创建一个类，实现只会实例化一次。

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication15
{
    class Program
    {
        static void Main(string[] args)
        {
            //Singleton stest = new Singleton();

            Singleton.CreateInstance();
            Singleton.CreateInstance();
            Console.WriteLine("OK");

        }

     
           
     
    }

   public class Singleton
    {
        private static Singleton objSingleton = null;
        private Singleton() { }
        public static Singleton CreateInstance()
        {
            if (null == objSingleton)
            {
                objSingleton = new Singleton();
            }
            return objSingleton;
        }
    }

}