多线程问题--- 死锁

过多的锁定也会有麻烦。在死锁中，至少有两个线程被挂起，并等待对方解除锁定。由于两个线程都在等待对方，就出现了死锁，线程将无限等待下去。

看下面的示例:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace ThreadingIssues
{
    public class StateObject
    {
        private int state = 5;
        private object sync = new object();
        /// <summary>
        /// 当state等于5时，改变state的值
        /// </summary>
        /// <param name="loop"></param>
        public void ChangeState(int loop)
        {
            lock (sync)
            {
                if (state == 5)
                {
                    state++;
                    Trace.Assert(state == 6, "Race condition occurred after " + loop + " loops");
                }
            }
            state = 5;
        }
    }

    public class SampleThread
    {
        private StateObject s1;
        private StateObject s2;

        public SampleThread(StateObject s1,StateObject s2)
        {
            this.s1 = s1;
            this.s2 = s2;
        }

        public void Deadlock1()
        {
            int i = 0;
            while (true)
            {
                lock (s1)
                {
                    lock (s2)
                    {
                        s1.ChangeState(i);
                        s2.ChangeState(i++);
                        Console.WriteLine("still running,{0}",i);
                    }
                }
            }
        }

        public void Deadlock2()
        {
            int i = 0;
            while (true)
            {
                lock (s2)
                {
                    lock (s1)
                    {
                        s1.ChangeState(i);
                        s2.ChangeState(i++);
                        Console.WriteLine("still running,{0}", i);
                    }
                }
            }
        }
    }

   class Program
    {
        static void Main(string[] args)
        {
            var state1 = new StateObject();
            var state2 = new StateObject();
            new Task(new SampleThread(state1, state2).Deadlock1).Start();
            new Task(new SampleThread(state1, state2).Deadlock2).Start();
        }
    }
}