private static void Start()
{
            Stopwatch sw = new Stopwatch();
            sw.Start();

            Action action = () =>
            {
                using (var db = new EFDBContext())
                {
                    db.Database.ExecuteSqlCommand("select num from stock where id=1 for update;update stock set num=num-1 where id=1");
                }
            };

            List<Task> tasks = new List<Task>();
            for (var i = 0; i < 50; i++)
            {
                tasks.Add(new Task(action));
            }
            tasks.ForEach((obj) => { obj.Start(); });
            Task.WaitAll(tasks.ToArray());

            sw.Stop();
            Console.WriteLine("总耗时：" + sw.Elapsed.TotalSeconds + " 秒");
}