Q173252: FIX: CArchive::GetObjectSchema Returns Incorrect Schema

Article: Q173252
Product(s): Microsoft C Compiler
Version(s): WINDOWS NT:4.2,5.0
Operating System(s): 
Keyword(s): kbVC420bug kbVC500bug kbVS97sp3fix
Last Modified: 17-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

CArchive::GetObjectSchema returns the schema of the object loading the
serialized information rather than the schema of the object being loaded. This
occurs only when an object with a schema greater than 0 loads an object with a
schema equal to 0 and the object being loaded is not the first of this type.

CAUSE
=====

When the first object of a type that has a schema of 0 is loaded, the value of
the CArchive member, m_nObjectSchema, is set to a value of 0. When further
objects of the same type are loaded, m_nObjectSchema is set to the schema of the
object loading the serialized information.

This behavior is due to the way CArchive::ReadClass determines the schema of a
type that has already been stored. This is done in the following code located
toward the end of CArchive::ReadClass:

     // determine schema stored against objects of this type
     void* pTemp;
     if (m_pSchemaMap != NULL &&
        (pTemp = m_pSchemaMap->GetValueAt(pClassRef)) != NULL)
        nSchema = (UINT)pTemp;
     else
     nSchema = pClassRef->m_wSchema & ~VERSIONABLE_SCHEMA;

When the schema of a type that has already been loaded is 0 the value returned
from GetValueAt is 0 and the else part of the if statement is executed. This
sets the value of the schema for the object being loaded as the schema of the
existing object.

RESOLUTION
==========

There are two ways to work around this issue:

- Do not use a schema of 0 to serialize object information. This is the
  simplest solution but may not be practical if objects of schema 0 must be
  maintained.

-or-

- Set a flag when you load objects of schema 0. This flag should be considered
  when testing for schema number when using Archive::GetObjectSchema. The time
  to set this flag is when the first object with schema 0 is being loaded.

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 Studio 97 Service
Pack 3.

For more information, please see the following article in the Microsoft Knowledge
Base:

  Q170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why

Additional query words:

======================================================================
Keywords          : kbVC420bug kbVC500bug kbVS97sp3fix 
Technology        : kbAudDeveloper kbMFC
Version           : WINDOWS NT:4.2,5.0
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.