interface
    procedure doit;
  end;

  tframe1 = class (tframe,ImyInterf)
   ....
   procedure doit;
  end;

  tframe2 = class (tframe,ImyInterf)
   ....
   procedure doit;
  end;

Now I'd like to iterate on the form components to activate every frame
:

procedure  TMainForm.ActivateFrame;
var   I : integer;
      F : ImyInterf;
begin
  for I := 0 to ComponentCount -1 do begin
    if Components[i] is tFrame then begin
      F := Components[i] as ImyInterf;   // ---- Error
      F := ImyInterf(Components[i]);     // ---- Error
      F.doit;  // --- this is what i'd like to call
    end;
  end;
end;

However The compiler (d7) complains in the above statements:

The first gives the error:
  operator not applicable to this operand type

the second the error
  incompatible types ImyInterf and tcomponent

What can I do ?
Regards
Maurizio