Imports System

' Here's a basic interface for geometric shapes.
Public Interface IGeometry
    Function Area() As Double
    Function Perim() As Double
End Interface

' For our example we'll implement this interface on
' Rectangle and Circle types.
Public Class Rectangle
    Implements IGeometry

    Public Property Width As Double
    Public Property Height As Double

    ' To implement an interface in VB.NET, we need to
    ' implement all the methods in the interface.
    Public Function Area() As Double Implements IGeometry.Area
        Return Width * Height
    End Function

    Public Function Perim() As Double Implements IGeometry.Perim
        Return 2 * Width + 2 * Height
    End Function
End Class

Public Class Circle
    Implements IGeometry

    Public Property Radius As Double

    ' The implementation for Circle.
    Public Function Area() As Double Implements IGeometry.Area
        Return Math.PI * Radius * Radius
    End Function

    Public Function Perim() As Double Implements IGeometry.Perim
        Return 2 * Math.PI * Radius
    End Function
End Class

Public Module Program
    ' If a variable has an interface type, then we can call
    ' methods that are in the named interface. Here's a
    ' generic Measure function taking advantage of this
    ' to work on any IGeometry.
    Sub Measure(g As IGeometry)
        Console.WriteLine(g)
        Console.WriteLine(g.Area())
        Console.WriteLine(g.Perim())
    End Sub

    Sub Main()
        Dim r As New Rectangle With {.Width = 3, .Height = 4}
        Dim c As New Circle With {.Radius = 5}

        ' The Circle and Rectangle types both
        ' implement the IGeometry interface so we can use
        ' instances of these types as arguments to Measure.
        Measure(r)
        Measure(c)
    End Sub
End Module