Polymorphism basics
Learn that the same call behaves differently on different types.
What is polymorphism?
Polymorphism means the same call behaves differently on different types. If a base Shape class marks its Draw method as virtual, a derived Circle can replace it with override.
The key point: when you write Shape s = new Circle();, the call s.Draw() looks at the object's runtime type and runs Circle.Draw. This delayed choice is the heart of polymorphism. The same code, different objects, different results.
A base reference holds a derived object
The practical side of polymorphism is this: a base class reference can hold a derived object. A List<Shape> can contain Circle, Square and Triangle.
Then a single foreach loop calls Draw on each. Every object runs its own drawing method. This makes the code type-agnostic: when a new shape is added, you do not touch the loop.
Exercise Polymorphism with Shape and Circle
Define a Shape class with a virtual void Draw() method that prints Console.WriteLine("Drawing shape"). Define Circle : Shape that overrides Draw and prints Console.WriteLine("Drawing circle"). In Main put both in a List<Shape> and call Draw in a foreach.
Show example solution
Shape { public override void Draw() { Console.WriteLine("Drawing circle"); } }. In Main build a List<Shape> and call Draw in a foreach.
Example solution:
using System;
using System.Collections.Generic;
class Shape
{
public virtual void Draw()
{
Console.WriteLine("Drawing shape");
}
}
class Circle : Shape
{
public override void Draw()
{
Console.WriteLine("Drawing circle");
}
}
class Program
{
static void Main()
{
var shapes = new List<Shape> { new Circle(), new Shape() };
foreach (var s in shapes)
{
s.Draw();
}
}
}Test yourself
-
What does this code print?
Shape s = new Circle(); s.Draw(); // Circle.Draw overrides- Circle.Draw runs
- Shape.Draw runs
- A compile error occurs
- A runtime error is thrown
Show answer
Circle.Draw runs — Correct. The runtime type of the object is selected: Circle.Draw.
-
The overridable keyword
public ____ void Draw() { }Show answer
public virtual void Draw() { } -
Match each keyword to its meaning
- virtual
- Can be overridden
- override
- Replaces base method
- sealed
- Stops further deriving
- base
- Calls the base class