function WndProc(hWindow: HWND; dwMsg: ZDWORD; wParam: WPARAM; lParam: LPARAM): LRESULT; stdcall;
  var
    wnd: PWindowBase;
    msg : TMSG;
  begin
    if dwMsg = WM_NCCREATE then
    begin
      wnd := PWindowBase(PCreateStruct(nlParam)^.lpCreateParams);
      SetWindowLongPtr(hWindow, GWL_USERDATA, Integer(wnd));
    end;
    wnd:= PWindowBase(GetWindowLongPtr(hWindow, GWL_USERDATA));

    if Assigned(wnd) then
    begin
      ZeroMemory(@msg, SizeOf(msg));
      msg.hwnd := hWindow;
      msg.message := dwMsg;
      msg.wParam := wParam;
      msg.lParam := lParam;
      wnd^.EventHandler(msg); // in WindowBase is diese funktion virtuell (hier crash es
      Exit;
    end;
    Result := DefWindowProc(hWindow, dwMsg, nwParam, nlParam);
  end;

  WindowBase = class
     ..
  public
     function EventHandler(msg: TMsg): Boolean; virtual;
  end;