Understanding Liskov Substitution

Introduction

Here I am going to discuss Liskov substitution principle of SOLID.

Background

If you read my previous two articles it will be better for you to understand SRP,OCP,ISP,DIP of SOLID.

• Understanding Open Closed Principle and Dependency Inversion
• Understanding Single Responsibility and Interface Segregation

Using the Code

Before start technical discussion I want to answer the below questions:

What is Liskov Substitution principle?

Answer:"Objects  in a program should be replaceable with their subtypes without alerting the correctness of that program"

Is my answer tough to understand? OK,make it easier.

"It means that we must make sure that new derived classes are extending the base classes without changing their behavior".

Let's consider an example to make it better for understanding.Suppose I buy a computer(desktop) from a Computer Shop after view desktop and laptop. Now  I ask for desktop screen size. Let's convert my requirements into code.

using System;

namespace LiskovSubstitution
{
    public class Computer
    {
        public string GetComputerDiscripution()
        {
            return "You get a desktop";
        }
        public string GetColor()
        {
            return "Color is white";
        }
        public virtual string GetScreen()
        {
            return "LCD 19 inch";
        }
    }

    public class Laptop:Computer
    {
        public new string GetComputerDiscripution()
        {
            return "You get a laptop";
        }
        public string GetColor()
        {
            return "Color is black";
        }
        public override string GetScreen()
        {
            return "LED 19 inch";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Computer desktop = new Laptop();
            desktop.GetComputerDiscripution();//You get a desktop
            desktop.GetColor();//Color is white
            desktop.GetScreen();//LED 19 inch--------------violation liskov
        }
    }
}

What "LED 19 inch"? It should be "LCD 19 inch"(violation of LSP).
Now I told the shop manager that your promotion shows that desktop computer have LCD screen.Please give me the right product. So how to do it in code?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace LiskovSubstitutionObey
{
    public abstract class Computer
    {
        public abstract string GetComputerDescription();
        public abstract string GetColor();
        public abstract string GetScreen();        
    }

    public class Desktop:Computer
    {
        public override string GetComputerDescription()
        {
            return "You get a desktop";
        }

        public override string GetColor()
        {
            return "Color is white";
        }

        public override string GetScreen()
        {
            return "LCD 19 inch";
        }
    }

    public class Laptop:Computer 
    {
        public override string GetComputerDescription()
        {
            return "You get a laptop";
        }

        public override string GetColor()
        {
            return "Color is black";
        }

        public override string GetScreen()
        {
            return "LED 19 inch";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Computer computer = new Laptop();
            computer.GetComputerDescription();//You get a laptop
            computer.GetScreen();//LED 19 inch
        }
    }
}

Yes finally I get it.
So we can say that the new derived classes are extending the base classes without changing their behavior or objects  in a program should be replaceable with their subtypes' without alerting the correctness of that program".So our code satisfy LSP rule.