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Q151407: PRB: DAO 3.0 Must Be Used in Primary Thread

Article: Q151407
Product(s): Microsoft C Compiler
Version(s): 4.0 4.1 4.2
Operating System(s): 
Keyword(s): kbprogramming kbusage kbDAOsearch kbDatabase kbMFC kbVC
Last Modified: 30-JUL-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
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SYMPTOMS
========

Attempting to use either the MFC DAO classes or the DAO SDK dbDao classes in any
thread but the primary one causes unexpected behavior.

CAUSE
=====

DAO 3.0 is not marked as apartment-model or free-threaded in the registry. No
multithreading testing has been done on DAO 3.0. Any documentation should say
that DAO 3.0 is not thread-safe and should only be used in the primary thread in
a process.

RESOLUTION
==========

Do not use MFC DAO or the DAO SDK dbDao classes in any thread but the primary
thread of an application.

STATUS
======

This behavior is by design.

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

Internally, the Jet engine is multithreaded, and the registry setting
\HKEY_LOCALMACHINE\Software\Microsoft\Jet\3.0\Engines\Jet\Threads controls the
number of threads that the engine uses internally. The Jet engine is
multithreaded internally, but neither DAO nor the Jet engine are thread- safe.

For more information on OLE threading models, see the following article in the
Microsoft Knowledge Base:

  Q150777 Descriptions and Workings of OLE Threading Models

Tech Note #64: Apartment-Model Threading in OLE Controls

REFERENCES
==========

  Q147629 PRB: Problems if ODBC or DAO in DLL's InitInstance or DLLMain

======================================================================
Keywords          : kbprogramming kbusage kbDAOsearch kbDatabase kbMFC kbVC 
Technology        : kbAudDeveloper kbMFC
Version           : 4.0 4.1 4.2
Issue type        : kbprb

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