c#-Polymorphism

using System;
using System.Collections.Generic;
using System.IO;
namespace CSharpPolymorphism
{
    class Animal{
    public string name;
    public string sound;
    public Animal(string name, string sound)
    {
        this.name=name;
        this.sound=sound;
    }
    public virtual string OldMcDonald(){
        return null;
    }


    }
    class Dog : Animal {
        public Dog(string name,string sound) : base(name,sound){

        }
        public override string OldMcDonald(){
            var OldMcDonaldLyrics=$@"
                Old MACDONALD had a farm
                E-I-E-I-O
                And on his farm he had a {name}
                E-I-E-I-O
                With a {sound} {sound} here
                And a {sound} {sound} there
                Here a {sound}, there a {sound}
                Everywhere a {sound} {sound}
                Old MacDonald had a farm
                E-I-E-I-O";
                return OldMcDonaldLyrics;
        }


    }
   class Cat : Animal {
        public Cat(string name,string sound) : base(name,sound){

        }
        public override string OldMcDonald(){
            var OldMcDonaldLyrics=$@"
                Old MACDONALD had a farm
                E-I-E-I-O
                And on his farm he had a {name}
                E-I-E-I-O
                With a {sound} {sound} here
                And a {sound} {sound} there
                Here a {sound}, there a {sound}
                Everywhere a {sound} {sound}
                Old MacDonald had a farm
                E-I-E-I-O";
                return OldMcDonaldLyrics;
        }


    }    class CSharpPolymorphism
    {
        static void Main(string[] args)
        {
            var dog =new Dog("dog", "bark");
            var cat=new Cat("cat", "meow");


            Console.WriteLine( dog.OldMcDonald());
            Console.WriteLine();
            Console.WriteLine( cat.OldMcDonald());
                    }
    }
}