Microsoft KB Archive/172329

= PRB: Error 459 Trying to Use Alternate Object Interface =

Article ID: 172329

Article Last Modified on 5/13/2003

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APPLIES TO


 * Microsoft Visual Basic 6.0 Learning Edition
 * Microsoft Visual Basic 5.0 Learning Edition
 * Microsoft Visual Basic 6.0 Professional Edition
 * Microsoft Visual Basic 5.0 Professional Edition
 * Microsoft Visual Basic 6.0 Enterprise Edition
 * Microsoft Visual Basic 5.0 Enterprise Edition
 * Microsoft Visual Basic 5.0 Control Creation Edition

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This article was previously published under Q172329



SYMPTOMS
You get run-time error 459 "Object or Class does not support the set of elements" when assigning an object of class "A" to an object variable of class "B" in order to use an alternate interface.



CAUSE
The base class you are implementing in your object defines an Event object and your object variables are declared using the WithEvents keyword. The Visual Basic versions listed above do not support events in inherited interfaces.



RESOLUTION
This is a limitation of the versions of Visual Basic listed above.



STATUS
This behavior is by design.



Steps to Reproduce Behavior
 Create a form with a command button and two class modules (Base_Class and Derived_Class).  Add the following code to the Base_Class class module: Event Alert(ByVal Msg As String)

Public Sub PostAlert(ByVal Msg As String) End Sub

  Add the following code to the Derived_Class class module: Implements Base_Class

Event Alert(ByVal Msg As String)

Private Sub Base_Class_PostAlert(ByVal Msg As String) RaiseEvent Alert(Msg) End Sub

  In the form module, add the following code: Dim WithEvents objDC As Derived_Class, WithEvents objBC As Base_Class

Sub Command1_Click Set objDC = New Derived_Class Set objBC = objDC             ' error 459 occurs on this line objBC.PostAlert "Some Message" Set objBC = Nothing Set objDC = Nothing End Sub

Sub DC_Alert(ByVal Msg As String) MsgBox Msg End Sub



Notes:

<ul> If you remove the WithEvents keyword from the objBC variable declaration, the code will run.</li> If you remove the Event declaration in the Base_Class class module, the compiler will also require you to remove the WithEvents keyword on the variable declaration, and the code will run.</li> If you remove the Event declaration in the Derived_Class class module, you will get a compile error.</li>  You cannot implement the Derived_Class event handler as a Private event or derived event as you can with methods. For example, either of the following in the Derived_Class class module will result in a compile error: Private Event Base_Class_Alert(ByVal Msg As String) Event Base_Class_Alert(ByVal Msg As String)

</li></ul>

Additional query words: WithEvents Implements kbdse kbDSupport kbVBp kbVBp500 kbVBp600

Keywords: kbprb KB172329

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