FIX: Drop-down Combobox Does Not Move with Property Sheet

WINDOWS    | WINDOWS NT

The information in this article applies to:

• Microsoft Visual C++ for Windows, versions 1.52, 1.52b
• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2

SYMPTOMS

When a combo box on the active page of a CPropertySheet object is opened and the property sheet is moved by dragging it with the mouse, the drop-down portion of the combo box stays at its current screen position; it does not move with the underlying property sheet's dialog box.

NOTE: This bug consistently occurs in 16-bit applications running under Windows 95 or Windows version 3.x. However, it does not occur when running under Windows NT, and it occurs intermittently in 32-bit applications running under Windows 95.

CAUSE

The _AfxCancelModes internal MFC function is not called from CPropertySheet::PreTranslateMessage() as it is for CFrameWnd. In 32-bit Visual C++, _AfxCancelModes is defined as AfxCancelModes.

RESOLUTION

To work around this problem, derive a class from CPropertySheet and call _AfxCancelModes from the derived class's PreTranslateMessage when the WM_LBUTTONDOWN or WM_NCLBUTTONDOWN messages are received. See the "Step- By-Step Workaround" section in this article for an example.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

Step-By-Step Workaround

1. Create a new class derived from CPropertySheet.

   a. Open the ClassWizard.

   b. Click the Add Class button.

   c. Choose a Class Type of generic CWnd. This is necessary because

         ClassWizard does not support creating a CPropertySheet class in
         16-bit Visual C++. If you are using 32-bit Visual C++, you should be
         able to derive a class from CPropertySheet directly.
   

   d. Enter a Class Name such as CMySheet.

   e. Click the Create Class button.

2. If you are using 32-bit Visual C++, please skip steps 2.a, 2.b, and 2.c. In the new .h file (mysheet.h), make these changes:

   a. Change this:

            class CMySheet : public CWnd
   
         To this:
   
            class CMySheet : public CPropertySheet
   
      b. Add the following as the first line in the class definition:
   
         DECLARE_DYNAMIC(CMySheet)
   
      c. Delete the default constructor declaration and add the following two
         constructors in its place:
   
         CMySheet(UINT nIDCaption, CWnd *pParentWnd = NULL,
            UINT iSelectPage = 0 ) : CPropertySheet(nIDCaption,
            pParentWnd, iSelectPage) { }
         CMySheet(LPCTSTR pszCaption, CWnd *pParentWnd = NULL,
            UINT iSelectPage = 0 ) : CPropertySheet(pszCaption,
            pParentWnd, iSelectPage) { }
   
      d. Add a prototype for an override of PreTranslateMessage in a public
         section of the class definition:
   
         public:
             virtual BOOL PreTranslateMessage(MSG* pMsg);
   
   

3. If you are using 32-bit Visual C++, please skip step 3.a. In the new .cpp file (mysheet.cpp), make these changes:

   a. Delete the default constructor function body.

   b. Add the definition of PreTranslateMessage:

         BOOL CMySheet::PreTranslateMessage(MSG* pMsg)
         {
             // check for special cancel modes for ComboBoxes
             if (pMsg->message == WM_LBUTTONDOWN ||
             pMsg->message == WM_NCLBUTTONDOWN)
           _AfxCancelModes(pMsg->hwnd);    // filter clicks
   
          return CPropertySheet::PreTranslateMessage(pMsg);
         }
   
      c. Use The following sample code to define _AfxCanelModes. Place this
         code should be placed above the CMySheet::PreTranslateMessage
         definition. The code is needed because if the project is linking to
         the DLL version of MFC, you need to copy the code for this function
         and one other function that is called and insert it into the project.
         The code can be found in msvc\mfc\src\winutil.cpp. If the project
         links to a static MFC library, all you need to do is add a prototype
         for the function. The code handles both situations by using the
         #ifdef - #else - #endif preprocessor directives.
   
   
         /* Compile options needed: Default AppWizard options
         */
   
         #ifdef _AFXDLL  // define the functions
         BOOL PASCAL _AfxIsComboBoxControl(HWND hWnd, UINT nStyle)
         {
            if (hWnd == NULL)
            return FALSE;
            // do cheap style compare first
            if ((UINT)(::GetWindowLong(hWnd, GWL_STYLE) & 0x0F) != nStyle)
            return FALSE;
   
            // do expensive classname compare next
   
            // If using 16-bit Visual C++, use the following code
               static char BASED_CODE szComboBox[] = "combobox";
               char szCompare[sizeof(szComboBox) + 1];
               ::GetClassName(hWnd, szCompare, sizeof(szCompare));
   
             // else if using 32-bit Visual C++, use the following code
             // and comment out the code above.
             //
             //  static const TCHAR szComboBox[] = _T("combobox");
             //  TCHAR szCompare[sizeof(szComboBox)/sizeof(szCompare[0])+1];
             //  ::GetClassName(hWnd, szCompare,
             //                 sizeof(szCompare)/sizeof(szCompare[0]) );
   
             return (lstrcmpi(szCompare, szComboBox) == 0);
               }
   
             void PASCAL _AfxCancelModes(HWND hWndRcvr)
             {
             // if we receive a message destined for a window, cancel any
             // combobox popups that could be in toolbars or dialog bars
             HWND hWndCancel = ::GetFocus();
             if (hWndCancel == NULL)
             return;     // nothing to cancel
   
             if (hWndCancel == hWndRcvr)
             return;     // let input go to window with focus
   
             // focus is in part of a combo-box
             if (!_AfxIsComboBoxControl(hWndCancel, (UINT)CBS_DROPDOWNLIST))
             {
             // try as a dropdown
             hWndCancel = ::GetParent(hWndCancel);// parent of edit is combo
             if (hWndCancel == hWndRcvr)
             return;     // let input go to part of combo
   
             if (!_AfxIsComboBoxControl(hWndCancel, (UINT)CBS_DROPDOWN))
                return;     // not a combo-box that is active
             }
   
             // combo-box is active, but if receiver is a popup, do nothing
             if (hWndRcvr != NULL &&
               (::GetWindowLong(hWndRcvr, GWL_STYLE) & WS_CHILD) != 0 &&
               ::GetParent(hWndRcvr) == ::GetDesktopWindow())
             return;
   
             // finally, you should cancel the mode !
             ::SendMessage(hWndCancel, CB_SHOWDROPDOWN, FALSE, 0L);
         }
   
         #else   // just prototype it
         void PASCAL _AfxCancelModes(HWND hWndRcvr);
   
         #endif