Q156135: FIX: CRecordset::m_lCurrentRecord Gives Inaccurate Values
Article: Q156135
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s):
Keyword(s): kbDatabase kbMFC kbODBC kbVC kbVC500fix
Last Modified: 30-JUL-2001
-------------------------------------------------------------------------------
The information in this article applies to:
- The Microsoft Foundation Classes (MFC), included with:
- *EDITOR Please do not choose this product*Microsoft Visual C++ 32-bit Edition* use 241, 265, 225, 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); }
The value should be as follows:
_AFXDBCORE_INLINE void CRecordset::MovePrev()
{ ASSERT(IsOpen()); Move(-1, SQL_FETCH_PRIOR); }
However, since CRecordset::MovePrev() is not virtual, it is not possible to
correctly override this method.
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()
Use of some methods may cause the bookmark to be invalidated. To determine when a
bookmark is persistent and when it is not, see the following method:
CRecordset::GetBookmarkPersistence()
The workaround presented below, however, ensures that
CRecordset::m_lCurrentRecord is as accurate as possible.
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
Technology : kbAudDeveloper kbMFC
Version : winnt:
Issue type : kbbug
Solution Type : kbfix
=============================================================================
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.
Copyright Microsoft Corporation 1986-2002.