PRB: Assert in Wincore.cpp with MFC in an NT Service

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2

SYMPTOMS

When you use a Visual C++ 4.x MFC application spawned from a NT service or as a NT service, an assertion may occur in Wincore.cpp. Specifically, it occurs on the following line in _AfxActivationWndProc():

  LRESULT CALLBACK
 _AfxActivationWndProc(HWND hWnd, UINT nMsg, WPARAM wParam, LPARAM lParam)
 {
    WNDPROC oldWndProc = (WNDPROC)::GetProp(hWnd, szAfxOldWndProc);
    ASSERT(oldWndProc != NULL);  // <----- assert occurs here
    .
    .
    .
 }

CAUSE

MFC subclasses all non-MFC created windows to handle specific activation issues. While subclassing a non-MFC created window, the old window procedure is stored in the properties of the window. Logging off of an NT session causes the atoms used to identify the properties to be destroyed and the property cannot be retrieved. This causes the assertion to occur.

MFC was not designed for Windows NT services. As a result, if an MFC application is spawned from a Windows NT service, minimized, and then a user logs-off, then the assertion will occur.

Also keep in mind that there are other problems to consider when spawning an MFC application from a service or as a service. The OnEndSession() message handler for the main frame window closes out the CDocument object. So the WM_ENDSESSION message must be handled to prevent this from happening.

RESOLUTION

You can do one of the following things:

• Unsubclass all of the non-MFC windows at some time before logoff or during logoff such as in the WM_ENDSESSION handler.

• Modify the MFC code and rebuild the MFC libraries. In this case, you can modify the MFC code to prevent it from subclassing any non-MFC windows or modify the code so that you can store the old window procedure in a list rather than in properties for the window. As each window is destroyed (WM_NCDESTROY is received), look up the window handle in your list and unsubclass it like MFC does in the _AfxActivationWndProc() function.

• Separate the GUI part of the application from the service part. In other words, have a GUI client startup every time the user logs on. Then, have the GUI client talk to the service through some form of interprocess communication, such as named pipes or sockets. The service should maintain the data and the GUI starts up each time when you log on.

Unsubclassing the non-MFC Windows

The first technique listed above may be an easy workaround for developers who have already written an application and don't want to rework their design, rebuild the MFC libraries, or don't need to handle the activation issues MFC handles.

You can unsubclass non-MFC Windows in the WM_ENDSESSION handler of your main frame window. The following sample code demonstrates how to enumerate all of the windows for your process and unsubclass them:

 static const TCHAR szAfxOldWndProc[] = _T("AfxOldWndProc");

 BOOL CALLBACK EnumProc( HWND hWnd, LPARAM lParam)
 {
    //check for property and unsubclass if necessary
    WNDPROC oldWndProc = (WNDPROC)::GetProp(hWnd, szAfxOldWndProc);
    if (oldWndProc!=NULL)
    {
       SetWindowLong(hWnd, GWL_WNDPROC, (DWORD)oldWndProc);
       RemoveProp(hWnd, szAfxOldWndProc);
    }

    return TRUE;
 }

 void CMainFrame::OnEndSession(BOOL bEnding)
 {
    // unsubclass the non-MFC windows which MFC has subclassed
    DWORD dwProcessId;

    DWORD dwThreadId= GetWindowThreadProcessId(m_hWnd,&dwProcessId);
    EnumThreadWindows(dwThreadId, EnumProc,(LPARAM) dwThreadId);
 }