Two keywords are used to make Polymorphism happen: virtual and override.

You need to describe a method as virtual if you want the ability to override its capabilities.

class Rectangle:
    def constructor(width as single, height as single):
        _width = width
        _height = height

    virtual def GetArea():
        return _width * _height

    _width as single
    _height as single

class Square(Rectangle):
    def constructor(width as single):
        super(width, width)

    override def GetArea():
        return _width * _width

r = Rectangle(4.0, 6.0)
s = Square(5.0)

print r.GetArea()
print s.GetArea()
print cast(Rectangle, s).GetArea()

// Output:
// 24.0
// 25.0
// 25.0

Even when casted to a Rectangle, s's .GetArea() functioned like if it were a Square.

An easier example to see is this:

class Base:
    virtual def Execute():
        return 'From Base'

class Derived(Base):
    override def Execute():
        return 'From Derived'

b = Base()
d = Derived()

print b.Execute()
print d.Execute()
print cast(Base, d).Execute()

// Output:
// From Base
// From Derived
// From Derived

If I were to leave out the virtual and override keywords,

// Output:
// From Base
// From Derived
// From Base

This happens because unless the base method is virtual or abstract, the derived method cannot be declared as override.

In order to override, the base function must be declared as virtual or abstract, have the same return type, and accept the same arguments.

Polymorphism is very handy when dealing with multiple types derived from the same base.

interface IAnimal:
    def MakeNoise()

class Dog(IAnimal):
    def MakeNoise():
        print 'Woof'

class Cat(IAnimal):
    def MakeNoise():
        print 'Meow'

class Hippo(IAnimal):
    def MakeNoise():
        print '*Noise of a Hippo*'

list = []
list.Add(Dog())
list.Add(Cat())
list.Add(Hippo())

for animal as IAnimal in list:
    animal.MakeNoise()

// Output:
// Woof
// Meow
// *Noise of a Hippo*

Very handy.

Exercises

1. Figure out an exercise