using System;

namespace TestSealedClasses 

{

    public abstract class Base

    {

        public void Walk()

        {                      

            Console.WriteLine("This is Base Class walk.");

        }

        public virtual void Run()

        {                      

            Console.WriteLine("This is Base Class run.");

        }

        public abstract void Swim(); // no implementation yet - just abstraction

    }

​

    public class Derived : Base

    {

        public override void Run()

        {                      

            Console.WriteLine("This is Derived Class run.");

        }

        public override void Swim()

        {

            Console.WriteLine("This is implementation of Swim in derived class");   

        }

    }

    public class Program

    {

        public static void Main(string[] args)

        {   

            Base objBase = new Derived();

            objBase.Walk();// Output ---------> This is Base Class walk.

            objBase.Run();// Output ---------> This is Derived Class run.

            objBase.Swim();// Output ---------> This is implementation of Swim in derived class

            // Base objBase1 = new Base(); // this is not allowed, abstract classes could not be created

        }

    }

}