Interfaces and Casting, Forms and Panels

SA -


I appreciate your comment on my technique. However, I do not know what the "suspect" part of code would be that I have not presented. To that end, I created a virgin VB Express project and modified the code for class Form1 as follows:

Public Class Form1
Implements IMyControlInterface

'...

End Class

Public Interface IMyControlInterface

End Interface

Public Class MyControl
Inherits Control

Private MyParentControl As IMyControlInterface


Protected Overrides Sub OnParentChanged(ByVal e As System.EventArgs)

If MyBase.Parent IsNot Nothing Then

Me.MyParentControl = TryCast(MyBase.Parent, IMyControlInterface)

If Me.MyParentControl Is Nothing Then

Throw New Exception("Control can only be added to an object that implements IMyControlInterface.")

End If
End If
MyBase.OnParentChanged(e)
End Sub

End Class

Public Class MyControlPanel
Inherits Panel
Implements IMyControlInterface


'...
End Class

This is basically the same as the original code snippets above, except that I added inheritance for class MyControl (as Control) and included a blank implementation of IMyControlInterface (as it is, so far, in my own code). I still get the same results: dragging MyControl to Form1 invokes the exception, and dragging it to MyControlPanel1 on Form1 does not.

Perhaps Manfred's answer is correct, although I do not pretend to understand its reasoning. I have also responded to that answer, to see if Manfred (or anyone else) can offer an alternative...