Q139593: BUG: DDX Problem with Combo Box in Win32s

Article: Q139593
Product(s): Microsoft C Compiler
Version(s): winnt:2.0,2.1,2.2,4.0,4.1
Operating System(s): 
Keyword(s): kbprogramming kbui kbnokeyword kbDlg kbMFC kbVC kbOSWin32s kbGrpDSMFCATLkbbuglist
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1 
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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:

1. Add the following function to your class header file:

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

2. 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.

3. In your implementation file, use #include to include the Afxpriv.h file.

4. 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.

5. 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);
              }
         }
     }

Additional query words:

======================================================================
Keywords          : kbprogramming kbui kbnokeyword kbDlg kbMFC kbVC kbOSWin32s kbGrpDSMFCATL kbbuglist
Technology        : kbAudDeveloper kbMFC
Version           : winnt:2.0,2.1,2.2,4.0,4.1
Issue type        : kbbug

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