PRB: DAO 3.0 Must Be Used in Primary Thread

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This article was previously published under Q151407

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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.

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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.

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RESOLUTION

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

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STATUS

This behavior is by design.

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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:

150777 (http://support.microsoft.com/kb/150777/EN-US/ ) Descriptions and Workings of OLE Threading Models

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

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REFERENCES

147629 (http://support.microsoft.com/kb/147629/EN-US/ ) PRB: Problems if ODBC or DAO in DLL's InitInstance or DLLMain

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APPLIES TO

Microsoft Foundation Class Library 4.2, when used with:

Microsoft Visual C++ 4.0 Standard Edition
Microsoft Visual C++ 4.1 Subscription
Microsoft Visual C++ 4.2 Enterprise Edition
Microsoft Visual C++ 4.2 Professional Edition

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kbdatabase kbprb kbprogramming kbusage KB151407

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Retired KB Content Disclaimer

This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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