Q177101: FIX: Modal Dialogs in MFC Regular DLL Cause ASSERT in AfxWndProc

Article: Q177101
Product(s): Microsoft C Compiler
Version(s): 4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbDlg kbDLL kbMFC kbThread kbVC420bug kbVC500bug kbVC600bug kbGrpDSMFCATL kbNoUpdate
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

If two or more threads display modal dialog boxes at the same time inside an MFC
regular dynamic-link library (DLL) (USRDLL), the following ASSERT may be
generated in AfxWndProc, on Wincore.cpp, line 365 in Visual C++ 6.0 (line 368 in
Visual C++ 5.0, line 360 in Visual C++ 4.2):

  // All other messages route through message map.
  CWnd* pWnd = CWnd::FromHandlePermanent(hWnd);
  ASSERT(pWnd != NULL);

This ASSERT occurs only if one of the threads was created outside of the DLL.

The probability of this problem occurring increases with the number of threads
and modal dialog boxes.

CAUSE
=====

When AfxGetThread() is called in an MFC regular DLL from a secondary thread that
was not created inside the DLL, it returns the CWinApp object for the DLL
because a CWinThread object was not created for the thread in the context of the
DLL.

When a modal dialog box is displayed, CWnd::RunModalLoop() pumps messages by
calling AfxGetThread()->PumpMessage(). If two modal dialog boxes both call
the CWinApp object's PumpMessage at the same time, synchronization problems
cause the wrong message to get processed on the wrong thread.

RESOLUTION
==========

One possible work around is to spawn secondary threads, which in turn display
the modal dialog boxes. Each new thread created inside the MFC regular DLL will
have a new CWinThread object and a separate message pump.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

This problem was corrected in Microsoft Visual C++ .NET.

MORE INFORMATION
================

In Visual C++ versions earlier than Visual C++ 4.2, MFC regular DLLs could be
accessed only from the same thread that loaded the DLL. Support for external
secondary threads was added in Visual C++ 4.2.

The following sample code shows one possible workaround:

        UINT ShowMeTheDlg( LPVOID pParam )

     {

      // Create CWnd object from passed in HWND
      CFileSecurity FS(CWnd::FromHandle((HWND)pParam));
      FS.DoModal();
      return TRUE;
     }

     void CSubfolderPage::OnBreakit()
     {
     // Start MFC Worker thread for modal dialog box. Modal dialog box has
     // PumpMessage code so, user interface thread is not necessary.
     // Can't pass CWnds between threads, so pass HWND
     // After DoModal call above, function returns
     // and thread terminates.
      AfxBeginThread(ShowMeTheDlg, (LPVOID)this->GetSafeHwnd());
     }

REFERENCES
==========

For additional information, please see the following article(s) in the Microsoft
Knowledge Base:

  Q122676 Multiple Threads and MFC _USRDLLs

(c) Microsoft Corporation 1997, All Rights Reserved.
Contributions by Marie Ward, Microsoft Corporation


Additional query words:

======================================================================
Keywords          : kbDlg kbDLL kbMFC kbThread kbVC420bug kbVC500bug kbVC600bug kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : :4.2,5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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

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