一、单例模式的介绍

     单例模式简单说就是掌握系统的至高点，在程序中只实例化一次，这样就是单例模式，在系统比如说你是该系统的登录的第多少人，还有数据库的连接池等地方会使用，单例模式是最简单，最常用的模式之一，面试经常会问答，所以我们在程序考虑什么地方会用到单例模式，单例模式写法有很多种，这里我下面列举2种写法很普遍的写法，然后做个多线程计数的例子大家来理解一下单例模式的用法。

二、单例模式Demo

     /// <summary>

    /// 利用C#的特性单例模式

    /// </summary>

     public sealed  class SampleSingleton1

    {

        private int m_Counter = 0;

        private SampleSingleton1()

        {

            Console.WriteLine("初始化SampleSingle1");

        }

        public static readonly SampleSingleton1 singleton = new SampleSingleton1();

        /// <summary>

        /// 调用次数计数器

        /// </summary>

        public void Counter()

        {

            m_Counter++;

        }

    }

    /// <summary>

    /// 线程安全的单例模式

    /// </summary>

    public  class SampleSingleton2

    {

        private static volatile SampleSingleton2 m_Instance = null;

        private int m_Counter = 0;

        private SampleSingleton2()

        {

            Console.WriteLine("初始化SampleSingleton2.");

        }

        public static SampleSingleton2 Singleton

        {

            get

            {

                if (m_Instance == null)

                {

                    lock (typeof(SampleSingleton2))

                    {

                        if (m_Instance == null)

                        {

                            m_Instance = new SampleSingleton2();

                        }

                    }

                }

                return m_Instance;

            }

        }

        /// <summary>

        /// 调用计数器

        /// </summary>

        public void Couter()

        {

            m_Counter++;

        }

    }

     /// <summary>

    /// 测试2种模式的效率和调用的方式

    /// </summary>

            DateTime milestone;

            int maxCallTimes= 100000000;

            // 第一次调用SampleSingleton1的Counter方法

            Console.WriteLine("第一次调用SampleSingleton1的Counter方法");

            SampleSingleton1.singleton.Counter();

            // 计算10000次调用的耗费时间

            milestone = DateTime.Now;

            for (int i = 0; i < maxCallTimes; i++)

            {

                SampleSingleton1.singleton.Counter();

            }

            Console.WriteLine(maxCallTimes.ToString() + "次调用执行时间为：" + ((TimeSpan)(DateTime.Now - milestone)).TotalMilliseconds.ToString());

            Console.WriteLine("");

            // 第一次调用SampleSingleton2的Counter方法

            Console.WriteLine("第一次调用SampleSingleton2的Counter方法");

            SampleSingleton2.Singleton.Couter();

            // 计算10000次调用的耗费时间

            milestone = DateTime.Now;

            for (int i = 0; i < maxCallTimes; i++)

            {

                SampleSingleton2.Singleton.Couter();

            }

            Console.WriteLine(maxCallTimes.ToString() + "次调用执行时间为：" + ((TimeSpan)(DateTime.Now - milestone)).TotalMilliseconds.ToString());

            string str = Console.ReadLine();

        }

 三、GitHub的地址

       https://github.com/wangtongzhou520/Designpattern