Polymorphism

Syntax

BaseType variable = new DerivedType();

Examples

public class Animal
{
    public virtual void MakeSound() => Console.WriteLine("Some generic sound");
}

public class Dog : Animal
{
    public override void MakeSound() => Console.WriteLine("Woof!");
}

public class Cat : Animal
{
    public override void MakeSound() => Console.WriteLine("Meow!");
}

Animal[] animals = { new Dog(), new Cat(), new Animal() };
foreach (Animal a in animals)
{
    a.MakeSound();  // calls the correct version for each actual object type
}
// Woof!
// Meow!
// Some generic sound

public interface IShape
{
    double Area();
}

public class Circle : IShape
{
    private double _radius;
    public Circle(double radius) => _radius = radius;
    public double Area() => Math.PI * _radius * _radius;
}

public class Square : IShape
{
    private double _side;
    public Square(double side) => _side = side;
    public double Area() => _side * _side;
}

IShape[] shapes = { new Circle(3), new Square(4) };
foreach (IShape s in shapes)
{
    Console.WriteLine($"Area: {s.Area():F2}");
}

public class Employee
{
    public string Name;
    public Employee(string name) => Name = name;
    public virtual double CalculatePay() => 0;
}

public class FullTime : Employee
{
    public FullTime(string name) : base(name) { }
    public override double CalculatePay() => 5000;
}

public class Contractor : Employee
{
    public Contractor(string name) : base(name) { }
    public override double CalculatePay() => 3000;
}

void PrintPay(Employee e)  // works with ANY Employee subclass
{
    Console.WriteLine($"{e.Name}: ${e.CalculatePay()}");
}

PrintPay(new FullTime("Fola"));
PrintPay(new Contractor("Zain"));

Best practices

• Declare variables, parameters, and return types using the base type or interface when you want to write flexible, extensible code
• Rely on polymorphism instead of type-checking with "is" and manual casting where possible - let overridden methods handle the differences
• Remember polymorphism applies to virtual/overridden instance methods, not fields or static members, which are resolved based on the declared type instead
• Design base classes/interfaces around behavior that genuinely varies by subclass, so polymorphism adds real value rather than just ceremony

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation