详解C# 线程的挂起与唤醒

如果说C#和C++有什么不同，博主不得不说，对于异步的支持程度是C#的一一个伟大的进步。

其实早期的C++都没有异步，并发的概念。博主第一次使用C++创建异步程序的时候，是使用boost库的内容进行实现的。相对而言，C#对于异步的支持可以说是相当的好。相信很多名词大家都很耳熟能详，比如说Thread，BeginInvoke，Delegate，backgroundworker等等。。。其实楼主在使用了这么多的异步操作过程中，还是觉得backgroudworker比较好用。

当然，我们今天要说的和上面的无关。讲述的是如何在线程中进行挂起唤醒操作。

假设，有一个Thread现在需要挂起，等到合适的时候再唤醒那么这个线程（消费者模式）。如果大家需要用Suspend，Resume操作，我建议还是要思考再三。以下是msdn原话（https://msdn.microsoft.com/zh-cn/library/system.threading.thread.suspend(v=vs.110).aspx）：

Do not use the Suspend and Resume methods to synchronize the activities of threads. You have no way of knowing what code a thread is executing when you suspend it. If you suspend a thread while it holds locks during a security permission evaluation, other threads in the AppDomain might be blocked. If you suspend a thread while it is executing a class constructor, other threads in the AppDomain that attempt to use that class are blocked. Deadlocks can occur very easily.

本篇文章要说的线程挂起与继续的方式其实是利用AutoResetEvent和ManualResetEvent的方法进行堵塞和继续的。

在介绍AutoResetEvent和ManualResetEvent之前，先介绍一个概念，就是线程中Set()和Reset（）的区别。

• set：指的是将一个事件设置为有信号，那么被这个事件堵塞的线程就会继续下去。
• reset：指的是将一个事件设置为无信号，那么尝试继续的事件就会被堵塞。

一，AutoResetEvent类

这个类的字面意思就能够解释一切：自动reset的事件，就是这个事件一旦set之后，如果某个线程堵塞被继续了，那么就会自动reset。下一次如果尝试继续，依然会被堵塞。

其中AutoResetEvent类的构造函数有一个参数 是bool型。

MSDN的解释是：

Initializes a new instance of the AutoResetEvent class with a Boolean value indicating whether to set the initial state to signaled.

如果这个参数是true，那么第一次尝试继续就不会被阻塞。如果这个参数是false，那么第一次尝试继续就会被堵塞。

以下是测试代码，取自MSDN：

using System;
using System.Threading;

// Visual Studio: Replace the default class in a Console project with
//                the following class.
class Example
{
    private static AutoResetEvent event_1 = new AutoResetEvent(true);
    private static AutoResetEvent event_2 = new AutoResetEvent(false);

    static void Main()
    {
        Console.WriteLine("Press Enter to create three threads and start them.\r\n" +
                          "The threads wait on AutoResetEvent #1, which was created\r\n" +
                          "in the signaled state, so the first thread is released.\r\n" +
                          "This puts AutoResetEvent #1 into the unsignaled state.");
        Console.ReadLine();

        for (int i = 1; i < 4; i++)
        {
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;
            t.Start();
        }
        Thread.Sleep(250);

        for (int i = 0; i < 2; i++)
        {
            Console.WriteLine("Press Enter to release another thread.");
            Console.ReadLine();
            event_1.Set();
            Thread.Sleep(250);
        }

        Console.WriteLine("\r\nAll threads are now waiting on AutoResetEvent #2.");
        for (int i = 0; i < 3; i++)
        {
            Console.WriteLine("Press Enter to release a thread.");
            Console.ReadLine();
            event_2.Set();
            Thread.Sleep(250);
        }

        // Visual Studio: Uncomment the following line.
        //Console.Readline();
    }

    static void ThreadProc()
    {
        string name = Thread.CurrentThread.Name;

        Console.WriteLine("{0} waits on AutoResetEvent #1.", name);
        event_1.WaitOne();
        Console.WriteLine("{0} is released from AutoResetEvent #1.", name);

        Console.WriteLine("{0} waits on AutoResetEvent #2.", name);
        event_2.WaitOne();
        Console.WriteLine("{0} is released from AutoResetEvent #2.", name);

        Console.WriteLine("{0} ends.", name);
    }
}

其中，AutoResetEvent.WaitOne()这个方法就是线程中尝试继续。如果没有SET信号，那么就会一直阻塞，如果收到Set信号该线程就会继续。但是因为是AutoResetEvent，所以下一次waitOne依然会被阻塞。

上面代码的输出结果是：

Press Enter to create three threads and start them.
The threads wait on AutoResetEvent #1, which was created
in the signaled state, so the first thread is released.
This puts AutoResetEvent #1 into the unsignaled state.

