using System;

    using System.Collections.Generic;

    using System.Linq;

    public class Program

    {

        public static void Main()

        {

            IList<object> foo = new List<object> {

                new Bird(),

                new Cat(),

                new Cat(),

                new Dog(),

                new Bird(),

                new Poodle(),

                new Cat(),

                new Dog(),

                new Husky(),

                new Dachshund(),

                new Poodle(),

                new Cat(),

                new Dog()

            };

            var dogCount = foo.Count(x => x is Dog);

            Console.WriteLine($"{dogCount} dogs.");

        }

    }

​

    public class Dog

    {

        public virtual string Sound { get { return "Woof"; } }

    }

​

    public class Dachshund : Dog

    {

        public override string Sound { get { return "Ruff"; } }

    }

​

    public class Husky : Dog

    {

        public override string Sound { get { return "Yarp"; } }

    }

​

​

    public class Poodle : Dog

    {

        public override string Sound { get { return "Yip"; } }

    }

​

    public class Cat

    {

        public string Sound { get { return "Meow"; } }

    }

​

    public class Bird

    {

        public string Sound { get { return "Tweet"; } }

    }

​

​