FIX: Cannot Receive WM_HELP for a Subclassed Control (145865)
The information in this article applies to:
- The Microsoft Foundation Classes (MFC), when used with:
- Microsoft Visual C++, 32-bit Editions 4.0
This article was previously published under Q145865 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);
}
////////////////////////////////////////////////////////////////////////
| Modification Type: | Major | Last Reviewed: | 10/24/2003 |
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| Keywords: | kbArchitecture kbbug kbfix kbNoUpdate KbUIDesign kbVC410fix KB145865 |
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