FIX: CRecordset::m_lCurrentRecord Gives Inaccurate Values

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++ 32-bit Edition, versions 4.2, 4.2b

SYMPTOMS

CRecordset::m_lCurrentRecord does not correctly decrement when CRecordset::MovePrev() is invoked.

CAUSE

CRecordset::m_lCurrentRecord tracks the absolute position of a record within a recordset. It is modified using a value passed to the CRecordset::Move() method. The CRecordset::MovePrev() method incorrectly increments the value of m_lCurrentRecord instead of decrementing it.

The source for CRecordset::MovePrev(), from Afxdb.inl, line 83-84 is shown below:

   _AFXDBCORE_INLINE void CRecordset::MovePrev()
      { ASSERT(IsOpen()); Move(1, SQL_FETCH_PRIOR); } 

   _AFXDBCORE_INLINE void CRecordset::MovePrev()
      { ASSERT(IsOpen()); Move(-1, SQL_FETCH_PRIOR); } 

WORKAROUND

The solution is to override the CRecordset::SetRowsetCurrencyStatus() so that it properly decrements the m_lCurrentRecord member for a MovePrev() call. This is done in the switch statement for wFetchType == SQL_FETCH_PRIOR as shown in the sample code below.

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 5.0.

MORE INFORMATION

The use of CRecordsetStatus should be restricted to information display, and should not be relied on in a robust environment. m_lCurrentRecord is not relational, and is not be accurate in a multi-user environment. A better approach is to use bookmarks, which are more persistent. See the following methods:

   CRecordset::SetBookmark() 

   CRecordset::GetBookmark() 

  CRecordset::GetBookmarkPersistence() 

Sample Code

   extern void AFXAPI AfxSetCurrentRecord(long* plCurrentRecord,
                                            long nRows, RETCODE nRetCode);

   extern void AFXAPI AfxSetRecordCount(long* plRecordCount,
        long lCurrentRecord, long nRows, BOOL bEOFSeen, RETCODE nRetCode);

    void CMyRecSet::SetRowsetCurrencyStatus(RETCODE nRetCode,
                         UWORD wFetchType, long nRows, DWORD dwRowsFetched)
    {
        // dwRowsFetched is not used, avoid warning with dummy call
        dwRowsFetched = 0;

        int nDirection;

        // Set the fetch direction
        switch (wFetchType)
        {
        case SQL_FETCH_FIRST:
            nDirection = 1;
            if (nRetCode == SQL_NO_DATA_FOUND)
            {
                m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;
                m_lRecordCount = 0;
            }
            else
                m_lCurrentRecord = 0;
            break;

        case SQL_FETCH_NEXT:
            nDirection = 1;
            AfxSetCurrentRecord(&m_lCurrentRecord, nRows, nRetCode);
            AfxSetRecordCount(&m_lRecordCount, m_lCurrentRecord, 1,
                m_bEOFSeen, nRetCode);

            // This is the only way to know you've hit the end (m_bEOFSeen)
            if (!m_bEOFSeen && nRetCode == SQL_NO_DATA_FOUND
             && m_lRecordCount == m_lCurrentRecord + 1)
                m_bEOFSeen = TRUE;
            break;

        case SQL_FETCH_LAST:
            nDirection = -1;
            if (nRetCode == SQL_NO_DATA_FOUND)
            {
                m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;
                m_lRecordCount = 0;
            }
            else if (m_bEOFSeen)
                m_lCurrentRecord = m_lRecordCount - 1;
            else
                m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;
            break;

        case SQL_FETCH_PRIOR:
            nDirection = -1;
            AfxSetCurrentRecord(&m_lCurrentRecord, -1, nRetCode);
            break;

        case SQL_FETCH_RELATIVE:
            nDirection = nRows;
            AfxSetCurrentRecord(&m_lCurrentRecord, nRows, nRetCode);
            AfxSetRecordCount(&m_lRecordCount, m_lCurrentRecord, 1,
                m_bEOFSeen, nRetCode);
            break;

        case SQL_FETCH_ABSOLUTE:
            nDirection = nRows;
            if (nRetCode != SQL_NO_DATA_FOUND)
            {
                if (nRows > 0)
                    m_lCurrentRecord = nRows - 1;
                else if (m_bEOFSeen)
                    m_lCurrentRecord = m_lRecordCount + nRows;
                else
                    m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;
            }
            else
                m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;

            AfxSetRecordCount(&m_lRecordCount, m_lCurrentRecord, 1,
                m_bEOFSeen, nRetCode);
            break;

        case SQL_FETCH_BOOKMARK:
            nDirection = 0;
            m_lCurrentRecord = AFX_CURRENT_RECORD_UNDEFINED;
            break;
        }

        // Set the BOF/EOF flags
        if (nRetCode == SQL_NO_DATA_FOUND)
        {
            if (wFetchType == SQL_FETCH_FIRST
             || wFetchType == SQL_FETCH_LAST
             || wFetchType == SQL_FETCH_BOOKMARK)
            {
                // If MoveFirst/MoveLast fails, result set is empty
                // If SetBookmark fails, currency undefined
                m_bEOF = m_bBOF = TRUE;
            }
            else
            {
                m_bEOF = nDirection >= 0 ? TRUE : FALSE;
                m_bBOF = !m_bEOF;
            }
        }
        else
        {
            m_bEOF = m_bBOF = FALSE;
        }
    } 

Additional query words: kbVC420bug

Keywords : kbDatabase kbMFC kbODBC kbVC kbVC500fix
Version : winnt:
Platform : winnt
Issue type : kbbug