VB.Net/Language Basics/Is

Demonstrate "is a" relationship

Imports System
Public Class MainClass
    Shared Sub Main(ByVal args As String())
      Dim point1, point2 As Point
      Dim circle1, circle2 As Circle
      point1 = New Point(30, 50)
      circle1 = New Circle(120, 89, 2.7)
      Console.WriteLine( "Point point1: " & point1.ToString() & _
         vbCrLf & "Circle circle1: " & circle1.ToString() )
      " use is-a relationship to assign Circle to Point reference
      point2 = circle1
      Console.WriteLine( "Circle circle1 (via point2): " & point2.ToString())
      " downcast (cast base-class reference to derived-class 
      " data type) point2 to circle2
      circle2 = CType(point2, Circle) " allowed only via cast
      Console.WriteLine( "Circle circle1 (via circle2): " & circle2.ToString() )
      Console.WriteLine( "Area of circle1 (via circle2): " & _
         String.Format("{0:F}", circle2.Area()) )
      " assign Point object to Circle reference
      If (TypeOf point1 Is Circle) Then
         circle2 = CType(point1, Circle)
         Console.WriteLine( "cast successful" )
      Else
         Console.WriteLine( "point1 does not refer to a Circle" )
      End If
    End Sub
End Class

Public Class Circle
   Inherits Point " Circle Inherits from class Point
   Private mRadius As Double
   " default constructor
   Public Sub New()
      " implicit call to Point constructor occurs here
      Radius = 0
   End Sub " New
   " constructor
   Public Sub New(ByVal xValue As Integer, _
      ByVal yValue As Integer, ByVal radiusValue As Double)
      " use MyBase reference to Point constructor explicitly
      MyBase.New(xValue, yValue)
      Radius = radiusValue
   End Sub " New
   " property Radius
   Public Property Radius() As Double
      Get
         Return mRadius
      End Get
      Set(ByVal radiusValue As Double)
         If radiusValue >= 0 Then " mRadius must be nonnegative
            mRadius = radiusValue
         End If
      End Set
   End Property " Radius
   " calculate Circle diameter
   Public Function Diameter() As Double
      Return mRadius * 2
   End Function " Diameter
   " calculate Circle circumference
   Public Function Circumference() As Double
      Return Math.PI * Diameter()
   End Function " Circumference
   " calculate Circle area
   Public Overridable Function Area() As Double
      Return Math.PI * mRadius ^ 2
   End Function " Area
   " return String representation of Circle
   Public Overrides Function ToString() As String
      " use MyBase reference to return Circle String representation
      Return "Center= " & MyBase.ToString() & _
         "; Radius = " & mRadius
   End Function " ToString
End Class
Public Class Point
   " point coordinate
   Private mX, mY As Integer
   " default constructor
   Public Sub New()
      " implicit call to Object constructor occurs here
      X = 0
      Y = 0
   End Sub " New
   " constructor
   Public Sub New(ByVal xValue As Integer, _
      ByVal yValue As Integer)
      " implicit call to Object constructor occurs here
      X = xValue
      Y = yValue
   End Sub " New
   " property X
   Public Property X() As Integer
      Get
         Return mX
      End Get
      Set(ByVal xValue As Integer)
         mX = xValue " no need for validation
      End Set
   End Property " X
   " property Y 
   Public Property Y() As Integer
      Get
         Return mY
      End Get
      Set(ByVal yValue As Integer)
         mY = yValue " no need for validation
      End Set
   End Property " Y
   " return String representation of Point
   Public Overrides Function ToString() As String
      Return "[" & mX & ", " & mY & "]"
   End Function " ToString
End Class