BUG: Modal Dialogs in MFC Regular DLL Cause ASSERT in AfxWndProc

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft Visual C++, 32-bit Editions, versions 4.2, 5.0
  

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 368 (line 360 in VC++ 4.2):

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

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. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

In Visual C++ versions earlier than VC++ 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:

   ARTICLE-ID: Q122676
   TITLE     : Multiple Threads and MFC _USRDLLs

Keywords          : MfcDLL MFCThreadIss vcbuglist420 vcbuglist500
Version           : WINNT:4.2,5.0
Platform          : winnt
Issue type        : kbbug