HideAndSeek.cs

using System;
using System.Collections.Generic;

namespace HideAndSeek
{
    class Program
    {
        static void Main(string[] args)
        {
            foreach(var s in countDownTo(10)){
                Console.WriteLine(s);
            }
        }

        static IEnumerable<string> countDownTo(int startAt, int stopAt=0)
        {
            int counter = startAt;
            while(true)
            {
                switch(counter)
                {
                    case 2:
                        yield return "Ready!";
                        break;
                    case 1:
                        yield return "Set!";
                        break;
                    case 0:
                        yield return "Go!";
                        break;
                    default:
                        yield return counter.ToString();
                        break;
                }

                counter--;
                
                if(counter < stopAt){
                    yield break;
                }
            }
        }
    }
}