Polymorphism in Visual Basic

Overview

Polymorphism allows objects to be treated as instances of their base type while still exhibiting behavior defined by their derived types. In Visual Basic, polymorphism is achieved through inheritance, Overrides, Overloads, and the Implements keyword.

Key Concepts

Late Binding – Determined at runtime using Object or Dynamic types.
Early Binding – Determined at compile time through inheritance hierarchies.
Method Overriding – Derived class provides a new implementation of a base class method using Overrides.
Method Overloading – Multiple methods with the same name but different signatures using Overloads.

Example: Method Overriding

Public Class Animal
    Public Overridable Sub Speak()
        Console.WriteLine("The animal makes a sound.")
    End Sub
End Class

Public Class Dog
    Inherits Animal

    Public Overrides Sub Speak()
        Console.WriteLine("Woof!")
    End Sub
End Class

Module Program
    Sub Main()
        Dim a As Animal = New Dog()
        a.Speak()   ' Outputs: Woof!
    End Sub
End Module

Example: Method Overloading

Public Class Calculator
    Public Overloads Function Add(a As Integer, b As Integer) As Integer
        Return a + b
    End Function

    Public Overloads Function Add(a As Double, b As Double) As Double
        Return a + b
    End Function
End Class

Module Demo
    Sub Main()
        Dim calc As New Calculator()
        Console.WriteLine(calc.Add(3, 4))          ' Integer result: 7
        Console.WriteLine(calc.Add(2.5, 3.1))      ' Double result: 5.6
    End Sub
End Module

Interface Implementation

Implementing an interface provides another form of polymorphism. Objects that implement the same interface can be used interchangeably.

Public Interface IShape
    Function Area() As Double
End Interface

Public Class Circle
    Implements IShape
    Private ReadOnly _radius As Double

    Public Sub New(radius As Double)
        _radius = radius
    End Sub

    Public Function Area() As Double Implements IShape.Area
        Return Math.PI * _radius * _radius
    End Function
End Class

Public Class Rectangle
    Implements IShape
    Private ReadOnly _width As Double
    Private ReadOnly _height As Double

    Public Sub New(width As Double, height As Double)
        _width = width
        _height = height
    End Sub

    Public Function Area() As Double Implements IShape.Area
        Return _width * _height
    End Function
End Class

Module ShapesDemo
    Sub Main()
        Dim shapes As IShape() = {New Circle(3), New Rectangle(4, 5)}
        For Each s In shapes
            Console.WriteLine(s.Area())
        Next
    End Sub
End Module