线程同步之Monitor

1、Monitor.Enter(obj);申请对obj（非静态字段）的独立访问权;

2、对静态字段加锁时，使用Monitor.Enter(typeof(共享类名));

示例：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
 
namespace Thread_Monitor
{
    class ShearedResource               //多线程共享的资源
    {
        public int instanceCount = 0;          //!实例字段
        public static int counter = 0;         //!静态字段
    }
 
    class Program
    {
        private static void IncreaseCount(Object obj)
        {
            Monitor.Enter(obj);
            int beginNum = (obj as ShearedResource).instanceCount;
            for (int i = 0; i < 100000; i++)
                beginNum++;
            (obj as ShearedResource).instanceCount = beginNum;
            Console.WriteLine("Thread ID:{0},  Count: {1}", Thread.CurrentThread.ManagedThreadId, beginNum);
            Monitor.Exit(obj);            
        }
                
        private static void IncreaseStaticCount()
        {
            Monitor.Enter(typeof(ShearedResource));
            int beginNum = ShearedResource.counter;
            for (int i = 0; i < 100000; i++)
                beginNum++;            
            ShearedResource.counter = beginNum;
            Console.WriteLine("Thread ID:{0},  Count: {1}", Thread.CurrentThread.ManagedThreadId, beginNum);
            Monitor.Exit(typeof(ShearedResource));
        }

        static void Main(string[] args)
        {
            const int threadCount = 15;
            ShearedResource sr = new ShearedResource();
            Thread[] threads = new Thread[threadCount];
            for (int i = 0; i < threadCount; i++)
            {
                threads[i] = new Thread(new ParameterizedThreadStart(IncreaseCount)).Start(sr);      //!线程带参
                //threads[i] = new Thread(IncreaseStaticCount).Start();
            }
            Console.ReadKey();
        }
    }
}

访问静态字段

        private static void IncreaseCount(Object obj)
        {
            Monitor.Enter(typeof(ShearedResource));
            int beginNum = ShearedResource.counter;
            for (int i = 0; i < 100000; i++)
                beginNum++;
            ShearedResource.counter = beginNum;
            Console.WriteLine("Thread ID:{0},  Count: {1}", Thread.CurrentThread.ManagedThreadId, beginNum);
            Monitor.Exit(typeof(ShearedResource));
        }
3、使用Monitor控制线程的推进顺序

（1）B执行Monitor.Wait(obj)后阻塞，等待A执行Monitor.Pulse(obj)。一旦A执行Monitor.Pulse(obj)，将通知B让其继续走下去。

（2）必须确保线程B先执行，否则会产出死锁，线程B将永远等不到A的消息。

（3）每调用一次Monito.Pulse()方法，等待队列里的一个线程被激活；调用Monitor.PulseAll()方法，激活等待队列里的所有线程。

//线程A的执行代码
lock(obj)
{
    //访问共享资源obj...
    Monitor.Pulse(obj);
}
//线程B的执行代码
lock(obj)
{
    Monitor.Wait(obj);    
    //访问共享资源obj...
}

4、用Attribute实现

        [MethodImpl(MethodImplOptions.Synchronized)]
        private static void IncreaseCount(Object obj)
        {
            //访问共享资源...
        }

好处，不需要改内部代码。
需引入命名空间：using System.Runtime.CompilerServices;
此方法是锁定类中一个方法，不是锁定整个类
5、指定超时选项以避免死锁
基本用法：
bool lockTaken = false;
Monitor.TryEnter(lockObj, 3000, ref lockTaken);    //超时3秒，   lockTaken为引用类型
if(lockTaken)
{
    try
    {    //访问共享资源....    }
    finally
    {
        Monitor.Exit(lockObj);
    }
}