Question

I have a DLL which exectues some code at its entry point, i.e.

procedure MainDLL(Reason: Integer);
begin
  { ... Code here ... }
end;

begin
  DLLProc := @MainDLL;
end.

Now, I would like to pass some values to the DLL’s entry point from an external application. I have tried creating a hidden window inside the DLL, like that:

const
  WM_JAJCO = WM_USER + 1024;

type
  TWnd = class(TObject)
    class procedure DLLWndProc(var Msg: TMessage);
  end;

{ ... }

class procedure TWnd.DLLWndProc(var Msg: TMessage);
var
  Tmp: DWORD;
begin
  if (Msg.Msg = WM_JAJCO) then
  begin
    PNewHandle := Msg.LParam;
    CreateThread(nil, 0, @Starter, nil, 0, Tmp);

    Msg.Result := 0;
  end else
    Msg.Result := DefWindowProc(MyHnd, Msg.Msg, Msg.WParam, Msg.LParam);
end;

// in the entry point
MyHnd := AllocateHWND(TWnd.DLLWndProc);

Then, after I initialize the DLL in the caller application, I use:

SendMessage(FindWindow('TPUtilWindow', nil), WM_USER + 1024, 0, wi.WndHandle);
Application.ProcessMessages();

But the window created inside the DLL does not seem to receive the message. Do you happen to know why?

If that’s a bad method and you have a different solution, please let me know.

Answer 1

First make sure that the injected DLL really does create your window handle. WinSight or Spy++ should help you there. Once you know the window really does exist make sure FindWindow find your window handle and not another one with the same class name. IIRC, even the Delphi IDE itself creates window handles using this class name.