PRB: Focus/Activation Problems with MFC Control on VB Forms

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0
• Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SYMPTOMS

You have an ActiveX control created in Visual C++ using the Microsoft Foundation Classes (MFC). When two of these controls reside on different Visual Basic MDI child forms, and you attempt to switch from one form to another by clicking on the controls, focus shifts to the control, but the expected Activate/Deactivate form events do not occur. Code in the events does not run, and the Active Title Bar does not switch as expected.

CAUSE

Windows MDI child activation is based on the MDI client window getting the WM_PARENTNOTIFY message. A control created in MFC has a window style of WS_EX_NOPARENTNOTIFY by default. This suppresses the parent notification message, causing the problem described above.

RESOLUTION

To resolve this problem, change the window style of the control. There are two ways to accomplish this. Resolution #1 requires alteration of the control's source code. It is the recommended solution, but does require source code access. Resolution #2 presents a way to resolve this situation in the Visual Basic client application. Resolution #2 has one requirement: The control must expose an hWnd property.

NOTE: The hWnd property is a stock property of an MFC control, but by default it is private to the control.

If you do not have access to the control's source code, and the hWnd property has not been exposed, then neither of these solutions works.

Resolution #1

Sample Code

   BOOL CPushCtrl::PreCreateWindow(CREATESTRUCT& cs)
   {
     cs.lpszClass = _T("BUTTON");
     cs.style |= BS_PUSHBUTTON | BS_OWNERDRAW;

      // The next statement allows the Visual Basic Child forms to get
      // focus correctly.
     if (cs.dwExStyle & WS_EX_NOPARENTNOTIFY)
        cs.dwExStyle -= WS_EX_NOPARENTNOTIFY;

     return COleControl::PreCreateWindow(cs);
   } 

Resolution #2

1. Add the following code to a BAS module, and add the module to any project that uses the ActiveX control:

   
         Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
          (ByVal hwnd As Long, ByVal nIndex As Long, _
            ByVal dwNewLong As Long) As Long
         Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
          (ByVal hwnd As Long, ByVal nIndex As Long) As Long
   
         Public Sub fixWindowStyle(handle As Long)
            Dim ret As Long
            Dim b As Boolean
            ret = GetWindowLong(handle, -20)
            ret = ret - 4
            b = SetWindowLong(handle, -20, ret)
         End Sub
    




2. Add the following code to the Load Event for any form hosting the ActiveX control. (The control name is Push1 in this example):

   
         fixWindowStyle (Push1.hwnd)
    

STATUS

This behavior is by design.

Keywords : kbcode kbActiveX kbMFC kbVBp500 kbVBp600 kbVC500 kbVC600 kbGrpVB
Version : WINDOWS:5.0,6.0; WINNT:5.0,6.0
Platform : WINDOWS winnt
Issue type : kbprb