Microsoft KB Archive/166756

Knowledge Base

FIX: Error in Requery When Set Null Date Parameter to Not Null

Article ID: 166756

Article Last Modified on 11/21/2006

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APPLIES TO

• Microsoft Foundation Class Library 4.2, when used with:
  • Microsoft Visual C++ 5.0 Enterprise Edition
  • Microsoft Visual C++ 6.0 Enterprise Edition
  • Microsoft Visual C++ 5.0 Professional Edition
  • Microsoft Visual C++ 6.0 Professional Edition
  • Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q166756

SYMPTOMS

When you set a previously Null date parameter to no longer be Null, using either CRecordset::SetParamNull() or CRecordset::SetFieldNull(), it may cause a subsequent call to CRecordset::Requery() to generate the following exception:

Invalid string or buffer length

CAUSE

During a call to CRecordset::Open, no memory is allocated for a Null date field. However, CRecordset::Requery expects the memory to exist.

RESOLUTION

Create a replacement for the RFX_Date function which always allocates memory for the date field. See the MORE INFORMATION section.

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.

MORE INFORMATION

This problem only occurs with the version of RFX_Date that takes a CTime argument.

Steps to Reproduce Behavior

The following code displays the problem for a predefined query named Query1, which takes one date parameter:

    CMySet rs;
    try
    {
        rs.SetParamNull( 0, TRUE );
        rs.Open( CRecordset::snapshot, "{Call Query1(?)}" );
        rs.m_paramDate = CTime( 1996, 10, 13, 0, 0, 0 );
        rs.SetParamNull( 0, FALSE );
        rs.Requery();
    }
    catch( CDBException* e )
    {
        AfxMessageBox( e->m_strError );
        e->Delete();
    }
    rs.Close();
                

Steps to Replace the RFX_Date Function

The following steps detail one method of making the needed changes in RFX_Date:

1. Copy the implementation of RFX_Date() (DEVSTUDIO\VC\MFC\SRC\DBRFX.CPP) into a new .cpp file, and rename the function to something such as RFX_Date2(). Create a header file for the .cpp file and include that in your recordset .cpp file. Be sure to type:

         #include "stdafx.h"
                               

   at the top of your new .cpp file, and include the .cpp file in your project.

2. Change all RFX_Date() calls for date parameters in your recordset's DoFieldExchange() to calls to RFX_Date2().

3. In your new RFX_Date2() implementation, comment out the following lines, which appear under the CFieldExchange::BindParam case:

          if (pFX->m_prs->IsParamStatusNull(nField - 1))
          {
              pts = NULL;
              *plLength = SQL_NULL_DATA;
          }
          else
                           

4. To reduce the amount of code, you may want to remove all of the cases from the select statement except the case for CFieldExchange::BindParam, and add the following lines to the top of your replacement function:

          if( CFieldExchange::BindParam != pFX->m_nOperation ) {
              RFX_Date(pFX, szName, value);
              return;
          }

Additional query words: kbVC500bug kbVC600bug kbmfc kbdatabase kbodbc

Keywords: kbbug kbfix KB166756

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