C#中lock关键字

时间：2010-09-27 来源：holywolf

lock关键字可以用来确保代码块完成运行，而不会被其他线程中断。这是通过在代码块运行期间为给定对象获取互斥锁来实现的。

首先定义Account类,如下所示:

public class Account
{
        private int balance = 500;

        public void WithDraw(int amount)
        {
                        
                        if (balance - amount > 0)
                        {
                                Console.WriteLine("{1}:before drawing:balance:{0},amount:{2}", balance, Thread.CurrentThread.Name, amount);
                                Thread.Sleep(500);
                                balance -= amount;
                                Console.WriteLine("{1}:after drawing:balance:{0},amount:{2}", balance, Thread.CurrentThread.Name, amount);
                                Console.WriteLine();

                        }
                        else
                        {
                                Console.WriteLine("{0}:no money", Thread.CurrentThread.Name);
                                Thread.CurrentThread.Abort();
                        }
                        
        }

        public void DoTrans()
        {
                Random ran = new Random();

                for (int i = 0; i < 10; i++)
                {
                        WithDraw(ran.Next(50, 100));
                                
                }
        }
}

该类中定义了某个帐户初使金额是500元,WithDraw方法用于从帐户中取钱,并在取钱之前判断要取的金额是否大于余额,在判断后如果满足条件则执行减操作,此处使用Thread.Sleep方法模拟操作过程中一些耗时操作以允许其他线程有机会执行.

DoTrans方法用于分10次随机从帐户中取款.接下来定义Main方法使用两个线程取款:

static void Main(string[] args)
{                       

        Account a1 = new Account();

        Thread t1 = new Thread(new ThreadStart(a1.DoTrans));
        Thread t2 = new Thread(new ThreadStart(a1.DoTrans));

        t1.Name = "Thread 11111";
        t2.Name = "Thread 22222";

        t1.Start();
        t2.Start();

        Console.ReadLine();
}

运行项目会发现由于两个线程同时取款,在其中一个线程判断满足条件后休眠,由另外一个线程取款,取完款后再返回来执行第一个线程的取款操作,此时一线程已经执行过判断,继承取款会导致负数余额的产生.

因此解决方法可以使用lock关键字,将Account的WithDraw方法修正如下:

public void WithDraw(int amount)
{
        lock (this)
        {
                if (balance - amount > 0)
                {
                        Console.WriteLine("{1}:before drawing:balance:{0},amount:{2}", balance, Thread.CurrentThread.Name, amount);
                        Thread.Sleep(500);
                        balance -= amount;
                        Console.WriteLine("{1}:after drawing:balance:{0},amount:{2}", balance, Thread.CurrentThread.Name, amount);
                        Console.WriteLine();

                }
                else
                {
                        Console.WriteLine("{0}:no money", Thread.CurrentThread.Name);
                        Thread.CurrentThread.Abort();
                }
        }
}

可以保证该代码块不能被中断,从而使帐户金额不会产生负数的情况