PRB: Assert When Creating MFC Windows in Printer Drivers

MORE INFORMATION

In an MFC program, when the Printer Properties dialog box is displayed, the following function is called:

   void CPrintDialog::OnPrintSetup()
   {
      ASSERT_VALID(this);

      CPrintDialog* pDlgSetup = AttachOnSetup();
      ASSERT(pDlgSetup != NULL);

      _AfxHookWindowCreate(pDlgSetup);
      Default();
      _AfxUnhookWindowCreate();

      delete pDlgSetup;
   }
				

Before calling Default(), MFC installs a WH_CALLWNDPROC hook function (_AfxSendMsgHook) through _AfxHookWindowCreate. Default() invokes the callback function in the printer driver. If the printer driver displays an MFC window (usually a property sheet), _AfxHookWindowCreate is called again and the subclassing needed in _AfxSendMsgHook is done improperly. In most cases, an assertion occurs in Wincore.cpp line 210:

   LRESULT CALLBACK AFX_EXPORT
   AfxWndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
   {
      CWnd* pWnd;

      pWnd = CWnd::FromHandlePermanent(hWnd);
      ASSERT(pWnd != NULL);  // LINE 210
      ASSERT(pWnd->m_hWnd == hWnd);

      LRESULT lResult = _AfxCallWndProc(pWnd, hWnd, message,
                                         wParam, lParam);

      return lResult;
   }
				

This problem occurs when you use 16-bit drivers with MFC and with MFC as the calling program. Note that the problem can still occur if the calling program is a 32-bit application running on Windows 95 or Win32s.