PRB: MFC Message Handler Not Called with PostThreadMessage()

The information in this article applies to:

  - Microsoft Visual C++ 32-bit, versions 1.0, 2.0, 2.1, 2.2, 4.0, 4.1

SYMPTOMS

When code uses the PostThreadMessage() Win32 function, the MFC message handlers are not called.

CAUSE

When you call PostThreadMessage(), the message is placed in the thread's message queue. However, because messages posted this way are not associated with a window, MFC will not dispatch them to message or command handlers. In order to handle these messages, override the PreTranslateMessage() function of your CWinApp-derived class, and handle the messages manually.

RESOLUTION

The following code demonstrates how to call PostThreadMessage() in the InitInstance() of the CWinApp-derived class using a single-threaded application. The principle is the same for secondary threads except that the code shown here would be put in your alternate CWinThread-derived class.

Visual C++ 4.2 does support handling messages posted to threads. For more information, see ON_THREAD_MESSAGE in the Visual C++ 4.2 documentation.

NOTE: MFC worker threads do not have a message loop/pump associated with them so you must use a user-interface thread.

Sample Code

/* Compile options needed:

   standard MFC project generated by AppWizard */

    // Is it the Message you want?
    // You can use a switch statement but because this is
    // only looking for one message, you can use the if/else
    if (pMsg->message == WM_USER+2268)
    {
        // Call the function to handle this message
   OnReceivedCommand(pMsg->wParam,pMsg->lParam);
        // Tell MFC no more processing is needed
        return TRUE;
    }
    else
        // Call MFC to have it continue processing the message
        return CWinThread::PreTranslateMessage(pMsg);
}

    WPARAM wParam;
    LPARAM lParam;
    wParam = MAKEWPARAM(0,0); // We can put whatever we
    lParam = MAKELPARAM(0,0); // want in wParam & lParam

    // Send the user-defined Thread Message
    // m_nThreadID is a member of CWinThread that holds the thread ID
    PostThreadMessage(m_nThreadID, WM_USER+2268, wParam, lParam);

    return TRUE;

void CThreadMsgApp::OnReceivedCommand(WPARAM wParam, LPARAM lParam)

    // You can do whatever you want in here, this is simply
    // sending output to the debug window
    TRACE0("Received WM_USER+2268!!\n");

REFERENCES

For more information on the CWinThread object, please see the following: MFC Encyclopedia, Multithreading: Creating User-Interface Threads.