using System using System Collections Generic using System Linq using

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            var a = OnlyOne.GetInstance();
            a.Value = "a";

            var b = OnlyOne.GetInstance();
            b.Value = "b";

            Console.WriteLine(a.Value);
            Console.WriteLine(b.Value);

            Console.ReadLine();
        }
    }

    class OnlyOne
    {
        private static OnlyOne _instance;

        public string Value;

        private OnlyOne() { }

        public static OnlyOne GetInstance()
        {
            if (_instance == null)
            {
                _instance = new OnlyOne();
            }

            return _instance;
        }
    }
}