//在方法里new  ,这里使用 双if-lock
    public  class SingleFirst
    {
        private static SingleFirst _SingleFirst;
        private static readonly object _Single_Lock = new object();
        private SingleFirst() { }
        public static SingleFirst GetInstance()
        {
            if(_SingleFirst == null)
            {
                lock (_Single_Lock)
                {
                    if(_SingleFirst == null)
                    {
                        _SingleFirst = new SingleFirst();
                    }
                }
            }
            return _SingleFirst;
        }
        
    }

    /// <summary>
    /// 在构造函数里new(), 在方法返回
    /// </summary>
    public class SingleSecond
    {
        private static SingleSecond _SingleSecond = null;
        private SingleSecond() { }
        static SingleSecond()
        {
            _SingleSecond = new SingleSecond();
          
        }
        public  static  SingleSecond GetInstance()
        {
            return _SingleSecond;
        }
    }

    /// <summary>
    /// 在静态字段里new（）,方法里返回
    /// </summary>
    public class SingleThrid
    {
        private static SingleThrid _SingleThird = new SingleThrid();
        private SingleThrid() { }
        public static SingleThrid GetInstance()
        {
            return _SingleThird;
        }
    }

                var a1 = SingleFirst.GetInstance();
                var a2 = SingleFirst.GetInstance();
                Console.WriteLine($"a1 和 a2 是同一实例：{ object.ReferenceEquals(a1, a2)}");
                ;
                var b1 = SingleSecond.GetInstance();
                var b2 = SingleSecond.GetInstance();
                Console.WriteLine($"b1 和 b2 是同一实例：{ object.ReferenceEquals(a1, a2)}");
                var c1 = SingleThrid.GetInstance();
                var c2 = SingleThrid.GetInstance();
                Console.WriteLine($"c1 和 c2 是同一实例：{ object.ReferenceEquals(a1, a2)}");