PRB: DAO 3.0 Must Be Used in Primary Thread

 Last reviewed: February 12, 1998
Article ID: Q151407
The information in this article applies to:
  • The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

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: 

   ARTICLE_ID:  Q150777
   TITLE     :  Descriptions and Workings of OLE Threading Models
Tech Note #64: Apartment-Model Threading in OLE Controls

REFERENCES

ARTICLE_ID: Q147629 

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

Keywords          : dbDao MfcDAO kbprg kbusage
Technology        : kbMfc
Version           : 4.0 4.1 4.2
Platform          : NT WINDOWS
Issue type        : kbprb

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Last reviewed: February 12, 1998
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