BUG: DDX Problem with Combo Box in Win32s (139593)

SYMPTOMS

The DDX function DDX_CBString does not work correctly under Win32s for combo boxes that use the drop list style. If you try to use DDX to retrieve the string associated with the static control of a drop list style combo box under Win32s, an empty string is returned.

CAUSE

This problem is the result of an improperly sign extended return value from the Windows SDK GetWindowTextLength function. The MFC function DDX_CBString uses GetWindowTextLength to try to determine how many characters to copy from the edit control of a combo box with a drop down style or the static control of a combo box with a drop list style. The GetWindowTextLength function fails to retrieve a valid text length for a combo box with a drop list style under Win32s. When GetWindowTextLength is called for a combo box with a drop list style under Win32s, a value of 0xFFFF is returned. Because this return value isn't properly sign extended to 0xFFFFFFFF, the DDX_CBString function doesn't work correctly.

RESOLUTION

It is possible to work around this problem by providing your own implementation of the DDX_CBString function for the class that has the DDX_CBString call in its DoDataExchange function. If you provide your own DDX_CBString member function, it will override the standard MFC DDX_CBString function. To provide your own version of DDX_CBString follow these steps:

Add the following function to your class header file:

   void DDX_CBString(CDataExchange *pDX, int
                     nIDC, CString& value);

Add the implementation for this function to your classes implementation file. See the "Sample Code" section in this article for an example of a revised implementation of DDX_CBString for a class called CMyTryDlg.
In your implementation file, use #include to include the Afxpriv.h file.
Inside the implementation for this function, copy all the MFC code for the standard MFC DDX_CBString function. The standard MFC DDX_CBString function can be found in the MFC source file Dlgdata.cpp.

Change this line:

      if (nLen != -1)

to this line: if (nLen!= -1 && nLen!=0xFFFF)

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

Visual C++ versions 4.2 and later do not support building Win32s applications.

MORE INFORMATION

Sample Code

The following is a sample of an override of the DDX_CBString function for the class called CMyTryDlg:


   void AFXAPI CMyTryDlg::DDX_CBString(CDataExchange* pDX, int nIDC,
                                       CString& value)
   {
        HWND hWndCtrl = pDX->PrepareCtrl(nIDC);
        if (pDX->m_bSaveAndValidate)
        {
             // Get current edit item text (or drop list static)
             int nLen = ::GetWindowTextLength(hWndCtrl);
             if (nLen != -1 && nLen != 0xFFFF)
             {
                  // Get known length
                  ::GetWindowText(hWndCtrl, value.GetBufferSetLength(nLen),
                                  nLen+1);
            }
            else
            {
                 // GetWindowTextLength does not work for drop lists assume
                 // max of 255 characters
                 ::GetWindowText(hWndCtrl, value.GetBuffer(255), 255+1);
            }
            value.ReleaseBuffer();
       }
       else
       {
            // Set current selection based on model string
            if (::SendMessage(hWndCtrl, CB_SELECTSTRING, (WPARAM)-1,
                 (LPARAM)(LPCTSTR)value) == CB_ERR)
            {
                 // Set the edit text (will be ignored if DROPDOWNLIST)
                 AfxSetWindowText(hWndCtrl, value);
            }
       }
   }