Singleon类：
    //应对于单线程
    public class SingletonA
    {
        private static SingletonA instance;

        //一个私有的构造器，禁用构造器
        private SingletonA() {}

        public static SingletonA Instance
        {
            get
            {
                if (instance == null)
                {
                    instance = new SingletonA();
                }
                return instance;
            }
        }
    }

    //应对于多线程
    public class SingletonB
    {
        //volatile关键字用于禁止编译器对指定的顺序进行微调。
        private static volatile SingletonB instance = null;

        private static object lockHelper = new object();

        //一个私有的构造器，禁用构造器
        private SingletonB() {}

        public static SingletonB Instance
        {
            get
            {
                //双锁，用于保证只一个线程使用这里，防止多线程产生多个实例。
                if (instance == null)
                {
                    lock (lockHelper)
                    {
                        if (instance == null)
                            instance = new SingletonB();
                    }
                    
                }
                return instance;
            }
        }
    }

主程序：
        public static void Main()
        {
            SingletonA t1 = SingletonA.Instance;

            SingletonA t2 = SingletonA.Instance;

            Console.WriteLine(object.ReferenceEquals(t1, t2) == true);
        }