【剑指Offer | C# 】面试题二：实现Singleton模式

题目：设计一个类，我们只能生成该类的一个实例。

属于是设计模式之类的面试题了
可以学习下大话设计模式：
https://blog.youkuaiyun.com/Ejzq1/article/details/125111181?spm=1001.2014.3001.5501

C# 代码逐行注释讲解：

using System;

namespace _02_Singleton
{
	//解法一：只能
    public sealed class Singleton1 //sealed类不能被继承或重写
    {
        private Singleton1() //私有构造函数，禁止创建
        {
        }

        private static Singleton1 instance = null; //静态实例
        public static Singleton1 Instance
        {
            get
            {
                if (instance == null)
                    instance = new Singleton1();

                return instance;
            }
        }
    }
	//解法二：可多线程低效解法
    public sealed class Singleton2
    {
        private Singleton2()
        {
        }

        private static readonly object syncObj = new object();

        private static Singleton2 instance = null;
        public static Singleton2 Instance
        {
            get
            {
            	//每个线程都加锁，低效
                lock (syncObj)//一般给对象加锁
                {
                    if (instance == null)
                        instance = new Singleton2();
                }

                return instance;
            }
        }
    }
	//解法三：加锁前判断是否存在
    public sealed class Singleton3
    {
        private Singleton3()
        {
        }

        private static object syncObj = new object();

        private static Singleton3 instance = null;
        public static Singleton3 Instance
        {
            get
            {//可读性较差
                if (instance == null)
                {
                    lock (syncObj)
                    {
                        if (instance == null)
                            instance = new Singleton3();
                    }
                }

                return instance;
            }
        }
    }
	//解法四：静态变量只在第一次用到Singleton4初始化，过早创建实例
    public sealed class Singleton4
    {
        private Singleton4()
        {
            Console.WriteLine("An instance of Singleton4 is created.");
        }

        public static void Print()
        {
            Console.WriteLine("Singleton4 Print");
        }

        private static Singleton4 instance = new Singleton4();//静态变量
        public static Singleton4 Instance
        {
            get
            {
                return instance;
            }
        }
    }
	//解法五：嵌套类，第一次用到才会调用静态构造函数去初始化静态变量
    public sealed class Singleton5
    {
        Singleton5()
        {
            Console.WriteLine("An instance of Singleton5 is created.");
        }

        public static void Print()
        {
            Console.WriteLine("Singleton5 Print");
        }

        public static Singleton5 Instance
        {
            get //让用到Singleton但是没用到Instance的地方不用去实例化
            {
                return Nested.instance;
            }
        }

        class Nested
        {
            static Nested()
            {
            }
			
            internal static readonly Singleton5 instance = new Singleton5();
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // 也会打印An instance of Singleton4 is created.
            Singleton4.Print();

            // 不会打印An instance of Singleton5 is created.
            Singleton5.Print();
        }
    }
}