Polymorphism (VB.NET)

Module Module1

    Sub Main()
        ' Create some objects
        Dim dObj() = New DrawingObject(1) {}

        dObj(0) = New Circle(50)
        dObj(1) = New Square(10, 10)

        For Each drawObj As DrawingObject In dObj

            Select Case drawObj.GetType().Name
                Case "Circle"
                    Dim circle As Circle = DirectCast(drawObj, Circle)
                    Console.WriteLine(String.Format("I'm a {0} with area of {1} derived from {2}", circle.Draw(), circle.Area(), circle.DrawBase()))
                Case "Square"
                    Dim square As Square = DirectCast(drawObj, Square)
                    Console.WriteLine(String.Format("I'm a {0} with area of {1} derived from {2}", square.Draw(), square.Area(), square.DrawBase()))
            End Select

        Next
    End Sub

    Public MustInherit Class DrawingObject
    ' Area() function must be over ridden in derived classes to provide specific functionality
        Public MustOverride Function Area() As Double
    ' DrawBase() function is available from all derived classes
        Public Function DrawBase() As String
            Return "DrawingObject"
        End Function

    End Class

    Public Class Square
        Inherits DrawingObject

        Public Property Width() As Integer
        Public Property Length() As Integer

        Public Sub New(w As Integer, l As Integer)
            width = w
            length = l
        End Sub

        Public Function Draw() As String
            Return "Square"
        End Function

        Public Overrides Function Area() As Double
            Return Width * Length
        End Function
    End Class

    Public Class Circle
        Inherits DrawingObject

        Public Property Radius() As Integer

        Public Sub New(r As Integer)
            Radius = r
        End Sub

        Public Function Draw() As String
            Return "Circle"
        End Function

        Public Overrides Function Area() As Double
            Return (Math.Pow(Radius, 2) * Math.PI)
        End Function
    End Class

End Module