描述C#多线程中lock关键字的使用分析

using System;
using System.Threading;
namespace ThreadSimple
{
    internal class Account
    {
        int balance; //余额
        Random r=new Random();
        internal Account(int initial) 
        { 
            balance=initial;
        }
        internal int Withdraw(int amount) //取回、取款
        {
            if(balance<0)
            { 
                //如果balance小于0则抛出异常 
                throw new Exception("NegativeBalance");//负的 余额
            }
            //下面的代码保证在当前线程修改balance的值完成之前
            //不会有其他线程也执行这段代码来修改balance的值 
            //因此，balance的值是不可能小于0的 
            lock(this) 
            {
                Console.WriteLine("CurrentThread:"+Thread.CurrentThread.Name);
                //如果没有lock关键字的保护，那么可能在执行完if的条件判断(成立)之后 
                //另外一个线程却执行了balance=balance-amount修改了balance的值
                //而这个修改对这个线程是不可见的，所以可能导致这时if的条件已经不成立了
                //但是，这个线程却继续执行 balance=balance-amount，所以导致balance可能小于0
                if(balance>=amount)
                {
                    Thread.Sleep(5);
                    balance=balance-amount;
                    return  amount;
                }  else
                {
                    return 0;
                    //transactionrejected
                }
            } 
        }
        internal void DoTransactions()//取款事务
        {
            for (int i = 0; i < 100; i++)
            {
                Withdraw(r.Next(-50, 100));
            }
        }
    }  
    internal class Test 
    { 
        static internal Thread[] threads=new Thread[10];
        public static void Main() 
        { 
            Account acc=new Account(0);
            for(int i=0;i<10;i++)
            { 
                Thread t=new Thread(new ThreadStart(acc.DoTransactions));
                threads[i]=t;
            }
            for (int i = 0; i < 10; i++)
            {
                threads[i].Name = i.ToString();
            }
            for (int i = 0; i < 10; i++)
            {
                threads[i].Start();
                Console.ReadLine();
            }
        }
    }
}