public class Demo
    {
        private static Demo _demo = null;

        /// <summary>
        /// 构造函数
        /// </summary>
        private Demo()
        {
            Console.WriteLine("构造了{0}", GetType().Name);
        }

        /// <summary>
        /// 获取该类的唯一实例
        /// </summary>
        /// <returns>该类的唯一实例</returns>
        public static Demo GetInstance()
        {
            if (_demo == null)
                _demo = new Demo();
            return _demo;
        }
    }

                Demo d1 = null;
                Demo d2 = null;
                //多线程创建对象实例
                var t1 = new Thread(() => { d1 = Demo.GetInstance(); });
                var t2 = new Thread(() => { d2 = Demo.GetInstance(); });

                t1.Start();
                t2.Start();
                Thread.Sleep(1000);//主线程等待子线程执行完成，为d1和d2变量赋值
                Console.WriteLine("d1 == d2 {0}", object.ReferenceEquals(d1, d2));
                Console.Read();
            

                for (int i = 0; i < 2; i++)
                {
                    new Action(() => { Demo.GetInstance(); }).BeginInvoke(null, null);
                }
                Console.Read();