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标题: C#的lock语句

作者: dnawo

日期: 2009-09-27 10:52 AM

分类: Win编程

内容: 复制内容到剪贴板 程序代码using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Thread thread1 = new Thread(new ThreadStart(ThreadStart1));
            thread1.Name = "Thread1";
            Thread thread2 = new Thread(new ThreadStart(ThreadStart2));
            thread2.Name = "Thread2";
            Thread thread3 = new Thread(new ThreadStart(ThreadStart3));
            thread3.Name = "Thread3";
            thread1.Start();
            thread2.Start();
            thread3.Start();
            Console.ReadKey();
        }
        static object _object = new object();
        static void Done(int millisecondsTimeout)
        {
            Console.WriteLine(string.Format("{0} -> {1}.Start", DateTime.Now.ToString("HH:mm:ss"), Thread.CurrentThread.Name));
            //下边代码段同一时间只能由一个线程在执行
            lock (_object)
            {
                Console.WriteLine(string.Format("{0} -> {1}进入锁定区域.", DateTime.Now.ToString("HH:mm:ss"), Thread.CurrentThread.Name));
                Thread.Sleep(millisecondsTimeout);
                Console.WriteLine(string.Format("{0} -> {1}退出锁定区域.", DateTime.Now.ToString("HH:mm:ss"), Thread.CurrentThread.Name));
            }
        }
        static void ThreadStart1()
        {
            Done(5000);
        }
        static void ThreadStart2()
        {
            Done(3000);
        }
        static void ThreadStart3()
        {
            Done(1000);
        }
    }
}
运行效果如下：
如上边例子所示，lock语句保证了某个代码段同一时间只能由一个线程在执行，这在多线程编程中经常用到。msdn说明：lock 确保当一个线程位于代码的临界区时，另一个线程不进入临界区，如果其他线程试图进入锁定的代码，则它将一直等待（即被阻止），直到该对象被释放。
注意：应避免锁定 public 类型，否则实例将超出代码的控制范围。常见的结构 lock (this)、lock (typeof (MyType)) 和 lock ("myLock") 违反此准则：
·如果实例可以被公共访问，将出现 lock (this) 问题；
·如果 MyType 可以被公共访问，将出现 lock (typeof (MyType)) 问题；
·由于进程中使用同一字符串的任何其他代码将共享同一个锁，所以出现 lock("myLock") 问题；
最佳做法是定义 private 对象来锁定, 或 private static 对象变量来保护所有实例所共有的数据。
不使用lock示例
复制内容到剪贴板 程序代码using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;
namespace ConsoleApplication1
{
    class Program
    {
        static string _str = string.Empty;
        static void Main(string[] args)
        {
            Thread thread1 = new Thread(new ThreadStart(ThreadStart1));
            thread1.Name = "Thread1";
            Thread thread2 = new Thread(new ThreadStart(ThreadStart2));
            thread2.Name = "Thread2";
            Thread thread3 = new Thread(new ThreadStart(ThreadStart3));
            thread3.Name = "Thread3";
            thread1.Start();
            thread2.Start();
            thread3.Start();
            Console.ReadKey();
        }
        static object _object = new object();
        static void Done(int millisecondsTimeout)
        {
            //不锁定
            //lock (_object)
            //{
                _str = Thread.CurrentThread.Name;
                Console.WriteLine(string.Format("{0} -> {1}线程将_str赋值为{1}.", DateTime.Now.ToString("HH:mm:ss"), Thread.CurrentThread.Name));
                Thread.Sleep(millisecondsTimeout);
                Console.WriteLine(string.Format("{0} -> {1}线程输出_str:{2}.", DateTime.Now.ToString("HH:mm:ss"), Thread.CurrentThread.Name, _str));
            //}
        }
        static void ThreadStart1()
        {
            Done(5000);
        }
        static void ThreadStart2()
        {
            Done(3000);
        }
        static void ThreadStart3()
        {
            Done(1000);
        }
    }
}
谁都可以修改_str的值，程序运行结果将没有保障！

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