Microsoft KB Archive/143261

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

Article ID: 143261

Article Last Modified on 10/15/2002

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APPLIES TO


 * Microsoft Visual Basic 4.0 32-Bit Enterprise Edition

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This article was previously published under Q143261



SYMPTOMS
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.



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.

  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.   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:

 Start a new project in Visual Basic. Form1 is created by default. 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. For both Class Modules, change the Instancing property to 2-Createable MultiUse and the Public property set to True.  In Class1, place the following line of code in the General Declarations section: Public MemberObj as New Class2

</li>  Inside the standard code module place the following two lines of code: Sub Main End Sub

</li> 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.</li> Compile the project as an OLE DLL.</li></ol>

Create the Client Application
 Start a new Project in Visual Basic. Form1 is created by default.</li>  In the Form_Click event, place the following code: Dim x as Object Dim y as New DLLGPF.Class1

Set x = y.MemberObj

</li> Choose Tools.References, and select the checkbox corresponding to DLLGPF.</li> From the menu, choose File.Make EXE and call the executable Client.exe.</li></ol>

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).

<div class="status_section">

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.

<div class="moreinformation_section">

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

Here x is a member object (of type Class2). However, it will not be instantiated until a property or method of x is invoked.

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.

Keywords: kbbug kbfix kbvbp600fix KB143261

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