using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public interface Animal

    {

        string Sound { get; set; }

        AnimalBase Instance();

    }

    public abstract class AnimalBase : Animal

    {

        public string Sound { get; set; } = "Hmmm...";

​

        protected abstract void MakeSound();

        public static AnimalBase GetAnimal(string type)

        {

            AnimalBase ab = null;

​

            if(type == "dog")

            {

                ab = new Dog();

            }

            else if(type == "cat")

            {

                ab = new Cat();

            }

            ab.MakeSound();

​

            return ab;

        }

        public AnimalBase Instance()

        {

            return this;

        }

    }

    public class Dog : AnimalBase

    {

        public Dog()

        {

            Sound = "Dog Sound";

        }

        protected override void MakeSound()

        {

            Sound = "Woof!";

        }

    }

    public class Cat : AnimalBase

    {

        public Cat()

        {

            Sound = "Cat Sound";

        }

​

        protected override void MakeSound()

        {

            Sound = "Meow!";

        }

    }   

    public static void Main()

    {       

        List<Animal> animals = new List<Animal>();

        animals.Add(new Dog());

        animals.Add(new Cat());

​

        Console.WriteLine(animals.ElementAt(0).Sound);

        Console.WriteLine(animals.ElementAt(1).Sound);

        Console.WriteLine(AnimalBase.GetAnimal("dog").Sound);

        Console.WriteLine(AnimalBase.GetAnimal("cat").Sound);

    }

}