Q160074: SetFieldNull Asserts or Crashes When Called for Parameter

Article: Q160074
Product(s): Microsoft C Compiler
Version(s): winnt:4.2
Operating System(s): 
Keyword(s): kbfile kbDatabase kbMFC kbODBC kbVC kbVC500fix
Last Modified: 04-MAY-2001

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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, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SUMMARY
=======

CRecordset::SetFieldNull() causes an assert or an access violation if called for
a parameter. Even if no assert or access violation occurs, the parameter may not
be set to Null.

If SetFieldNull() is called before the recordset is open, the assert occurs in
DBRFX.CPP at line 48:

     ASSERT(prs->m_hstmt != SQL_NULL_HSTMT);

If SetFieldNull() is called after the recordset is open, then a call to
CRecordset::Requery() may assert in DBCORE.CPP at line 4055 or line 4062:

     ASSERT(nParam < m_nParams);

If SetFieldNull() is called after the recordset is closed and before it is
reopened, you will see an access violation in DBCORE.CPP at line 4057:

      m_pbParamFlags[nParam] |= AFX_SQL_FIELD_FLAG_NULL;

or at line 4064:

      m_pbParamFlags[nParam] &= ~AFX_SQL_FIELD_FLAG_NULL;

NOTE: Visual C++ 5.0 has a new function called SetParamNull() that you can use to
avoid this problem. SetFieldNull() in Visual C++ 4.2 was not designed to be
called before calling CRecordset::Open(), and was not designed to handle
parameters. The following files are available for download from the Microsoft
Download Center:

Paramset.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.


Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. The problem has been fixed with Visual C++ version
5.0 by adding a new function called SetParamNull().

MORE INFORMATION
================

Once you have downloaded Paramset.exe and run the EXE to extract the CParamSet
class files, add CParamSet.CPP to your project.

Change your recordsets to be derived from CParamSet rather than CRecordset.

CRecordset::SetFieldNull() cannot be called before the recordset is open. If you
want to call SetFieldNull() to set parameters to Null before calling
CRecordset::Open(), then call CParamSet::SetParamNull() instead. You will have
to change the parameters that are passed to SetParamNull() because
SetFieldNull() requires a pointer to the parameter and SetParamNull() requires a
zero-based index.

In your implementation of the DoFieldExchange() function, change each call to an
RFX function for a parameter to a call to the corresponding Param_RFX function.

REFERENCES
==========

Visual C++ Books Online: CRecordset::SetFieldNull

Additional query words: kbVC420bug kbDSupport assert gpf crash

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Keywords          : kbfile kbDatabase kbMFC kbODBC kbVC kbVC500fix 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.2
Issue type        : kbbug
Solution Type     : kbfix

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