Q178967: FIX: CRecordset Does Not Set HSTMT Attributes for Updates

Article: Q178967
Product(s): Microsoft C Compiler
Version(s): 5.0,6.0
Operating System(s): 
Keyword(s): kbDatabase kbMFC kbODBC kbVC kbVC600bug kbNoUpdate
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
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SYMPTOMS
========

When updating using the cursor library or when using a cursor that causes MFC to
use SQL statements to update, CRecordset errors can occur when using strings
containing ODBC escape sequences. For example, the string [ASCII
147]{d2fcfefb-6126-11d1-84c1-0020afd6c52f}[ASCII 148] may cause a data
conversion error because [ASCII 147]{d[ASCII 148] is the ODBC escape sequence
for a date. The error occurs whether the SQLSetStmtOption() attribute SQL_NOSCAN
is set to SQL_NOSCAN_ON or SQL_NOSCAN_OFF.

CAUSE
=====

In the CRecordset::PrepareUpdateHstmt() function, CRecordset allocates a handle
to use to update m_hstmtUpdate. However, CRecordset does not apply the HSTMT
attributes on its default HSTMT, m_hstmt, to m_hstmtUpdate. Therefore any
statement attributes set by the user will not be seen in an update.

For example, given the code:

     void CSampleSet::OnSetOptions(HSTMT hstmt)
     {
        CRecordset::OnSetOptions(hstmt);

        SQLSetStmtOption(hstmt, SQL_NOSCAN, SQL_NOSCAN_ON);
     }

updates may fail for fields that contain ODBC escape because m_hstmtUpdate will
have the SQL_NOSCAN option turned off. For example you might get a data
conversion error from the ODBC driver.

RESOLUTION
==========

It is possible to work around this bug by taking the following steps:

For dynamically linking the MFC library:

1. Copy the implementations of CRecordset::UpdateInsertDelete() and
  CRecordset::PrepareUpdateHstmt() to your CRecordset derived class.

2. If you haven't already implemented the virtual CRecordset::Update() function,
  copy the MFC implementation of that method.

3. In the copied CRecordset::PrepareUpdateHstmt(), change the block of code that
  reads

        if (m_hstmtUpdate == SQL_NULL_HSTMT)
        {
           AFX_SQL_SYNC(::SQLAllocStmt(m_pDatabase->m_hdbc, &m_hstmtUpdate));
           if (!Check(nRetCode))
           {
              TRACE0("Error: failure to allocate update statement.\n");
              AfxThrowDBException(nRetCode, m_pDatabase, m_hstmtUpdate);
           }
        }

  to the following:

        if (m_hstmtUpdate == SQL_NULL_HSTMT)
        {
           AFX_SQL_SYNC(::SQLAllocStmt(m_pDatabase->m_hdbc, &m_hstmtUpdate));
           if (!Check(nRetCode))
           {
              TRACE0("Error: failure to allocate update statement.\n");
              AfxThrowDBException(nRetCode, m_pDatabase, m_hstmtUpdate);
           }

           OnSetOptions(m_hstmtUpdate);
        }

This will ensure that the options set in the virtual function OnSetOptions() are
applied to the member m_hstmtUpdate.

For statically linking the MFC library, you can either follow the steps above or
you can modify your version of MFC sources directly by following step 3 above in
the CRecordset class and rebuilding.

It is highly recommended that developers do not directly modify the MFC source.
For more information on rebuilding the MFC static library, see the README.TXT
file in the MFC\SRC directory.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

This problem was corrected in Microsoft Visual C++ .NET.

REFERENCES
==========

For more information on ODBC escape sequences, see Chapter 8 and Appendix C,
ODBC Programmers Reference.

Additional query words: scalar functions kbVC500bug kbVC600bug kbmfc kbdatabase kbodbc

======================================================================
Keywords          : kbDatabase kbMFC kbODBC kbVC kbVC600bug kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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