Q255625: BUG: Err Object Returns Zero Number and Empty Description.

Article: Q255625
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbActiveX kbCOMt kbVBp kbVBp600bug kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, version 6.0 
- Microsoft Visual Basic Professional Edition for Windows, version 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
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SYMPTOMS
========

After an object raises a trappable error, the client checks the Err object and
finds that the error number is zero and the error description is empty.

CAUSE
=====

This problem occurs only when the client application is compiled to native code
and when the client does not have a local or global variable that stores a
reference to the object that raises the error.

Assume that you have an object called Object1 and it has a property called
ChildObject that returns a reference to another object, Object2. If you call a
method on Object2 using a syntax such as:

Object1.ChildObject.Method

you see this type of behavior because no variable holds a reference to Object2.

RESOLUTION
==========

The workaround for this problem is to assign an object to a variable and avoid
using the preceding syntax. Do the following:

  Dim MyObject as Object2

  Set MyObject = Object1.ChildObject
  ' call the method using the MyObject variable
  MyObject.Method

Another workaround is to compile your project to p-code.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

1. Start a new ActiveX DLL project. Class1 is created by default.

2. From the Project menu, select Project1 Properties, and then set the name to
  "MyDll" (without the quotation marks).

3. From the Project menu, add a new class module to this project. This is called
  Class2 by default.

4. Paste the following code into the Class1 code module:

  Option Explicit
  Public Sub DoSomething
      Err.Raise 101
  End Sub

  Private Sub Class_Terminate
      Dim i as Long

      ' if you comment the following line, the error object is not reset
      On Error Resume Next
  	
      i=0
  End Sub

5. Paste the following code into the Class2 code module:

  Option Explicit
  Public Property Get ChildObject() As Class1
      Set ChildObject = New Class1
  End Property

6. Save the project and compile it as "MyDll.Dll" (without the quotation marks).

7. Start a new Standard EXE project. Form 1 is created by default.

8. From the Project menu, select References, select MyDll, and then click OK to
  add a reference to the DLL you just created.

9. Add a CommandButton to Form1. This is Command1 by default.

10. Paste the following code into Form1's code module:

  Option Explicit
  Private Sub Command1_Click()
     Dim MyObj As New MyDll.Class2
     On Error GoTo p1
     MyObj.ChildObject.DoSomething
     Exit Sub
  p1:
     MsgBox "Err.Number=" & Err.Number
  End Sub

11. From the Project menu, select Project1 Properties, and then select the
  Compile tab. Select Compile to P-Code.

12. From the File menu, select Make to compile the Standard EXE project to
  p-code. Run the executable and click on Command1. Note that you get a
  message box with "Err.Number=101" as expected.

13. Dismiss the message box and stop the project to return to the design
  environment.

14. From the Project menu, select Project1 Properties, and select the Compile
  tab. Select Compile to Native Code.

15. From the File menu, select Make to compile the project to native code. Run
  the executable and click on Command1. Note that you get a message box with
  "Err.Number=0" which is not as expected.

The problem is caused by the Class_Terminate event. When you use the syntax
Object1.ChildObject.Method, Object2 gets released before the next line of code
is executed, which triggers the Class_Terminate event, and the Err object is
reset in the terminate event procedure. If you comment out the line that uses
the Err object in this event, the problem does not happen.

Additional query words:

======================================================================
Keywords          : kbActiveX kbCOMt kbVBp kbVBp600bug kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600
Version           : WINDOWS:6.0
Issue type        : kbbug
Solution Type     : kbnofix

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