单例模式

单例模式（Singleton）：

  保证一个类仅有一个实例，并提供一个访问它的全局访问点。

一、UML结构图

二、示例代码【重点】

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

namespace 单例模式
{
    class Program
    {
        static void Main(string[] args)
        {
            SingleModelOne.GetInstance().Show();
            SingleModelTwo.GetInstance().Show();
            SingleModelThree.GetInstance().Show();

            Console.Read();
        }
    }

    /// <summary>
    /// 单线程单例模式(懒汉式单例)
    /// </summary>
    public class SingleModelOne
    {
        private static SingleModelOne m_SingleModeOne = null;        

        /// <summary>
        /// 单例模式的构造函数一定要【私有化】
        /// </summary>
        private SingleModelOne() { }

        public static SingleModelOne GetInstance()
        {
            if (m_SingleModeOne == null)
                m_SingleModeOne = new SingleModelOne();

            return m_SingleModeOne;
        }
        public void Show()
        {
            Console.WriteLine("1、我是懒汉式、单线程单例模式");
        }
    }

    /// <summary>
    /// 多线程单例模式(懒汉式单例)
    /// </summary>
    public class SingleModelTwo
    {
        private static SingleModelTwo m_SingleModeTwo = null;
        private static object m_LockObj = new object();

        /// <summary>
        /// 单例模式的构造函数一定要【私有化】
        /// </summary>
        private SingleModelTwo() { }

        public static SingleModelTwo GetInstance()
        {
            //双重加锁 一 ：是为了防止线程阻塞，若对象已创建，此时在lock内部判断，就会导致其它线程在lock外等待。
            if (m_SingleModeTwo == null)
            {
                lock (m_LockObj)
                {
                    if (m_SingleModeTwo == null)
                        m_SingleModeTwo = new SingleModelTwo();
                }
            }
            return m_SingleModeTwo;
        }
        public void Show()
        {
            Console.WriteLine("2、我是懒汉式、多线程单例模式");
        }
    }

    /// <summary>
    /// 单线程单例模式(饿汉式单例)
    /// 饿汉式：【sealed】防止类被派生，派生就可能会增加实例。
    ///        【readonly】对象只读，交由OCL负责初始化变量
    /// </summary>
    public sealed class SingleModelThree
    {
        //首先将对象初始化完毕
        private readonly static SingleModelThree m_SingleModelThree = new SingleModelThree();        

        /// <summary>
        /// 单例模式的构造函数一定要【私有化】
        /// </summary>
        private SingleModelThree() { }

        public static SingleModelThree GetInstance()
        {           
            return m_SingleModelThree;
        }
        public void Show()
        {
            Console.WriteLine("3、我是饿汉式、单线程单例模式");
        }
    }

}