TargetInvocationException when handling a COM Event

Question

I am developing a COM dll library, and I have a little vb.net (vs 2005) application just to test it.

I had my object declared in the application as

Private m_VarName As MyLib.CMyComClass

So far, so good.

But now, I need an event to inform the application of some things, so I implemented such event in the COM dll, and changed the declaration to

Private WithEvents m_VarName As MyLib.CMyComClass

So far, so good, again. But if I add a Sub to handle my event:

Private Sub m_VarName_OnCaptureStop() Handles m_VarName.MyEvent

...

End Sub

The first time I create the object, nothing bad happens, but if I reinstantiate it

If (Not m_VarName Is Nothing) Then ReleaseComObject(m_VarName)

m_VarName= New MyLib.CMyComClass

then I get a cryptic TargetInvocationException, seemingly related to reflection (which, AFAIK, I am not using).

If I remove the "Handles m_VarName.MyEvent" part, everything seems to work. In case it matters, I am not firing any event, for now. Any idea about why this happens?

Answer 1

The reason why has to do with the implementation of WithEvents and Handles in VB.Net. When you declare a field with the WithEvents modifier in VB.net it will be generated as a property.

Whenever that property is updated via an assignment operation the property setter will unsubscribe from the old event handler and then subscribe to the event handler on the new value. In effect it looks like this

Property m_VarName as SomeType 
  Set   
    if _m_VarName isNot Nothing Then
      RemoveHandler _m_VarName.SomeEVent,m_VarName_OnCaptureStop
    End If
    _m_VarName = Value
    AddHandler _m_VarName.SomeEvent,m_VarName_OnCaptureStop
  End Set 
End Property

If you consider that in the context of your code what's essentially happening is the following

ReleaseComObject(m_VarName)
RemoveHandler m_VarName.SomeEvent,m_VarName_OnCaptureStop

So you're calling RemoveHandler on a COM object which is already released. Hence it throws an Exception.

The fix is simple, don't call ReleaseComObject. The number of situations where you actually need to call this method are very small and almost certainly not applicable to this situation. Instead just let the GC collect it normally.