'Visual Basic 2010 Redefinition - Invariance, Covariance en Contravariance Static Typing en Dynamic Binding
Class Fruit
    Public Function MethodA() As String
    End Function
End Class
Class Apple : Inherits Fruit
    Public Function MethodB() As String
    End Function
End Class
Class GrannySmith : Inherits Apple
    Public Function MethodC() As String
    End Function
End Class
Class JonaGold : Inherits Apple
    Public Function MethodD() As String
    End Function
End Class

Class BaseClass
    Public Overridable Sub Method1(ByVal argument As Apple)                ' (3)
    End Sub
End Class
Class DerivedClass : Inherits BaseClass
    ' invariance ( unchanged parametertype ) : possible
    Public Overrides Sub Method1(ByVal argument As Apple)                  ' (6)
    End Sub
    ' contravariant ( supertype ) parametertype : expected, but not possible
    Public Overrides Sub Method1(ByVal argument As Fruit)                  ' (1)
        Console.WriteLine(argument.MethodA())                              ' (4)
    End Sub
    ' covariant ( subtype ) parametertype : as expected not possible
    'Public Overrides Sub Method1(ByVal argument As GrannySmith)           ' (7)
    'End Sub
End Class

Class ExpectedContraVariance
    Public Shared Sub Main()
        Dim grannySmith1 As GrannySmith = New GrannySmith
        Dim jonaGold1 As JonaGold = New JonaGold
        '
        Dim object1 As BaseClass = New DerivedClass                        ' (5)
        '
        object1.Method1(grannySmith1)                                      ' (2)
        'object1.Method1(jonaGold1)                                        ' (8)
    End Sub
End Class
Partial Class BaseClass
    Public Overridable Function Method2() As Apple
        Method2 = New Apple
    End Function
End Class
Partial Class DerivedClass : Inherits BaseClass
    ' invariance ( unchanged returntype ) : possible
    Public Overrides Function Method2() As Apple
        Method2 = New Apple
    End Function
    ' covariant ( subtype ) returntype : expected, but not possible
    Public Overrides Function Method2() As GrannySmith
        Method2 = New GrannySmith
    End Function
    ' contravariant ( supertype ) returntype : as expected not possible
    'Public Overrides Function Method2() As Fruit
    '    Method2 = New Fruit                                               ' (1)
    'End Function
End Class

Class ExpectedCoVariance
    Public Shared Sub Main()
        Dim grannySmith1 As GrannySmith = New GrannySmith
        Dim jonaGold1 As JonaGold = New JonaGold
        '
        Dim object1 As BaseClass = New DerivedClass                        ' (5)
        '
        Console.WriteLine(object1.Method2().MethodB())
    End Sub
End Class
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