The mysterious case of the unexpected implicit interface variable

Question

I recently came across some behaviour that I simply could not and cannot explain, related to Delphi interface variables.

Essentially, it boils down to an implicit in

Answer 1

Uwe Raabe is correct, if you look at the code further up:

Project4.dpr.51: Listener := FListeners[i] as IListener;
00441C16 8D4DE0           lea ecx,[ebp-$20]
00441C19 8B55F4           mov edx,[ebp-$0c]
00441C1C 8B45FC           mov eax,[ebp-$04]
00441C1F 8B4004           mov eax,[eax+$04]
00441C22 8B18             mov ebx,[eax]
00441C24 FF530C           call dword ptr [ebx+$0c]
00441C27 8B55E0           mov edx,[ebp-$20]
00441C2A 8D45F0           lea eax,[ebp-$10]
00441C2D B9A81C4400       mov ecx,$00441ca8
00441C32 E8A573FCFF       call @IntfCast

You can see how the result of the FListeners[i] call is placed in [ebp-$20] and then procedure _IntfCast(var Dest: IInterface; const Source: IInterface; const IID: TGUID); is called on that (eax being the target, [ebp-$10], edx the source, [ebp-$20], and ecx the address where the appropriate guid can be found.

You can fix your code by changing the Broadcast method to:

procedure TBroadcaster.Broadcast(Msg: Integer);
var
  i: Integer;
  Intf: IInterface;
  Listener: IListener;
begin
  for i := 0 to FListeners.Count-1 do begin
    Intf := FListeners[i];
    if Supports(Intf, IListener, Listener) then
      Listener.HandleMessage(Msg);
  end;
  Listener := nil;
  Intf := nil;

  FListeners.Clear;
  FreeAndNil(FListener);
end;//method epilogue: why is there a call to IntfClear and then TComponent._Release?

Answer 2

Just a guess, but perhaps the FListeners[i] as IListener uses a temporary variable for FListeners[i]. After all it is the result of a function call.