Microsoft KB Archive/142415

= PRB: MFC Message Handler Not Called with PostThreadMessage =

Article ID: 142415

Article Last Modified on 12/9/2003

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 2.2
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 4.1 Subscription

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This article was previously published under Q142415



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 */

BOOL CThreadMsgApp::PreTranslateMessage(MSG* pMsg) {   // 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); }

BOOL CThreadMsgApp::InitInstance {   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"); }

