Q143261: FIX: Error When Referring to Member Object/Collection in OLE DLL
Article: Q143261
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0
Operating System(s):
Keyword(s): kbnokeyword kbVBp kbVBp400bug kbVBp600fix kbGrpDSVB kbDSupport
Last Modified: 22-APR-2000
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The information in this article applies to:
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0
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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.
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).
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
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.
Additional query words:
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Keywords : kbnokeyword kbVBp kbVBp400bug kbVBp600fix kbGrpDSVB kbDSupport
Technology : kbVBSearch kbAudDeveloper kbVB400Search kbVB400
Version : WINDOWS:4.0
Issue type : kbbug
Solution Type : kbfix
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