使用线程 Monitor.TryEnter()

xiaoxiao2021-02-28  13

Monitor 类的TryEnter() 方法在尝试获取一个对象上的显式锁方面和 Enter() 方法类似。然而,它不像Enter()方法那样会阻塞执行。如果线程成功进入关键区域那么TryEnter()方法会返回true.

TryEnter()方法的三个重载方法中的两个以一个timeout类型值作为参数,表示按照指定时间等待锁。我们来看一个关于如何使用TryEnter()方法的例子,MonitorTryEnter.cs:

/************************************* /* Copyright (c) 2012 Daniel Dong * * Author:oDaniel Dong * Blog:o www.cnblogs.com/danielWise * Email:o guofoo@163.com * */ using System; using System.Collections.Generic; using System.Text; using System.Threading; namespace MonitorTryEnter { public class TryEnter { public TryEnter() { } public void CriticalSection() { bool b = Monitor.TryEnter(this, 1000); Console.WriteLine("Thread " + Thread.CurrentThread.GetHashCode() + " TryEnter Value " + b); if (b) { for (int i = 1; i <= 3; i++) { Thread.Sleep(1000); Console.WriteLine(i + " " + Thread.CurrentThread.GetHashCode() + " "); } } if (b) { Monitor.Exit(this); } } public static void Main() { TryEnter a = new TryEnter(); Thread t1 = new Thread(new ThreadStart(a.CriticalSection)); Thread t2 = new Thread(new ThreadStart(a.CriticalSection)); t1.Start(); t2.Start(); Console.ReadLine(); } } }

一个可能的输出结果如下:

当发生资源争夺而你又不像让线程睡眠一段不可预期的时间时TryEnter()方法很有用。向ISP拨号的例子很好的解释这个。假设有两个程序A和B,它们都想使用同一个调制解调器向ISP拨号。而一旦连接建立那么只会有一个网络连接,我们不知道已有的应用程序将会连接多长时间。假设程序A首先向ISP拨号,然后程序B也向ISP拨号;毫无疑问程序B将会一直等待,因为我们不知道程序A将连接多久。在这种情况下,程序B可能使用TryEnter()来确定调制解调器是否已经被另外一个应用程序锁定(本例中是程序A),而不是使用Enter()方法导致一直等待。

 

lock 关键字

lock 关键字可以作为Monitor类的一个替代。下面两个代码块是等效的:

Monitor.Enter(this); //... Monitor.Exit(this); lock (this) { //... }

下面的例子, Locking.cs, 使用lock 关键字而不是Monitor方法:

/************************************* /* copyright (c) 2012 daniel dong * * author:daniel dong * blog: www.cnblogs.com/danielwise * email: guofoo@163.com * */ using System; using System.Collections.Generic; using System.Text; using System.Threading; namespace Lock { class LockWord { private int result = 0; public void CriticalSection() { lock (this) { //Enter the Critical Section Console.WriteLine("Entered Thread " + Thread.CurrentThread.GetHashCode()); for (int i = 1; i <= 5; i++) { Console.WriteLine("Result = " + result++ + " ThreadID " + Thread.CurrentThread.GetHashCode()); Thread.Sleep(1000); } Console.WriteLine("Exiting Thread " + Thread.CurrentThread.GetHashCode()); } } public static void Main(string[] args) { LockWord e = new LockWord(); Thread t1 = new Thread(new ThreadStart(e.CriticalSection)); t1.Start(); Thread t2 = new Thread(new ThreadStart(e.CriticalSection)); t2.Start(); //Wait till the user enters something Console.ReadLine(); } } }

Locking.cs 的输出与MonitorEnterExit(需要提供一个参数)一样:

 

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