FIX: Error When Referring to Member Object/Collection in OLE DLL (143261)

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

Create the OLE Server Application:

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.

Upon running Client.exe and clicking on Form1, a General Protection Fault occurs.

On Windows 95 or Windows 98, a General Protection Fault generally occurs (in one test this occurred in Vb40032.dll at 0137:0f7269d9). On Windows NT 3.51, a General Protection Fault can occur, and also a Run-time error '-2146500594 (800f000e)' can occasionally occur.

Notably, the General Protection Fault or OLE Automation Error does not occur if the client application is run from the Design Environment, or if the OLE Server is implemented as an Out-of-Proc (EXE) OLE server.

Another example of reproducing a General Protection Fault can occur if a Collection object is a member object of the OLE DLL, and if the Client application refers to the Count property of the Collection object before the member collection object has been instantiated (the count of the Collection object will be zero when this is the case).

   Public x as New Class2