DOCUMENT:Q198607 11-JAN-2001 [vbwin] TITLE :PRB: Access Violation in VB Run-Time Using AddressOf PRODUCT :Microsoft Visual Basic for Windows PROD/VER:WINDOWS:6.0 OPER/SYS: KEYWORDS:kbAPI kbThread kbVBp kbVBp600 kbGrpDSVBDB ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Visual Basic Learning Edition for Windows, version 6.0 - Microsoft Visual Basic Professional Edition for Windows, version 6.0 - Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 ------------------------------------------------------------------------------- SYMPTOMS ======== When running your Visual Basic application, you may encounter an error similar to the following: The instruction at 0x660bd3b1 referenced memory at 0x0000009c. The memory could not be written. This error may occur immediately following a declared API function call within a Visual Basic callback function. CAUSE ===== When the error occurs, the callback function is called by a thread that was not created by Visual Basic. For instance, certain API calls such as CreateThread() and RegisterServiceCtrlHandler() create an additional thread before invoking the callback function that was previously setup by using the AddressOf operator. Although the same code may have worked in previous versions of Visual Basic, calling API's of this nature are currently unsupported in Visual Basic. MORE INFORMATION ================ These API functions use a Free threading model, but Visual Basic only supports Apartment-model threading. Furthermore, additional care must be taken with regards to what code can be executed within the callback function. Any use of the following within the callback function may cause undesirable results: - File I/O. - Error handling. - Fixed size arrays. - Set statements. - COM method calls that return HRESULTs (such as any Visual Basic ActiveX object). - Declare calls. - Global objects such as the Application object. - Most of the Visual Basic run-time files. REFERENCES ========== For more information on using threads in Visual Basic, please see the following: Visual Basic Books Online, "Apartment-Model Threading in Visual Basic" Pattison, Ted, Programming Distributed Applications with COM and Microsoft Visual Basic 6.0. Microsoft Press, 1999, Chapter 7, "Exploring Apartments and Multithreading". McKinney, Bruce, HardCore Visual Basic. Microsoft Press, 1997 Chapter 11, "Threads and Synchronization" Appleman, Daniel, A Thread to Visual Basic article located at the following Web URL: http://www.desaware.com For additional information, please see the following articles in the Microsoft Knowledge Base: Q189133 Make C DLL More Accessible to VB with a Type Library Q143258 How to Create Constants and DLL Declarations in a Type Library Additional query words: ====================================================================== Keywords : kbAPI kbThread kbVBp kbVBp600 kbGrpDSVBDB Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600 Version : WINDOWS:6.0 Issue type : kbprb ============================================================================= 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 2001.