Inheritance

Inheritance #

Inheritance is an important feature of Object Oriented Programming. It enables you to create a new class while reusing the members of other classes. The class whose members are inherited is called the base class and the class that inherits those members is called the derived class.


Duck and Penguin classes inherit from Bird class, both have the Fly method which is derived from the Bird class.

Note that we can re-use common methods in the derived classes without having to rewrite them.

public class Bird{
    public void Fly(){
        Console.WriteLine("A bird is flying");
    }
}

public class Duck : Bird{
    public void Quack(){
        Console.WriteLine("A duck is quacking");
    }
}

public class Penguin : Bird{
    public void Swim(){
        Console.WriteLine("A penguin is swimming");
    }
}


public class Program{
    public static void Main(string[] args){
        var bird = new Bird();
        bird.Fly();

        var duck = new Duck();
        duck.Quack();   // output: A duck is quacking
        duck.Fly();     // output: A bird is flying

        var penguin = new Penguin();
        penguin.Swim();
        penguin.Fly();
    }
}

The following example shows that Car, Truck, and Bycicle classes inherit the Vehicle class.

  • They overrides the Move method, overriding is a rewriting of the method in the base class.
  • Only virtual methods can be overridden.
  • we can call members of the base class with base keyword. It’s equivalent to super keyword in Java.
public class Vehicle
{
    public int Speed { get; set; }
    public virtual void Move()
    {
        Console.WriteLine($"Vehicle move {Speed} per second");
    }
}

public class Car : Vehicle
{
    public Car()
    {
        Speed = 10;
    }
    public override void Move()
    {
        Console.WriteLine($"Car move {Speed} per second");
        base.Move();
    }
}

public class Truck : Vehicle
{
    public Truck()
    {
        Speed = 5;
    }
    public override void Move()
    {
        Console.WriteLine($"Truck move {Speed} per second");
        base.Move();
    }
}

public class Bycicle : Vehicle
{
    public Bycicle()
    {
        Speed = 2;
    }
    public override void Move()
    {
        Console.WriteLine($"Bycicle move {Speed} per second");
        base.Move();
    }
}

They can be treated as Vehicle type.

I hope you already know about Polymorphism.

List<Vehicle> vehicles = new List<Vehicle>();
vehicles.Add(new Car());
vehicles.Add(new Truck());
vehicles.Add(new Bycicle());

foreach (var vehicle in vehicles) {
    vehicle.Move();
}
// output:
// Car move 10 per second
// Vehicle move is called
// Truck move 5 per second
// Vehicle move is called
// Bycicle move 2 per second
// Vehicle move is called

A class can only inherit from one base class. 

public class A {}
public class B : A {}
public class C : A {}

// ERROR! Multiple inheritance is not allowed
public class D : B, C {}