using System;

public abstract class Shape

{

}

​

public class Cube : Shape

{

}

​

public class Quad : Shape

{

}

​

public class Program

{

    public static void Main()

    {

        // uncomment line 21 and comment out line 22 to see the behaviour change

        //Shape shape = new Quad();

          Shape shape = new Cube();

​

        if (shape is Cube)

        {

          Console.WriteLine("it's a cube!");

        }        

        else

        {

            Console.WriteLine("not a cube!");

        }

​

        // you can also dynamic downcast: 

        var cubeCast = shape as Cube;

        if (cubeCast != null)

        {

          Console.WriteLine("it's a cube!");

        }

        else

        {

            Console.WriteLine("not a cube!");

        }

​

        // and in the most recent version of C#, you can combine the two:

        if (shape is Cube c)

        {

         Console.WriteLine("it's a cube!");

        }

        else

        {

            Console.WriteLine("not a cube!");

        }

    }

}