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In this article i would like to describe the people what are the basic concepts of Object Oriented Programming
The OOP stands on the following three pillars.

1. Object &Class (Encapsulation)
2. Inheritance
3. Polymorphism

1 Object & Class (Encapsulation): 

       

         Object:

The Object is an Instance in programming language which is made by inspiring from the real world which contains some properties and methods.

Lets take an example of a person(object) from the real world.

A person is an object, as i told you before that an object could have methodes(behaviour)& properties so here person(object) could have some properties like his black hair,fair color,tall,and so on..Simillarly it could have the methodes like Eat,Run,Play,Sleep and so on..All these properties & methodes are kept stored in a seperate class.
          

         Class:

A class is basically a container where object lies..In programming we define all the properties&methodes in a seperate class while we make object of that class in main class... It means we generate the objects from main class, lets have a look..

class cat

{

   public void eat()

         {

           console.writeline("cat eats mouse");

          }

    public void walk()

          {

            console.writeline("It walks slowly...");

          }

}

Here we have defined a class and in 'class cat' " public void eat() " and " public void walk() " are methods.now we make an object in main class and call these methods...

static void main ()

{

   cat mycat = new cat();

    mycat.eat();

    mycat.walk();

    console.readline();

}

This is the main  class in which we make an object named mycat of the classs cat.. after making the cat's class object we can call its methods&properties by writting "mycat." when we put dot after the object it calls all the methodes & properties from the related class...

Encapsulation in OOP?

Encapsulation is the word which came from the word "CAPSULE" which to put some thing in a kind of shell. In OOP we are capsuling our code not in a shell but in "Objects" & "Classes".  So it means to hide the information into objects and classes is called Encapsulation.

2 Inheritance:

The Inheritance is one of the main concepts of oop.Inheritance is just to adobe the functionality (methods & properties) of one class to another. This simple term is known as Inheritance.  

Lets have a look of code

public class cat
{
   public void eat()
   {
      console.writeline("cat can eat");
    }
   public void walk()
   {
    console.writeline("cat can walk");
   }

}

now we make another class of another cat and we inherit the methods of the first class to the new class.Do remember in programming we inherit one class to another by coding " : " colon between the parent and child class.
Lets have a look.. 

public class EgyptianCat : cat
{
  public void jump()
  {
     console.writeline("Egyptian cat can jump"); 
  }

 public void bite()
  {
    console.writeline("Egyptian cat can not bite"); 
   }
}

static void main()

{

 cat mycat=new cat();

 mycat.eat();

 mycat.walk();

 EgyptianCat yourcat=new EgyptianCat();

 yourcat.eat();

 yourcat.walk();

 yourcat.jump();

 yourcat.bite();

 console.readline();

}

this is the code in main class in which we are inheriting the class EgyptianCat to the class cat, so that the new class becomes the child class of the first class which is now called the parent class.

Basic Concepts of OOP : Polymorphism

 In this article i am describing Polymorphism for those who want to make their concepts clear.

Polymorphism

The word Polymorphism means of many forms.In programming this word is meant to reuse the single code multiple times. In object oriented programming its a big question that why the Polymorphism is done, what is the purpose of it in our code?

There are lots of people who don't even know the purpose and usage of Polymorphism.Polymorphism is the 3rd main pillar of OOP without it the object oriented programming is incomplete. Lets go in the depth of Polymorphism.

Why Polymorphism is done in OOP?

     
Its obvious that when we do inheritance between two classes, all the methods and properties of the first class are derived to the other class so that this becomes the child class which  adobes all the functionality of base class.It can also possesses its own separate methods.

But there is a big problem in inheriting the second class to the first class as it adobes all the methods same as the base class has,which  means that after inheritance both(base class& child class) have the methods of same name and same body as shown in this example:~

____Base Class___

public class fish
{
  public void eat()
   {
        console.writeline("fish eat");
    }
  public void swim()
   {
          console.writeline("fish swim");
    }
  }

______Derived Class______

class Dolphen:fish
{

  public void eat()
   {
        console.writeline("fish eat");
    }
  public void swim()
   {
          console.writeline("fish swim");
    }
}

In this example it is clear that when we Inherit two classes, all the methods with the same name and same body are adobed by the derived class as shown above.Both methods have the same name but if we change the body of the second method then it makes Compiler disturb whether to compile base class method or derived class's
method first... 

Lets have a look of this code..

____Base Class___

public class fish
{
  public void eat()
   {
        console.writeline("fish eat");
    }
  public void swim()
   {
          console.writeline("fish swim");
    }
  }

______Derived Class______

class Dolphin:fish
{

  public void eat()
   {
        console.writeline("Dolphin can eat");
    }
  public void swim()
   {
          console.writeline("Dolphin can swim");
    }
}

In this example we have changed the body of the methods of Derived class.Now this will make the Compiler disturb to compile which method first,because they both have same name but different bodies.

To remove this problem and to tell the compiler that which method is to be executed we need to use Polymorphism.

How the Polymorphism is done?

        
Polymorphism is used to remove the problem which is shown in the above code.It is done not by changing the name of the methods of both base & derived class.Infect it is done by adding the "virtual" keyword before the base class method, and the "override" keyword before the derived class method.As shown in this exmple:
 

____Base Class___

       

public class fish
        {
            public virtual void eat()
            {
                Console.WriteLine("fish eat");
            }
            public virtual void swim()
            {
                Console.WriteLine("fish swim");
            }
        }    

 

______Derived Class______

class Dolphin : fish
        {

            public override void eat()
            {
                Console.WriteLine("Dolphin can eat");
            }
            public override void swim()
            {
                Console.WriteLine("Dolphin can swim");
            }
        }

This is actually the Polymorphism in which we write virtual keyword with the base class method and we write override keyword with the derived class method as we did. It helps the compiler to select the method to be executed.

Here is the complete code in which Polymorphism has been applied.

class Program
    {
        public class fish
        {
            public virtual void eat()
            {  }
            public virtual void swim()
            { }
            public virtual void dive()
            {}

        }
       public class Dolphin : fish
         {
            public override void eat()
            { Console.WriteLine("Dolphin eats Plants"); }
            public override  void swim()
            { Console.WriteLine("Dolphin swims quickly"); }
            public override  void dive()
            { Console.WriteLine("Dolphin dive deeply "); }
           public void dance()
            { Console.WriteLine("Dolphin can Dance"); }

        }
        public class Shark : fish
        {
            public override  void eat()
            { Console.WriteLine("Shark eats dead animal"); }
            public override  void swim()
            { Console.WriteLine("Sharks swim fastest than Dolphin"); }
            public override  void dive()
            { Console.WriteLine("Sharks Dive deeper than Dolphin"); }

            public void kill()
            { Console.WriteLine("Shark kills Others"); }

        }
        
        static void Main(string[] args)
        {
            Dolphin D = new Dolphin();
            D.dance();
            D.dive();
            D.eat();
            D.swim();
            Shark S = new Shark();
            S.kill();
            S.dive();
            S.eat();
            S.swim();
            Console.ReadLine();
        }
    }