DOCUMENT:Q189352  11-SEP-2001  [visualc]
TITLE   :PRB: Assert When Creating MFC Windows in Printer Drivers
PRODUCT :Microsoft C Compiler
PROD/VER:winnt:
OPER/SYS:
KEYWORDS:kbprint kbCmnDlgPage kbCmnDlgPrint kbMFC kbPrinting kbPropSheet kbVC152 kbGrpDSMFCATL k

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - The Microsoft Foundation Classes (MFC), used 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.
(c) Microsoft Corporation 1998, All Rights Reserved.
Contributions by Adam Kim, Microsoft Corporation


Additional query words:

======================================================================
Keywords          : kbprint kbCmnDlgPage kbCmnDlgPrint kbMFC kbPrinting kbPropSheet kbVC152 kbGrpDSMFCATL kbArchitecture kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbprb

=============================================================================

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Copyright Microsoft Corporation 2001.