Q145865: FIX: Cannot Receive WM_HELP for a Subclassed Control

Article: Q145865
Product(s): Microsoft C Compiler
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbMFC KbUIDesign kbVC400 kbVC410fix kbDSupport kbGrpDSMFCATL kbArchitecture kbNoUpdate
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used 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().

The following code demonstrates how to work around this problem. In your class'
OnInitDialog() or OnInitialUpdate(), call the FixHelp() function with the this
pointer:

      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 kbSweptVC600

======================================================================
Keywords          : kbMFC KbUIDesign kbVC400 kbVC410fix kbDSupport kbGrpDSMFCATL kbArchitecture kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0
Issue type        : kbbug
Solution Type     : kbfix

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