VB.Net/Language Basics/Is

Demonstrate "is a" relationship

<source lang="vbnet"> 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

      </source>