详解C# 线程的挂起与唤醒
作者：jack_Meng
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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