Error When Referring to Member Object/Collection in OLE DLL

• Enterprise Edition of Microsoft Visual Basic, 32-Bit Edition, 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 an issue in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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

The problem discussed in this article is that x may not be correctly instantiated if a property or method of x is first used in the client application, and x is a member object contained in an OLE DLL. Under these circumstances, a General Protection Fault or OLE Automation Error may occur.

Workaround

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.

On Windows 95, 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).