Abstract Class

The abstract modifier can be used with classes, methods, properties, indexers, and events.

Use the abstract modifier in a class declaration to indicate that a class is intended only to be a base class of other classes.

Abstract classes have the following features:

• An abstract class cannot be instantiated.
• An abstract class may contain abstract methods and accessors.
• It is not possible to modify an abstract class with the sealed modifier, which means that the class cannot be inherited.
• A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.

Use the abstract modifier in a method or property declaration to indicate that the method or property does not contain implementation.

Abstract methods have the following features:

• An abstract method is implicitly a virtual method.
• Abstract method declarations are only permitted in abstract classes.
• Because an abstract method declaration provides no actual implementation, there is no method body; the method declaration simply ends with a semicolon and there are no braces ({ }) following the signature. For example: 

An abstract class cannot be instantiated. The purpose of an abstract class is to provide a common definition of a base class that multiple derived classes can share. For example, a class library may define an abstract class that is used as a parameter to many of its functions, and require programmers using that library to provide their own implementation of the class by creating a derived class.

Abstract classes may also define abstract methods. This is accomplished by adding the keyword abstract before the return type of the method. For example:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace AbstractCLass

{

    class AbstractClassDemo

    {

        public abstract void Animal(string strAnimal)

        {

            Console.WriteLine("Animal Name is :-" +strAnimal);

        }

    }

}

when u compile this u ll get error as

Can not declare a body because it marked as Abstract

Instead u use following code to make a abstract Function.

Second thing is class is not abstract so we have to make it abstract

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace AbstractCLass

{

    abstract class AbstractClassDemo

    {

        public abstract void Animal(string strAnimal);

    }

}

it will compile 

Now u Run below Code.

1)  Class 1

namespace AbstractCLass

{

    abstract class AbstractClassDemo

    {

        public abstract void Animal(string strAnimal);

    }

    class Derived_abstract : AbstractClassDemo

    {

        public override void Animal(string strAnimal)

        {

            Console.WriteLine("This animal is :=  "+strAnimal);

        }

    }

}

2)  Class 2

namespace AbstractCLass

{ 

    class Class_Main

    {

        static void Main(string[] args)

        {

            Derived_abstract objDerived_abstract = new Derived_abstract();

            objDerived_abstract.Animal("C# .Net");

            Console.ReadLine();

        }

    }

}

This is a simple Demo for abstract Class 

I also Refer Some concepts of Microsoft MSDN

Its helps me Lot to clear My Concepts