BUG: Crash When Closing Application That Uses ActiveX DLL

MORE INFORMATION

The steps below illustrate two sample ActiveX DLLs and a client application to use the DLLs. By following these steps, you will see how an error can occur when you close the client application. A workaround is also provided.

Steps to Reproduce Behavior

1. Start Visual Basic and create a new ActiveX DLL project. Class1 is created by default.
2. Change the name of the Class1 module to GlobalMethods.
3. Choose Properties from the Project menu, and set the Project Name to UBGlobal.
4. Put the following code in the GlobalMethods module:

       Option Explicit
   
       Public Sub gMethod1()
   
       End Sub
   
       Public Function getValidList(a As Object, b() As Collection) As Integer
       End Function
   
   						

5. Save the UBGlobal project and make UBGlobal.dll.
6. Close the UBGlobal project and start a new ActiveX DLL project. Class1 is created by default.
7. Choose References from the Project menu, and set a reference to UBGlobal.
8. Choose Properties from the Project menu, and set the Project Name to UBServer.
9. From the Project menu, add a new class module. Class2 is created.
10. Put the following code in the Class2 module:

          Option Explicit
    
          Public objGlobal As UBGlobal.GlobalMethods
          Public objParent As Class1
    
          Private Sub Class_Initialize()
             Set objGlobal = New UBGlobal.GlobalMethods
             objGlobal.gMethod1
          End Sub
    
          Private Sub Class_Terminate()
             Set objGlobal = Nothing
          End Sub
    
    						

11. Put the following code in the Class1 module:

          Option Explicit
    
          Private objChild As Class2
    
          Private Sub Class_Initialize()
             Set objChild = New Class2
             Set objChild.objParent = Me
          End Sub
    
          Private Sub Class_Terminate()
             Set objChild.objParent = Nothing
             Set objChild = Nothing
          End Sub
    
          Public Sub CleanUp()
             Set objChild.objParent = Nothing
          End Sub
    						

12. Save the UBServer project and make UBServer.dll.
13. Close the UBServer project, and start a new Standard EXE project. Form1 is created by default.
14. Choose References from the Project menu, and set a reference to both UBGlobal and UBServer.
15. Choose Properties from the Project menu, and set the Project Name to Client.
16. Put a CommandButton on Form1.
17. Put the following code in the form's code window:

          Option Explicit
    
          Dim objGlobal As UBGlobal.GlobalMethods
          Dim objParent As UBServer.Class1
    
          Private Sub Form_Load()
             Set objGlobal = New UBGlobal.GlobalMethods
             objGlobal.gMethod1
             Set objParent = New UBServer.Class1
          End Sub
    
          Private Sub Command1_Click()
             ' uncomment the following line for solution:
             'objParent.CleanUp
             Set objParent = Nothing
             Set objGlobal = Nothing
             Unload Me
          End Sub
    						

18. Run the project. The form with the CommandButton appears.
19. Click the CommandButton. Note that the application ends without an error.
20. Save the Client project and compile Client.exe.
21. Close Visual Basic.
22. Run Client.exe. The form with the CommandButton appears.
23. Click the CommandButton.

Result: The client application terminates with one of the errors described above.

Solution: Uncomment the objParent.CleanUp line in the client project. Recompile and run Client exe. Click the CommandButton.

Result: The application shuts down properly with no error.

The purpose of the CleanUp method in Class1 is to remove the circular reference between Class1 (parent) and Class2 (child). Until the reference to Class1 is removed, Class1 does not terminate properly when using the ActiveX DLL from a Visual Basic client EXE.