Casting Between Derived Types Throws InvalidCastException

The following Code throws the InvalidCastException error. What I wish to know is why and what the standard workaround is for this situation.

Code:

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Module Module1

    Interface IFooBar
    End Interface

    Class Foo : Implements IFooBar
        Public Shared Widening Operator CType(ByVal Val As Foo) As Bar
            Return New Bar
        End Operator
    End Class

    Class Bar : Implements IFooBar
        Public Shared Widening Operator CType(ByVal Val As Bar) As Foo
            Return New Foo
        End Operator
    End Class

    Sub Main()
        Dim A As Foo = New Foo
        Dim B As Bar = New Bar
        Dim C As IFooBar
        'these assignments work fine with implicit casting
        A = B
        B = A
        C = A
        'InvalidCastException Thrown Here
        B = C
        'this throws the same error
        B = CType(C, Bar)
        'this does not fail but requires pre-compile knowledge of the type of object in C
        B = CType(CType(C, Foo), Bar)
    End Sub

End Module

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What I would like is the line B = C to work without me having to know what type of object C currently is. I need to find a way to get this to work so that i can finish a function I ma working on so that it will excecute correctly on any new object which implements the interface and boasts an Operator CType function to cast it to the type used in the function.

I have been searching a while for a solution but I couldn't find one. It may be that I am missing a key term which would get me straight to the answer. Any help you can give me would be much appreciated. Although I would particularly appreciate some references to articles which might explain this problem and why it occurs as I am just learning Visual Basic.

I don't think what you're trying to do is possible. Interface types are generic. You can't assign one to a specific type (SpecificType=InterfaceType)

I've used Widening operators to do automatic type conversion, but I don't see why you'd ever need to do what you're trying to do. Wouldn't an inherited type be better? Regardless, you'll need to have your function take a base-type and check the type of what you're handing it to determine if it's a Foo or a Bar and how to proceed. If you have common functionality between them though, they should be handled in the basetype; perhaps overridable, but still defined in the basetype. Example:

Code:

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Public Class BaseFooBar
    Public Overridable Sub BaseClassFunction()
    End Sub
End Class

Public Class Foo
    Inherits BaseFooBar
    Public Shared Widening Operator CType(ByVal Val As Bar) As Foo
        Return New Foo
    End Operator

    Public Overrides Sub BaseClassFunction()
        MyBase.BaseClassFunction()
    End Sub
End Class

Public Class Bar
    Inherits BaseFooBar
    Public Shared Widening Operator CType(ByVal Val As Foo) As Bar
        Return New Bar
    End Operator

    Public Overrides Sub BaseClassFunction()
        MyBase.BaseClassFunction()
    End Sub
End Class

Public Module Blah
    Public Sub Main()
        Dim A As Foo = New Foo
        Dim B As Bar = New Bar
        Dim C As BaseFooBar

        A = B
        B = A
        C = A
        MyFunc(A)
        MyFunc(B)
        MyFunc(C)

    End Sub

    Public Sub MyFunc(ByVal Foobar As BaseFooBar)
        Foobar.BaseClassFunction()
    End Sub
End Module

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