FIX: DFX_Text() Sets CString Length Incorrectly in Unicode Build

• Microsoft Visual Studio versions 97, 97sp3
• Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 5.0sp1, 5.0sp2, 5.0sp3

SYMPTOMS

In a Unicode build of an MFC DAO project, the Record Field Exchange function DFX_Text() incorrectly sets the length of its CString argument to twice the actual length of the data fetched from the database. This action causes any concatenation operation with the CString to behave unexpectedly.

NOTE: This article contains the source code to corrected versions of DFX_Text() and DAOStringAllocCallback.

CAUSE

At line 906 in the Daodfx.cpp file within the DFX_Text() function, the length of the CString is set to the number of bytes rather than the number of characters returned from the database:

   value.GetBufferSetLength(
   pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] - 1); 

   pstr->ReleaseBuffer(dwLen); 

RESOLUTION

To resolve this problem, divide the string length by sizeof(TCHAR). In an ANSI build, a TCHAR is 1 byte in length; in a Unicode build, a TCHAR is 2 bytes in length.

This article contains the source code to corrected versions of DFX_Text() and DAOStringAllocCallback(), which contain the two fixes shown below:

Line 906, Daodfx.cpp, DFX_Text()

   pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] - 1); 

   (pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] / sizeof(TCHAR)) [ASCII 150]
      sizeof(TCHAR)); 

Line 1391, Daodfx.cpp, DAOStringAllocCallback()

   lpsz = pstr->GetBuffer(dwLen); 

   lpsz = pstr->GetBuffer(dwLen / sizeof(TCHAR)); 

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed in Visual C++ version 6.0

MORE INFORMATION

Copy and paste the following two functions and function declarations into your project:

//////////////////////////////////////////////// 

   STDAPI DaoStringAllocCallback2(DWORD dwLen, DWORD pData, void** ppv);

   STDAPI DaoStringAllocCallback2(DWORD dwLen, DWORD pData, void** ppv)
   {
      LPTSTR lpsz;
      CString* pstr = (CString*)pData;

   #ifndef _UNICODE
      // If using ANSI DAO interfaces, DAO reports field length
      // rather than data length. In this case there will not be space
      // for NULL terminator if data length equals field length. Make room.
      dwLen++;
   #endif

      TRY
      {
         // Only re-allocate if necessary
         lpsz = pstr->GetBuffer(dwLen / sizeof(TCHAR));
         pstr->ReleaseBuffer(dwLen);
         *ppv = (void*)(dwLen > 0 ? lpsz : NULL);
      }
      CATCH_ALL(e)
      {
         e->Delete();
         return E_OUTOFMEMORY;
      }
      END_CATCH_ALL

      return S_OK;
   }

   void AFXAPI DFX_Text2(CDaoFieldExchange* pFX, LPCTSTR lpszName,
      CString& value, int nPreAllocSize = AFX_DAO_TEXT_DEFAULT_SIZE,
      DWORD dwBindOptions = AFX_DAO_ENABLE_FIELD_CACHE);

   void AFXAPI DFX_Text2(CDaoFieldExchange* pFX, LPCTSTR lpszName,
      CString& value, int nPreAllocSize, DWORD dwBindOptions)
   {
      (pFX->m_nFieldType == CDaoFieldExchange::outputColumn) ?
         ++pFX->m_nField: ++pFX->m_nParam;

      // Do nothing if op not supported for outputColumn or param type
      if (!pFX->IsValidOperation())
         return;

      DWORD dwDAOType;

   #ifdef _UNICODE
      dwDAOType = DAO_WCHAR;
   #else
      dwDAOType = DAO_CHAR;
   #endif

      switch (pFX->m_nOperation)
      {
      case CDaoFieldExchange::BindField:
         {
            // Pre-allocate buffer to prevent needless re-allocations
            value.GetBuffer(nPreAllocSize);
            value.ReleaseBuffer();

            LPDAOCOLUMNBINDING pcb =
               &pFX->m_prs->m_prgDaoColBindInfo[pFX->m_nField-1];
            pcb->dwDataType = dwDAOType;
            pcb->dwBinding = DAOBINDING_DIRECT | DAOBINDING_CALLBACK;
            pcb->dwUser = (DWORD)&value;
            pcb->cbDataOffset = (DWORD)DaoStringAllocCallback2;
            pcb->cbMaxLen = INT_MAX;
         }
         // Fall through to finish setting up column binding struct

      default:
         pFX->Default(lpszName, (void*)&value, AFX_RFX_TEXT,
            dwBindOptions);
         return;

      case CDaoFieldExchange::Fixup:
         if (pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] == 0 ||
            pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] == DAO_NULL)
         {
            // If null or empty string set length zero
            value.GetBufferSetLength(0);
         }
         else
         {
            // Make sure length correct (and doesn't include NULL
            //terminator)
            value.GetBufferSetLength(

        (pFX->m_prs->m_pulColumnLengths[pFX->m_nField-1] / 
   sizeof(TCHAR))- sizeof(TCHAR));

         }

         pFX->Default(lpszName, (void*)&value,
            AFX_RFX_TEXT, dwBindOptions);
         return;

   #ifdef _DEBUG
      case CDaoFieldExchange::DumpField:
         *pFX->m_pdcDump << "\n" << lpszName << " = " << value;
         return;
   #endif //_DEBUG
      }
   }

//////////////////////////////////////////////  

   void CDaoSet::DoFieldExchange(CDaoFieldExchange* pFX)
   {
   #ifdef _UNICODE
      //{{AFX_FIELD_MAP(CDaoSet)
      pFX->SetFieldType(CDaoFieldExchange::outputColumn);
      DFX_Short(pFX, _T("[Hours]"), m_Hours);
      //}}AFX_FIELD_MAP
      DFX_Text2(pFX, _T("[CourseID]"), m_CourseID);
      DFX_Text2(pFX, _T("[CourseTitle]"), m_CourseTitle);
   #else
   //{{AFX_FIELD_MAP(CDaoSet)
      pFX->SetFieldType(CDaoFieldExchange::outputColumn);
      DFX_Text(pFX, _T("[CourseID]"), m_CourseID);
      DFX_Text(pFX, _T("[CourseTitle]"), m_CourseTitle);
      DFX_Short(pFX, _T("[Hours]"), m_Hours);
      //}}AFX_FIELD_MAP
   #endif

   } 

Additional query words: MFCDAO MFCDatabase kbDAO kbMFC kbVC600fix kbVC500bug kbdse kbDSupport

Keywords :
Version :
Platform : WINDOWS
Issue type : kbbug