C# 線程的掛起與喚醒 （AutoResetEvent,ManualResetEvent）

     如果說C#和C++有什么不同，博主不得不說，對於異步的支持程度是C#的一一個偉大的進步。

    其實早期的C++都沒有異步，並發的概念。博主第一次使用C++創建異步程序的時候，是使用boost庫的內容進行實現的。相對而言，C#對於異步的支持可以說是相當的好。相信很多名詞大家都很耳熟能詳，比如說Thread，BeginInvoke，Delegate，backgroundworker等等。。。其實樓主在使用了這么多的異步操作過程中，還是覺得backgroudworker比較好用。

    當然，我們今天要說的和上面的無關。講述的是如何在線程中進行掛起喚醒操作。

    假設，有一個Thread現在需要掛起，等到合適的時候再喚醒那么這個線程（消費者模式）。如果大家需要用Suspend，Resume操作，我建議還是要思考再三。以下是msdn原話（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.

     本篇文章要說的線程掛起與繼續的方式其實是利用AutoResetEvent和ManualResetEvent的方法進行堵塞和繼續的。

在介紹AutoResetEvent和ManualResetEvent之前，先介紹一個概念，就是線程中Set()和Reset（）的區別。

set：指的是將一個事件設置為有信號，那么被這個事件堵塞的線程就會繼續下去。

reset：指的是將一個事件設置為無信號，那么嘗試繼續的事件就會被堵塞。

一，AutoResetEvent類

     這個類的字面意思就能夠解釋一切：自動reset的事件，就是這個事件一旦set之后，如果某個線程堵塞被繼續了，那么就會自動reset。下一次如果嘗試繼續，依然會被堵塞。

      其中AutoResetEvent類的構造函數有一個參數 是bool型。

     MSDN的解釋是：

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

    如果這個參數是true，那么第一次嘗試繼續就不會被阻塞。如果這個參數是false，那么第一次嘗試繼續就會被堵塞。

    以下是測試代碼，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()這個方法就是線程中嘗試繼續。如果沒有SET信號，那么就會一直阻塞，如果收到Set信號該線程就會繼續。但是因為是AutoResetEvent，所以下一次waitOne依然會被阻塞。

上面代碼的輸出結果是：

 

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，最大的不同就是一個是自動reset一個是手動reset。也就是說，如果使用ManualResetEvent類，一旦Set之后，所有已經阻塞的線程（waitone（））都會繼續。而且之后調用waitone的線程也不會被堵塞，除非手動再次Reset。也就是說，這個類是手動開啟關閉信號的事件。

以下是測試代碼，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.");
    }
}

輸出結果是：

 

 

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類的輸出結果與AutoResetEvent輸出結果最大的不同是在於：

如果不手動Reset，一旦調用Set方法，那么ManualResetEvent.WaitOne(）就不會堵塞。

但是，AutoResetEvent會自動Reset，所以哪怕不手動Reset，每一次AutoResetEvent.WaitOne()都需要Set方法進行觸發以繼續線程。

 

 

出處：https://blog.csdn.net/zuoshengting/article/details/74156833