Class Inheritance Basics

Class Inheritance is a way to extend a class. The new class (child class) builds or overwrites logic from the parent class (super class).

The syntax for class extension is:

Overwriting methods:

Overwritten method is a method that has been inherited from the parent but it's logic is changed in the child.

Syntax for overwriting function:

Let's have a base class Vehicle and child classes - Car, Scooter, Bicycle. We'll set a simplified formula for the insurance cost.

using System;

namespace ConsoleApp1
{
    class Vehicle
    {
        public double getInsuranceCost(int months)
        {
            return 0;
        }
    }

// Scooter inherits Vehicle.
    class Scooter : Vehicle
    {
        // overwriting method
        public double getInsuranceCost(int months)
        {
            return months * 15;
        }
    }

// Car inherits Vehicle.
    class Car : Vehicle
    {
        // overwriting method
        public double getInsuranceCost(int months)
        {
            return months * 25;
        }
    }

// Bicycle inherits Vehicle.
    class Bicycle : Vehicle
    {
        // the class uses getInsuranceCost()
        // inherited from Vehicle.
    }

class Program
    {
        static void Main(string[] args)
        {
            Scooter scooter = new Scooter();
            Console.WriteLine("Scooter Year Insurance: " +
                    scooter.getInsuranceCost(12) + " $");

Car car = new Car();
            Console.WriteLine("Car Year Insurance: " +
                    car.getInsuranceCost(12) + " $");

Bicycle bike = new Bicycle();
            Console.WriteLine("Bicycle Year Insurance: " +
                    bike.getInsuranceCost(12) + " $");
        }
    }
}

Output:

Explanation:

At line 14 the class Scooter inherits the Vehicle class. And at lines 17 to 20 the function getInsuranceCost is overwritten.
At line 24 the class Car extends the Vehicle class. The Car class overwrites getInsuranceCost function at lines 27 - 30.
At line 34 the Bicycle class extends Vehicle. It doesn't overwrite the getInsuranceCost method. When called (line 54) the code from the base class is executed.

Extending functions and constructors:

Let's have a base class Vehicle and child class Car. The child class will add one more member and also extend a constructor and a function called representation.

using System;

namespace ConsoleApp1
{
    class Vehicle
    {
        protected string brand;
        protected string model;

public Vehicle(string brandValue, string modelValue)
        {
            brand = brandValue;
            model = modelValue;

Console.WriteLine("Vehicle constructor");
        }

public void representation()
        {
            Console.WriteLine( "Brand: " + brand);
        }
    }

// Scooter inherits Vehicle.
    class Car : Vehicle
    {
        private int horsePowers;

public Car(string brandValue, string modelValue, int horsePowersValue)
    : base(brandValue, modelValue)
        {
            Console.WriteLine("Car constructor");

horsePowers = horsePowersValue;
        }

public void representation()
        {
            base.representation();

Console.WriteLine("Models: " + model);
           Console.WriteLine("Horse Powers: " + horsePowers);
        }
    }

class Program
    {
        static void Main(string[] args)
        {
            Vehicle sampleBicycle = new Vehicle("Merida ", "ONE-FORTY 800");
            sampleBicycle.representation();
            Console.WriteLine("--------------------");

Car fordCar = new Car("Ford", "Fiesta", 80);
            fordCar.representation();
            Console.WriteLine("--------------------");

Car toyotaCar = new Car("Toyota", "Corolla", 100);
            toyotaCar.representation();
        }
    }
}

Output:

Explanation:

At line 27 instead of private the members are defined as protected. That's because we want to extend the class and inherit it's members. More details about it later in the lesson.
At line 25 the class Car inherits Vehicle.
The Car class has 3 members - horsePowers defined at line 27 and also brand and model (lines 7 and 8) due to the class extension.
The Car class constructor inherits the vehicle constructor at line 30. When car object is created (lines 54 and 58) the code from the parent's class constructor (lines 12 - 15) executes first and afterwards the code from child's constructor (lines 32 - 34).
At line 39 the representation function of the parent class is called. When representation function is called for a Vehicle object at line 51, only the brand is printed.
When representation function is called for a Car object at lines 55 and 59, all 3 members (make, model, horsePowers) are printed.

Private vs protected members

Both private and protected members can be accessed or modified only in a class body.

Private Members can be accessed or modified only in one class - the one they are defined in.

Protected members can be accessed or modified in the class they are defined in and the classes that extend it.

Let's look at the example:

Due to the extension class Car has 3 members - brand, model and horsePowers.
If at lines 5 and 6 protected is replaced with private, the class Car will have only one member - horsePowers and the code won't compile due to lines 15 and 16.