Thread_1 waits on AutoResetEvent #1.
Thread_1 is released from AutoResetEvent #1.
Thread_1 waits on AutoResetEvent #2.
Thread_3 waits on AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #1.
Press Enter to release another thread.

Thread_3 is released from AutoResetEvent #1.
Thread_3 waits on AutoResetEvent #2.
Press Enter to release another thread.

Thread_2 is released from AutoResetEvent #1.
Thread_2 waits on AutoResetEvent #2.

All threads are now waiting on AutoResetEvent #2.
Press Enter to release a thread.

Thread_2 is released from AutoResetEvent #2.
Thread_2 ends.
Press Enter to release a thread.

Thread_1 is released from AutoResetEvent #2.
Thread_1 ends.
Press Enter to release a thread.

Thread_3 is released from AutoResetEvent #2.
Thread_3 ends.

二，ManualResetEvent

ManualResetEvent和AutoResetEvent大部分概念都是相同的，最大的不同就是一个是自动reset一个是手动reset。也就是说，如果使用ManualResetEvent类，一旦Set之后，所有已经阻塞的线程（waitone（））都会继续。而且之后调用waitone的线程也不会被堵塞，除非手动再次Reset。也就是说，这个类是手动开启关闭信号的事件。

以下是测试代码，取自MSDN：

using System;
using System.Threading;

public class Example
{
    // mre is used to block and release threads manually. It is
    // created in the unsignaled state.
    private static ManualResetEvent mre = new ManualResetEvent(false);

    static void Main()
    {
        Console.WriteLine("\nStart 3 named threads that block on a ManualResetEvent:\n");

        for(int i = 0; i <= 2; i++)
        {
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;
            t.Start();
        }

        Thread.Sleep(500);
        Console.WriteLine("\nWhen all three threads have started, press Enter to call Set()" +
                          "\nto release all the threads.\n");
        Console.ReadLine();

        mre.Set();

        Thread.Sleep(500);
        Console.WriteLine("\nWhen a ManualResetEvent is signaled, threads that call WaitOne()" +
                          "\ndo not block. Press Enter to show this.\n");
        Console.ReadLine();

        for(int i = 3; i <= 4; i++)
        {
            Thread t = new Thread(ThreadProc);
            t.Name = "Thread_" + i;
            t.Start();
        }

        Thread.Sleep(500);
        Console.WriteLine("\nPress Enter to call Reset(), so that threads once again block" +
                          "\nwhen they call WaitOne().\n");
        Console.ReadLine();

        mre.Reset();

        // Start a thread that waits on the ManualResetEvent.
        Thread t5 = new Thread(ThreadProc);
        t5.Name = "Thread_5";
        t5.Start();

        Thread.Sleep(500);
        Console.WriteLine("\nPress Enter to call Set() and conclude the demo.");
        Console.ReadLine();

        mre.Set();

        // If you run this example in Visual Studio, uncomment the following line:
        //Console.ReadLine();
    }

    private static void ThreadProc()
    {
        string name = Thread.CurrentThread.Name;

        Console.WriteLine(name + " starts and calls mre.WaitOne()");

        mre.WaitOne();

        Console.WriteLine(name + " ends.");
    }
}

输出结果是：

Start 3 named threads that block on a ManualResetEvent:

Thread_0 starts and calls mre.WaitOne()
Thread_1 starts and calls mre.WaitOne()
Thread_2 starts and calls mre.WaitOne()

When all three threads have started, press Enter to call Set()
to release all the threads.

Thread_2 ends.
Thread_0 ends.
Thread_1 ends.

When a ManualResetEvent is signaled, threads that call WaitOne()
do not block. Press Enter to show this.

Thread_3 starts and calls mre.WaitOne()
Thread_3 ends.
Thread_4 starts and calls mre.WaitOne()
Thread_4 ends.

Press Enter to call Reset(), so that threads once again block
when they call WaitOne().

Thread_5 starts and calls mre.WaitOne()

Press Enter to call Set() and conclude the demo.

Thread_5 ends.

ManualResetEvent类的输出结果与AutoResetEvent输出结果最大的不同是在于：

如果不手动Reset，一旦调用Set方法，那么ManualResetEvent.WaitOne(）就不会堵塞。

但是，AutoResetEvent会自动Reset，所以哪怕不手动Reset，每一次AutoResetEvent.WaitOne()都需要Set方法进行触发以继续线程。

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