using System;

public class Program

{

    public static void Main()

    {

        var myObject = Activator.CreateInstance(typeof(Testclass)) as TestAbstract;

        myObject.HelloWorld();

    }

    public abstract class TestAbstract {

        public virtual void HelloWorld() { throw new NotImplementedException(); }

    }

    public class Testclass : TestAbstract

    {

        public override void HelloWorld()

        {

            Console.WriteLine("Hello World");

        }

    }

}