PRB: Assert When Creating MFC Windows in Printer Drivers

ID: Q189352

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++ for Windows, 16-bit edition, version 1.52

SYMPTOMS

When you display the Printer Properties dialog box for a printer based on a 16-bit MFC printer driver from an MFC program, you might experience various problems, such as an assertion in Wincore.cpp line 210.

CAUSE

MFC attempts to subclass the Printer Properties dialog box twice. This occurs only when the printer driver is using an MFC window such as a property sheet for the Printer Properties dialog box and when the dialog is invoked from an MFC program.

RESOLUTION

Do not display MFC windows from a printer driver.

STATUS

Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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.

Additional query words: kbMFC kbVC152

Keywords :
Version : winnt:
Platform : winnt
Issue type : kbprb