多任务—死锁问题

    class StateObject
    {
        private int State1 = 5;
        private int State2 = 5;

        Object _Lock1 = new object();
        Object _Lock2 = new object();

        public void ChangeState1()
        {
            lock (_Lock1)
            {
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第1把锁");
                lock (_Lock2)
                {
                    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第2把锁");
                    if (State1 == 5)
                    {
                        Thread.Sleep(1);
                        State1++;
                        State2++;

                        Console.WriteLine("State1: " + State1 + " : " + Thread.CurrentThread.ManagedThreadId);

                        Console.WriteLine("State2: " + State2 + " : " + Thread.CurrentThread.ManagedThreadId);
                        Thread.Sleep(1);
                    }
                    State1 = 5;
                    State2 = 5;
                }
            }
        }


        public void ChangeState2()
        {
            lock (_Lock2)
            {
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第2把锁");
                lock (_Lock1)
                {
                    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第1把锁");
                    if (State1 == 5)
                    {
                        Thread.Sleep(1);
                        State1++;
                        State2++;

                        Console.WriteLine("State1: " + State1 + " : " + Thread.CurrentThread.ManagedThreadId);

                        Console.WriteLine("State2: " + State2 + " : " + Thread.CurrentThread.ManagedThreadId);
                        Thread.Sleep(1);
                    }
                    State1 = 5;
                    State2 = 5;
                }
            }
        }
    }

        static void Main(string[] args)
        {
            StateObject State = new StateObject();
            for (int i = 0; i < 5; i++)
            {
                Task t1 = new Task(State.ChangeState1);
                t1.Start();

                Task t2 = new Task(State.ChangeState2);
                t2.Start();
            }
            Console.ReadKey();
        }

运行结果

10拿到了第1把锁
12拿到了第2把锁

会发现程序会出现卡死现象，因为当两个任务分别获取了lock1和lock2之后，就会无限等待另一个锁，这样就会造成两个任务都无法继续运行。为了避免这种情况出现，应该在编程开始的设计阶段，就设计好锁定顺序。

修改ChangeState2函数代码

        public void ChangeState2()
        {
            lock (_Lock1)
            {
                Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第1把锁");
                lock (_Lock2)
                {
                    Console.WriteLine(Thread.CurrentThread.ManagedThreadId + "拿到了第2把锁");
                    if (State1 == 5)
                    {
                        Thread.Sleep(1);
                        State1++;
                        State2++;

                        Console.WriteLine("State1: " + State1 + " : " + Thread.CurrentThread.ManagedThreadId);

                        Console.WriteLine("State2: " + State2 + " : " + Thread.CurrentThread.ManagedThreadId);
                        Thread.Sleep(1);
                    }
                    State1 = 5;
                    State2 = 5;
                }
            }

程序执行结果

6拿到了第1把锁
6拿到了第2把锁
State1: 6 : 6
State2: 6 : 6
12拿到了第1把锁
12拿到了第2把锁
State1: 6 : 12
State2: 6 : 12
10拿到了第1把锁
10拿到了第2把锁
State1: 6 : 10
State2: 6 : 10
11拿到了第1把锁
11拿到了第2把锁
State1: 6 : 11
State2: 6 : 11
...