Abstract Class In C#

·   Class cannot be instantiated but only used as based class.

·   But another class can inherit from an abstract class and can create their instances

·   we can implement methods, fields, and properties in the abstract class that can be used by the child class

·   Abstract class can't be static.

·   Abstract class is used for inheritance.

·   Abstract class can't be sealed.

·   Abstract or virtual members can't be private, as we can't override it.

·   Abstract class can be inherited from an abstract class but the methods in the base class have to be declared abstract.

·   Abstract method and properties do not have any functionality. The derived class defines their full functionality.

Ex:

namespace abstract_class

{

    class Program

    {

        public abstract class clsAbstract

        {

            public abstract int sum(int a, int b);

            public abstract int subt(int a, int b);

            public void Display()

            {

                Console.WriteLine("Abstract class");

            }

        }

         public class clsDerived : clsAbstract

        {

            // Method overriding

            public override int subt(int a, int b)

            {

                return (a - b);

            }

            // Method overriding

            public override int sum(int a, int b)

            {

                return (a + b);

            }

            public void Show()

            {

                Console.WriteLine("YY class method");

            }

        }

        static void Main(string[] args)

        {

            clsDerived  obj = new clsDerived  ();

            // here you can't create instance of a abstract class it is giving error

            // clsDerived  obj1 = new clsAbstract ();

            // clsAbstract obj2 = new clsAbstract ();

            // here we have used up level casting

            clsAbstract obj3 = new clsDerived  ();

                       

            // here you don't have access to the Show method of YY

            // Because you can only access methods of a reference object only

            //obj3.Show();

            Console.WriteLine("Calling sum method through reference of base class " + obj3.sum(2,2));

            obj.Display();

            Console.WriteLine("Subtraction : "+ obj.subt(12, 10));

            Console.WriteLine("Addition : " + obj.sum(10, 10));

            Console.ReadLine();

        }

    }

}

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