DOCUMENT:Q166756  11-FEB-2002  [visualc]
TITLE   :FIX: Error in Requery When Set Null Date Parameter to Not Null
PRODUCT :Microsoft C Compiler
PROD/VER::5.0,6.0
OPER/SYS:
KEYWORDS:kbVC500bug kbVC600bug kbDSupportkbbuglist

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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 
    - Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
    - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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

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Keywords          : kbVC500bug kbVC600bug kbDSupport kbbuglist
Technology        : kbAudDeveloper kbMFC
Version           : :5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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