Q237757: INFO: Leaking Handles When Creating COM Objects

Article: Q237757
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbActiveX kbCOMt kbVBp600 kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

-------------------------------------------------------------------------------
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 
-------------------------------------------------------------------------------

SUMMARY
=======

When a Visual Basic application creates COM objects that are instantiated in an
out-of-process server, observe the number of handles shown in the performance
tab of the NT task manager. You might see the number of handles increasing,
without returning to its initial level when the objects are released.

Although this behavior appears to be a leak, it is not. This behavior is by
design.

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

Visual Basic does not release the handles it creates immediately after it uses
them; it caches them. When the number of handles reaches a given level, they are
released and new allocations start again when they are needed. All handles are
recovered when the client application is closed.

This is the expected behavior. Visual Basic is just optimizing the
creation/release of handles internally.

You can observe this behavior by doing the following under Windows NT or Windows
2000:

1. Create a Visual Basic ActiveX EXE project. Class1 is created by default.

2. On the Project menu, click Project1 Properties. Set the contents of the
  Project Name field to MyServer.

3. Add the following Code to the General Declarations section of Class1:

  Public Function GetLong()
      GetLong=1
  End Function

4. Save the project and compile it.

5. Create a new Visual Basic Standard EXE project. Form1 is created by default.

6. From the Project menu, select References and add a reference to the server
  you just created.

7. Place a CommandButton on Form1.

8. Place a label beside the button and clear the Caption property.

9. Add the following code to the General Declarations section of Form1:

  Private Sub Command1_Click()
     Dim i As Long
     Dim MyObj As MyServer.Class1
     Label1.Caption = "Creating..."
     Form1.Refresh
     For i = 1 To 200
        Set MyObj = CreateObject("MyServer.Class1")
        Set MyObj = Nothing
     Next i
     Label1.Caption = "Done"
  End Sub

10. Save the project and compile it.

11. Close the Visual Basic Design Environment (IDE).

12. Open the NT or Windows 2000 task manager and click on the Performance tab.

13. Run the compiled client.

14. Click the button and observe the number of handles. It will grow and will
  not come back down after creating the objects.

15. Wait until the label displays "Done," indicating all objects were created.

16. Repeat this by clicking the button several times. You will see that after
  the third or fourth time the number of handles drops to a lower value. (This
  does not occur in Windows 2000.)

17. You could also do the same with 1000 iterations, by changing the code in the
  button's click event (for i=1 to 1000) and observe the number of handles
  while these 1000 objects get created. You should see the number of handles
  going up and down.

Additional query words:

======================================================================
Keywords          : kbActiveX kbCOMt kbVBp600 kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600
Version           : WINDOWS:6.0
Issue type        : kbinfo

=============================================================================

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 1986-2002.