方法一 ：一个静态的对象创建方法

public class Singleton
    {
        private Singleton()
        {
            Thread.Sleep(1000);//耗时
            string bigSize = "占用10M内存";//耗计算资源
            string resource = "占用多个线程和数据库连接资源";//耗有限资源
            Console.WriteLine("{0}被构造，线程id={1}", this.GetType().Name, 
                                       Thread.CurrentThread.ManagedThreadId);
        }
        private static Singleton _Singleton = null;
 
        private static object Singleton_Lock = new object();
 
        public static Singleton CreateInstance()
        {
            if (_Singleton == null)//保证对象初始化之后的所有线程，不需要等待锁
            {
                Console.WriteLine("准备进入lock");
                lock (Singleton_Lock)//保证只有一个线程进去判断
                {
                    //Thread.Sleep(1000);
                    if (_Singleton == null)//保证对象为空才真的创建
                    {
                        _Singleton = new Singleton();
                    }
                }
            }
            return _Singleton;
        }
 
        public void Show()
        {
            Console.WriteLine("这里调用了{0}.Show", this.GetType().Name);
        }
    }

方法二  ：私有化构造函数

public class SingletonSecond
    {
        private SingletonSecond()
        {
            Thread.Sleep(1000);//耗时
            string bigSize = "占用10M内存";//耗计算资源
            string resource = "占用多个线程和数据库连接资源";//耗有限资源
            Console.WriteLine("{0}被构造，线程id={1}", this.GetType().Name, 
                                       Thread.CurrentThread.ManagedThreadId);
        }
        private static SingletonSecond _Singleton = null;
 
         // 静态构造函数：由CLR保证，在第一次使用这个类之前，调用而且只调用一次
        static SingletonSecond()
        {
            _Singleton = new SingletonSecond();
        }
 
        public static SingletonSecond CreateInstance()
        {
            return _Singleton;
        }
 
        public void Show()
        {
            Console.WriteLine("这里调用了{0}.Show", this.GetType().Name);
        }
    }

方法三： 静态变量  

public class SingletonThird
    {
        private SingletonThird()
        {
            Thread.Sleep(1000);//耗时
            string bigSize = "占用10M内存";//耗计算资源
            string resource = "占用多个线程和数据库连接资源";//耗有限资源
            Console.WriteLine("{0}被构造，线程id={1}", this.GetType().Name,
                                       Thread.CurrentThread.ManagedThreadId);
        }
       
        // 静态变量：会在类型第一次使用的时候初始化，而且只初始化一次
        private static SingletonThird _Singleton = new SingletonThird();
 
        public static SingletonThird CreateInstance()
        {
            return _Singleton;
        }
        public void Show()
        {
            Console.WriteLine("这里调用了{0}.Show", this.GetType().Name);
        }
    }
                        
 static void Main(string[] args)
        {
            try
            {
                List resultList = new List();
                resultList.Add(new Action(() =>
                {
                    Singleton sin = Singleton.CreateInstance();
                    sin.Show();
                }).BeginInvoke(null,null));
                        
                for (int i = 0; i < 10; i++)
                {
                   resultList.Add( new Action(() =>
                    {
                        Singleton singleton = Singleton.CreateInstance();
                        singleton.Show();
                    }).BeginInvoke(null, null));//会启动一个异步多线程调用
                }
                        
                while (resultList.Count(r => !r.IsCompleted) > 0)
                {
                    Thread.Sleep(10);
                }
                        
                for (int i = 0; i < 10; i++)
                {
                    resultList.Add(new Action(() =>
                    {
                        SingletonSecond singleton = SingletonSecond.CreateInstance();
                        singleton.Show();
                    }).BeginInvoke(null, null));//会启动一个异步多线程调用
                }
                        
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            Console.Read();
        }