KB173026 - PRB: Message-Handlers for ActiveX Control Aren't Called

PSS ID Number: 173026

Article Last Modified on 12/1/2003

The information in this article applies to:

The Microsoft Foundation Classes (MFC), when used with:
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 4.0a
- Microsoft Visual C++, 32-bit Editions 4.1
- Microsoft Visual C++, 32-bit Editions 4.2b
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2
- Microsoft Visual C++, 32-bit Professional Edition 4.2
- Microsoft Visual C++, 32-bit Enterprise Edition 5.0
- Microsoft Visual C++, 32-bit Professional Edition 5.0
- Microsoft Visual C++, 32-bit Enterprise Edition 6.0
- Microsoft Visual C++, 32-bit Professional Edition 6.0
- Microsoft Visual C++, 32-bit Learning Edition 6.0

This article was previously published under Q173026

SYMPTOMS

When you create an ActiveX control dynamically via CWnd::CreateControl(), your window's message-handlers in the CWnd-derived class will not be called. For example, if you create a handler for WM_KILLFOCUS [OnKillFocus()] in the CWnd-derived class of a Microsoft Masked Edit control, it will not be called.

CAUSE

CWnd::CreateControl() doesn't subclass the HWND associated with the control.

RESOLUTION

For message-handlers to be called, the CWnd-derived class needs to subclass the HWND. You can do this by calling SubclassDlgItem(). Calling SubclassDlgItem() right after CreateControl() will cause an assert because CreateControl() previously called CWnd::Attach(). SubclassDlgItem() will call Attach() again. To avoid this assert, we need to call Detach() first before calling SubclassDlgItem() as follows:

   // m_MaskEdit is the Microsoft Masked Edit control and 100 is
   // its resource ID.
   m_MaskEdit.Create("", WS_VISIBLE | WS_CHILD | WS_TABSTOP,
      CRect(0,0,100,25), this, 100);
   m_MaskEdit.Detach();
   m_MaskEdit.SubclassDlgItem(100, this);

STATUS

This behavior is by design.

MORE INFORMATION

To set up a message handler for WM_KILLFOCUS in the CWnd-derived class of Microsoft Masked Edit control, you'll need to add the following lines to the header file:

   afx_msg void OnKillFocus(CWnd* pNewWnd);
   DECLARE_MESSAGE_MAP()

The following lines go in the .CPP file:

   BEGIN_MESSAGE_MAP(CMSMask, CWnd)
      ON_WM_KILLFOCUS()
   END_MESSAGE_MAP()

   void CMSMask::OnKillFocus(CWnd* pNewWnd)
   {
      TRACE ("CMSMask::OnKillFocus\n");
   }

When you insert an ActiveX control via Component Gallery, the Visual C++ wrapper class has Create() functions that end up calling CWnd::CreateControl().

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