FIX: Cannot Receive WM_HELP for a Subclassed Control

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

SYMPTOMS

WM_HELP is not received by the parent window of a control that has been subclassed by MFC.

CAUSE

CWnd, the base class of all MFC windows, has a handler for WM_HELP. This handler intercepts the WM_HELP message in the control itself and thus the message will not be sent to the parent window as expected.

RESOLUTION

Subclass all the controls in the CDialog, CFormView, CPropertyPage, or other CWnd derived class that contains the controls which have been subclassed by MFC. The "Sample Code" below iterates through the child windows of a given parent window and checks their WindowProc. If their WindowProc is AfxWndProc (MFC's WindowProc), it subclasses the controls again so that another window procedure is called first. In this WindowProc, we check for WM_HELP and call DefWindowProc instead of passing the message to MFC's AfxWndProc.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.

MORE INFORMATION

CWnd::OnHelpInfo (handler for WM_HELP) sends a WM_COMMAND with ID_HELP to the application's main frame window. This message can be intercepted in the main frame, but at this point, the HELPINFO structure has been lost. The main frame will not have any way of determining which help context should be used.

REFERENCES

- Win32 SDK documentation for WM_HELP

• MFC source code for CWnd::OnHelpInfo().

    FixHelp(this); 

Sample Code

/* Compile options needed:  Default
*/ 

//////////////////////////////////////////////////////////////////////// 
// FixHelp.h - Include this file in your dialog's or form view's .CPP

void FixHelp(CWnd* pWnd); // Call this from your OnInitDialog() for a
                          // dialog or OnInitialUpdate() for a CFormView

//////////////////////////////////////////////////////////////////////// 
// FixHelp.cpp - Insert this file into your project

#include "stdafx.h"

LRESULT CALLBACK ControlFixProc( HWND  hwnd, UINT  uMsg, WPARAM wParam,
                                                        LPARAM  lParam);

void FixHelp(CWnd* pWnd)
{
    // search all child windows.  If their window proc
    // is AfxWndProc, then subclass with our window proc
    CWnd* pWndChild = pWnd->GetWindow(GW_CHILD);
    while(pWndChild != NULL)
    {
        if (GetWindowLong(pWndChild->GetSafeHwnd(),
                                    GWL_WNDPROC) == (LONG)AfxWndProc)
        {
            SetWindowLong(pWndChild->GetSafeHwnd(), GWL_WNDPROC,
                                                (LONG)ControlFixProc);
        }
        pWndChild = pWndChild->GetWindow(GW_HWNDNEXT);
    }
}

LRESULT CALLBACK ControlFixProc(HWND  hwnd, UINT  uMsg, WPARAM wParam,
                                                        LPARAM  lParam)
{
    if (uMsg == WM_HELP)
    {
        // bypass MFC's handler, message will be sent to parent
        return DefWindowProc(hwnd, uMsg, wParam, lParam);
    }
    return AfxWndProc(hwnd,uMsg,wParam,lParam);
}

////////////////////////////////////////////////////////////////////////  

Additional query words: kbVC400bug 4.00 4.10 SubclassWindow SubclassDlgItem

Keywords : kbMFC KbUIDesign kbVC kbVC410fix
Version : 4.00
Platform : NT WINDOWS
Issue type :