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C# 多线程系列之Mutex使用

转发自:http://www.cnblogs.com/HouZhiHouJueBlogs/p/3945195.html

互斥量是一个内核对象,它用来确保一个线程独占一个资源的访问,并且互斥量可以用于不同进程中的线程互斥访问资源。

我们可以把Mutex看作一个出租车,乘客看作线程。乘客首先等车,然后上车,最后下车。当一个乘客在车上时,其他乘客就只有等他下车以后才可以上车。而线程与C# Mutex对象的关系也正是如此,线程使用Mutex.WaitOne()方法等待C# Mutex对象被释放,如果它等待的C# Mutex对象被释放了,或者它没有被任何对象有用,它就自动拥有这个对象,直到它调用Mutex.ReleaseMutex()方法释放这个对象,而在此期间,其他想要获取这个C# Mutex对象的线程都只有等待。

Msdn地址:http://msdn.microsoft.com/en-us/library/system.threading.mutex(v=vs.110).aspx

举MSDN上的例子说明:

using System;using System.Threading;class Example{    // Create a new Mutex. The creating thread does not own the mutex.     private static Mutex mut = new Mutex();    private const int numIterations = 1;    private const int numThreads = 3;    static void Main()    {        // Create the threads that will use the protected resource.         for(int i = 0; i < numThreads; i++)        {            Thread newThread = new Thread(new ThreadStart(ThreadProc));            newThread.Name = String.Format("Thread{0}", i + 1);            newThread.Start();        }        // The main thread exits, but the application continues to         // run until all foreground threads have exited.    }    private static void ThreadProc()    {        for(int i = 0; i < numIterations; i++)        {            UseResource();        }    }    // This method represents a resource that must be synchronized     // so that only one thread at a time can enter.     private static void UseResource()    {        // Wait until it is safe to enter, and do not enter if the request times out.        Console.WriteLine("{0} is requesting the mutex", Thread.CurrentThread.Name);        if (mut.WaitOne(1000)) {           Console.WriteLine("{0} has entered the protected area",                Thread.CurrentThread.Name);           // Place code to access non-reentrant resources here.            // Simulate some work.           Thread.Sleep(5000);           Console.WriteLine("{0} is leaving the protected area",                Thread.CurrentThread.Name);           // Release the Mutex.              mut.ReleaseMutex();           Console.WriteLine("{0} has released the mutex",                              Thread.CurrentThread.Name);        }        else {           Console.WriteLine("{0} will not acquire the mutex",                              Thread.CurrentThread.Name);        }    }}// The example displays output like the following: //       Thread1 is requesting the mutex //       Thread1 has entered the protected area //       Thread2 is requesting the mutex //       Thread3 is requesting the mutex //       Thread2 will not acquire the mutex //       Thread3 will not acquire the mutex //       Thread1 is leaving the protected area //       Thread1 has released the mutex

当一个线程占有Mutex后,代码就可以这样写:

 mutex.WaitOne(); mutex.WaitOne(); mutex.WaitOne(); mutex.WaitOne();。。。。

WaitOne这个方法被调用时 系统检查发现mutex没有被任何线程使用 故而将mutex分配给当前线程
因而就算继续调用WaitOne也没有影响 因为系统发现当前线程已经占有 就直接返回了。换言之,waitOne是获取
Mutex锁的方式。如果调用ReleaseMutex那么当前线程退出Mutex锁,其它线程便可进来申请。但是如果调用两次
ReleaseMutex 那么由于当前线程实际上是不占有锁的 那么会抛出异常。

所以不能这样写:

mutex.ReleaseMutex();//如果已经占有 那么OKmutex.ReleaseMutex();//调用第二次直接异常

Mutex是一个内核对象,所以,它是可以用作跨进程线程同步的。

A进程可以如此写:

Mutex mutex = new Mutex(true,"mutex1");

B进程则可以如此写:

Mutex mutex = Mutex.OpenExisting("mutex1")
OpenExisting这个方法签名我们后面也可以经常看到,作用就是获取一个已经存在的内核对象。
获取到之后的线程同步代码是跟单进程是一致的。

完毕。

C# 多线程系列之Mutex使用