Let's make a simple example for a Coordinate
The Base-Class
You can the that this must be a base-class by the presence of the MustInherit.
Public MustInherit Class Coordinate Protected Friend Sub New() End Sub Public MustOverride Overrides Function ToString() As String End Class
1D_X
1D_X <: Coordinate
This is a sub-class of the Coordinate base-class.
Public Class 1D_X
Inherits Coordinate
Private _X As Integer = 0
Public ReadOnly Property X As Integer
Get
Return _X
End Get
End Property
Public Sub New(X As Integer)
MyBase.New()
_X = X
End Sub
Public Overrides Function ToString() As String
Return String.Format("X:= {0}", Me.X)
End Function
End Class
Spoiler
2D_XY
2D_XY <: 1D_XY <: Coordinate
So this inherits from the features of the base-class 1D_XY which in turn inherits from the base-class Coordinate.
Public Class XY
Inherits 1D_X
Private _Y As Integer = 0
Public ReadOnly Property Y As Integer
Get
Return _Y
End Get
End Property
Public Sub New(X As Integer, Y As Integer)
MyBase.New(X) ' Call the Constructor of the base-class I'm inheriting from.
_Y = Y
End Sub
Public Overrides Function ToString() As String
Return MyBase.ToString() + " " + String.Format("Y:= {0}", Me.Y)
End Function
End Class
3D_XYZ
You'll notice that this example has the keyword NotInheritable modifier on the class name.
This means that the class can't be used as a base-class.
Public NotInheritable Class 3D_XYZ
Inherits 2D_XY
Private _Z As Integer = 0
Public Sub New(X As Integer, Y As Integer, Z As Integer)
MyBase.New(X, Y) ' Call the Constructor of the base-class I'm inheriting from.
End Sub
Public ReadOnly Property Z As Integer
Get
Return _Z
End Get
End Property
Public Overrides Function ToString() As String
Return MyBase.ToString() + " " + String.Format("Z:={0}", Me.Z)
End Function
End Class
Polar
Public Class Polar
Inherits Coordinate
Public Sub New(Angle As Double, Distance As Double)
MyBase.New()
_Angle = Angle : _Distance = Distance
End Sub
Private _Angle As Double
Public Property Angle() As Double
Get
Return _Angle
End Get
Set(ByVal value As Double)
_Angle = value
End Set
End Property
Private _Distance As Double
Public Property Distance() As Double
Get
Return _Distance
End Get
Set(ByVal value As Double)
_Distance = value
End Set
End Property
Public Overrides Function ToString() As String
Return String.Format("Angle:={0} Distance:={1}", Me.Angle, Me.Distance)
End Function
End Class
Let's add another function to the base-class coordinate.
Public Overridable Function What() As String
Return "Some Default Message"
End Function
You'll notice it has the modifier Overridable this means that it can be overriden by the sub-class, if you don't provide one the one provided by the base-class is used.
To see the effect of this add this to 2D_XY
Public Overrides Function What() As String
Return "Overridden What Method"
End Function
Sample Usage
A little Console Application demo.
Module Module1
Sub Main()
Dim x As New X(1)
Dim xy As New XY(2, 3)
Dim xyz As New XYZ(4, 5, 6)
Dim p As New Polar(90, 10)
Dim coords() As Coordinate = {x, xy, xyz, p}
For Each c In coords
Console.WriteLine(c.ToString())
Console.WriteLine(c.What())
Console.WriteLine()
Next
Console.ReadKey()
End Sub
End Module
Outputs
X:= 1 Some Default Message X:= 2 Y:= 3 Overridden What Method X:= 4 Y:= 5 Z:=0 Overridden What Method Angle:=90 Distance:=10 Some Default Message
You'll notice the two Overridden What Method when we only wrote one in the code.
Where did it come from?
Spoiler
Limitations
- Single Inheritance
You can only inherit from one base-class. - Non-Inherited Constructors
The Constructor(s) of the base-class are not inherited by the sub-class, thus you to implement them and call the base-class's constructor in the sub-class's constructor.
So Thank-You reading this tutorial on Inheritance.
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