BUG: Modal Dialogs in MFC Regular DLL Cause ASSERT in AfxWndProcLast reviewed: March 13, 1998Article ID: Q177101 |
The information in this article applies to:
SYMPTOMSIf 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);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.
CAUSEWhen 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.
RESOLUTIONOne 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.
STATUSMicrosoft 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 INFORMATIONIn 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()); } REFERENCESFor additional information, please see the following article(s) in the Microsoft Knowledge Base:
ARTICLE-ID: Q122676 TITLE : Multiple Threads and MFC _USRDLLs(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Kelly Marie Ward, Microsoft Corporation Keywords : MfcDLL MFCThreadIss vcbuglist420 vcbuglist500 Version : WINNT:4.2,5.0 Platform : winnt Issue type : kbbug |
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