FIX: Error When Referring to Member Object/Collection in OLE DLL

• Microsoft Visual Basic Enterprise Edition, 32-bit only, for Windows, version 4.0

SUMMARY

Assigning an object dimensioned in a client application to a Public member object inside an OLE DLL, or referencing a property of the member object may give a General Protection Fault or OLE Automation Error depending upon how the member object was declared and whether or not it has been instantiated.

The problem discussed in this article only occurs when the server is compiled as an OLE DLL (In-Proc server) and the client has been compiled as an EXE.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been fixed in Visual Basic 6.0.

MORE INFORMATION

The General Protection Fault can occur if the member object was constructed by declaring it as a property using the New keyword in the declarations section of a class module, and if the DLL code has not instantiated the object by referring to a property or method of that object before the client application is accessing the member object.

In a class module, it is possible to create a member object by placing a declaration like so:

   Public x as New Class2 

RESOLUTION

By implementing the member object property so that it is instantiated upon startup of the parent class to the member, this problem can be avoided.

1. To fix this problem, the MemberObj object can be dimensioned in the General Declarations section of a Class1 module as below:

   
      Public MemberObj as Class2
    

   Then, the Set statement is used to instantiate the member object in the Initialize event of the parent class. Also, the member object should be released in the Terminate event:

   
      Private Class1_Initialize()
      Set MemberObj = New Class2
   
      End Sub
   
      Private Class1_Terminate()
      Set MemberObj = Nothing
      End Sub
    

   Public Collection objects should also be implemented in the same fashion in an OLE DLL. For example, if there is to be a Collection object as a member object to a class, using the above workaround, the declaration below should NOT be used:

   
      Public x as New Collection
    

   Rather, the OLE DLL Server should ensure that the Collection object is instantiated before the client begins using it. It would be accomplished in the same way as the example above. In the General Declarations section of the Class1 module

   
      Public x as Collection
    

   and then, in the Initialize event of Class1, the Collection member object is instantiated:

   
      Private Class1_Initialize()
      Set x = New Collection
      End Sub
   
      Private Class1_Terminate()
      Set x = Nothing
      End Sub
    

   The latter method ensures that the member object will be correctly instantiated and avoids a potential problem of accessing it from a client.



2. A second workaround involves keeping the original declaration for a member object, but referring to a property or method of the member object within the Initialize event of the parent class. When the property or method of the member object is invoked, the object will be correctly instantiated.
   
   For example, to implement a Collection class, it could be declared within the parent class as

   
      Public x as New Collection
    

   and then referred to in the initialize event of the parent class, so that the member object becomes instantiated:

   
      Sub Class1_Initialize()
      Dim I As Integer
      I = x.Count
      End Sub
    

Steps to Reproduce Problem

1. Start a new project in Visual Basic. Form1 is created by default.



2. Insert two class modules into the project by choosing Insert.Class Module from the menu. Also, insert a standard code module by choosing Insert.Module.



3. For both Class Modules, change the Instancing property to 2-Createable MultiUse and the Public property set to True.



4. In Class1, place the following line of code in the General Declarations section:

   
      Public MemberObj as New Class2
    




5. Inside the standard code module place the following two lines of code:

   
      Sub Main()
      End Sub
    




6. From the menu choose Tools.Options, and in the Project tab select the OLE Server radio button, and change the name of the Project to DLLGPF. Also, for the Startup Form, select Sub Main() from the choices. Press the OK button.



7. Compile the project as an OLE DLL.

Create the Client Application

1. Start a new Project in Visual Basic. Form1 is created by default.



2. In the Form_Click event, place the following code:

   
      Dim x as Object
      Dim y as New DLLGPF.Class1
   
      Set x = y.MemberObj
    




3. Choose Tools.References, and select the checkbox corresponding to DLLGPF.



4. From the menu, choose File.Make EXE and call the executable Client.exe.

Additional query words: kbVBp400bug kbVBp600fix kbdss kbVBp kbDSupport kbNoKeyWord

Keywords : kbGrpVB
Version :
Platform : NT WINDOWS
Issue type : kbbug