FIX: Timer Callbacks Not Called in MFC 4.2 Applications

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

SYMPTOMS

An MFC 4.2 application that uses ::SetTimer to set up a timer that uses a timer callback function (for example, the hWnd argument in the call to SetTimer is NULL) never gets the callback function invoked.

If a menu, message box, or modal dialog box is active, then the timer callback function is called.

CAUSE

MFC 4.2 added the capability to add message map entries for messages that are posted to the queue but are not destined for any particular window (for example, messages posted with PostThreadMessage).

This can be done using the ON_THREAD_MESSAGE macro. To enable this capability, a change was made in CWinThread::PreTranslateMessage that prevents any message that does not have an HWND from being routed through the message pump. Consequently, messages that do not have an HWND are not passed to the Win32 DispatchMessage API call.

Timer callback functions are invoked through a posted WM_TIMER message. The WM_TIMER message has NULL as the hwnd member and the lParam points to the callback function. The callback function is invoked by Windows when a WM_TIMER message with a NULL hwnd is passed to DispatchMessage.

Because MFC 4.2 does not pass these messages to DispatchMessage, the timer callback function will not be called.

RESOLUTION

Override CWinThread::PreTranslateMessage in the thread that owns the timer.

Remember that CWinApp is derived from CWinThread, and if your timer is established in the primary thread then your application will need to override PreTranslateMessage in its CWinApp-derived class.

Implement this function as follows:

   BOOL CMyApp::PreTranslateMessage( MSG *pMsg )
   {
       // If this is a timer callback message let it pass on through to the
       // DispatchMessage call.
       if( (pMsg->message==WM_TIMER) && (pMsg->hwnd==NULL) )
           return FALSE;
       ...
       // The rest of your PreTranslateMessage goes here.
       ...

       return CWinApp::PreTranslateMessage(pMsg);
   } 

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been corrected in the Visual C++ 4.2b technology update.

MORE INFORMATION

Your callback might be called when executing a menu, dialog box, or a modal dialog box because these operations all use a different message loop. This message loop is provided by Windows, and it does not call PreTranslateMessage. Instead, it will just pass the messages on to DispatchMessage, and your timer callback will be invoked as expected.

Additional query words: kbVC400bug

Keywords : kbnokeyword kbMFC kbVC kbVC420fix
Version : 4.0 4.1 4.2
Platform : NT WINDOWS
Issue type : kbbug