DOCUMENT:Q241896  25-JUN-2002  [vbwin]
TITLE   :PRB: Threading Issues with Visual Basic 6.0 ActiveX Components
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER::6.0,6.0 SP3
OPER/SYS:
KEYWORDS:kbtophit kbActiveX kbThread kbVBp600 kbGrpDSVB kbWinDNA MSGRAPH

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

 - Microsoft Visual Studio, Enterprise Edition versions 6.0, 6.0 SP3 
 - Microsoft Visual Basic Professional Edition for Windows, version 6.0 
 - Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When using Visual Basic 6.0 ActiveX components in a multi-threaded environment,
you should be aware of the following potential problems:

ActiveX DLL Hosted in a Multi-threaded Client
---------------------------------------------

 - Access Violation inside MSVBVM60.DLL.
 - Client enters a deadlock state.

You may see these two symptoms if a Visual Basic ActiveX DLL is hosted in a
multi-threaded environment, for example, IIS, MTS, or a multi-threaded C client,
and the Retain In Memory option is not enabled. To enable this option, follow
the steps below:

1. From Project menu, select Project Properties.

2. On the General tab, make sure that the Threading Model is Apartment Threaded,
   then select the Unattended Execution and Retain In Memory checkboxes*.

3. Save the project and compile the DLL.

*NOTE: The Unattended Execution option is not available if the project contains
any user interface elements, such as forms or controls. The Retain In Memory
option is not available if Unattended Execution is not selected.

NOTE: Prior to Service Pack 3 for Visual Studio 6.0, it was possible to get an AV
during process shutdown with Retain in Memory enabled. This has been fixed in
Visual Studio 6, Service Pack 3
(http://msdn.microsoft.com/vstudio/sp/vs6sp3/default.asp).


If an ActiveX DLL or UserControl project contains API declarations, you may
experience deadlocks during process/thread shut down or object creation, even if
the Unattended Execution check box has been selected in the case of an ActiveX
DLL.


To workaround this problem, you can use a Type Library instead of Declare in
Visual Basic. For additional information on how to use a Type Library, click the
article number below to view the article in the Microsoft Knowledge Base:

   Q189133 HOWTO: Make C DLL More Accessible to VB with a Type Library

ActiveX EXE Accessed by a Multi-threaded Client or by Multiple Single- or Multi-threaded Clients:
-------------------------------------------------------------------------------------------------

   Runtime error '7': out of memory and sometimes followed by a disk operation
   error.

   Run-time error '430': Class does not support Automation or does not support
   expected interface.

   Run-time error '424': Object required.

   Run-time error '-2147023170 (800706be)': Automation error. The remote
   procedure call failed.

   Run-time error '-2147287010 (8003001e)': Automation error. This is a "A disk
   error occurred during a read operation." based on ErrLook.

   Additional server processes (ThreadTest.EXE) are created even though the
   Instancing property of Class1 is marked MultiUse.

You may see the error messages listed above if you have an ActiveX EXE server
with a thread pool greater than one (1), and a multi-threaded client or multiple
single- or multi-threaded clients rapidly creating and destroying objects inside
the server. To work around this problem, you can create an empty class in the
local server and have the client keep a reference to it as shown in the "More
Information" section below.

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

Steps to Reproduce the Behavior
-------------------------------

A: Creating the server:

1. Create an ActiveX EXE project and rename it ThreadTest.

2. From Project menu, select Project Properties and on the General tab, select a
   Thread Pool of two (2).

3. Add the following code to the default class (Class1):

   Private strClassName As String
   Public Property Let ClassName(ByVal vData As String)
      strClassName = vData
   End Property
   Public Property Get ClassName() As String
      ClassName = strClassName 
   End Property

4. Save and compile the project (ThreadTest.EXE).

B: Creating the client and testing:

1. Start a Standard EXE project and rename it Client.

2. Add a command button and a textbox to the default form (Form1).

3. Add the following code to Form1:

   Private Sub Command1_Click()
      Dim i As Long, j As Long
      Dim o As Object
      j = Val(Text1.Text)
      For i = 1 To j
         DoEvents
         Set o = CreateObject("ThreadTest.Class1")
         o.ClassName = i
         Me.Caption = o.ClassName
         Set o = Nothing
      Next
   End Sub
   Private Sub Form_Load()
      Text1.Text = 1000
      Command1.Caption = "Start"
   End Sub

4. Compile the project (Client.EXE).

5. Start three or more instances of Client.EXE and press the Start button on
   each form. Note that you see one or more of the error messages above.

C: Implementing the workaround:

1. Open the ThreadTest project.

2. Add another class module (Class2) with no code.

3. Save and re-compile the project (ThreadTest.EXE).

4. Open the Client project.

5. Replace the code in Form1 with the following:

   Private p As Object
   Private Sub Command1_Click()
      Dim i As Long, j As Long
      Dim o As Object
      j = Val(Text1.Text)
      For i = 1 To j
         DoEvents
         Set o = CreateObject("ThreadTest.Class1")
         o.ClassName = i
         Me.Caption = o.ClassName
         Set o = Nothing
      Next
   End Sub
   Private Sub Form_Load()
      Text1.Text = 1000
      Command1.Caption = "Start"   
      Set p = CreateObject("ThreadTest.Class2")
   End Sub
   Private Sub Form_Unload(Cancel As Integer)
      Set p = Nothing
   End Sub

6. Save and re-compile the project (Client.EXE).

7. Run three or more instances of Client.EXE and press the Start button on each
   form. Note that you should see no error messages.

When an ActiveX EXE with a thread pool greater than one is accessed by multiple
clients through DCOM, the workaround described in part C of the "More
Information" section does not work. As a consequence, Visual Basic 6.0 ActiveX
EXE servers are not suitable for DCOM servers with multiple clients rapidly
creating and destroying objects. If your server application needs to handle this
scenario, it is strongly recommended that you use ActiveX DLLs in MTS instead.
When designing ActiveX DLLs to be hosted in MTS, you should make sure that the
Unattended Execution and Retain In Memory checkboxes are both selected. These
checkboxes and located on the Project menu, when you choose Properties, and
select the General tab.

Do not use the GlobalMultiUse Instancing property for a class when you intend to
use the ActiveX component under MTS or COM+. The interface for the
GlobalMultiUse object is cached in a per thread-based table and is not freed
until the thread terminates. As a result, if a call comes in with a different
context (although on the same thread), it fails with RPC_E_WRONG_THREAD. To use
components in MTS and COM+, you should design your classes in such a way that
the objects are stateless.


REFERENCES
==========

For more information, refer to the following books:

   Ted Pattison Programming Distributed Applications with COM+ and Microsoft
   Visual Basic 6.0, Second Edition (http://go.microsoft.com/fwlink/?LinkId=8387
   ) Microsoft Press, 2001

   Francesco Balena Programming Microsoft Visual Basic 6.0
   (http://www.microsoft.com/redirect.asp?PageID=159&PARAM=BOKPSS62&TARGET=http://www1.fatbrain.com/asp/bookinfo/bookinfo.asp?theisbn=0735605580%2526p=netskills%2526s=69196)
   Microsoft Press, 2001

Additional query words:

======================================================================
Keywords          : kbtophit kbActiveX kbThread kbVBp600 kbGrpDSVB kbWinDNA MSGRAPH 
Technology        : kbVSsearch kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVB600 kbVS600SP3 kbVS600 kbVS600Search
Version           : :6.0,6.0 SP3
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 2002.