c# - C#锁构造误区(附msdn代码示例)

Question

我对使用 C# lock 构造的一件事很感兴趣现在从 MSDN 取样，然后是主要问题:

以下示例使用线程和锁。只要 lock 语句存在，语句 block 就是临界区，balance 永远不会变成负数。

class Account
{
    private Object thisLock = new Object();
    int balance;

    Random r = new Random();

    public Account(int initial)
    {
        balance = initial;
    }

    int Withdraw(int amount)
    {

        // This condition never is true unless the lock statement
        // is commented out.
        if (balance < 0)
        {
            throw new Exception("Negative Balance");
        }

        // Comment out the next line to see the effect of leaving out 
        // the lock keyword.
        lock (thisLock)
        {
            if (balance >= amount)
            {
                Console.WriteLine("Balance before Withdrawal :  " + balance);
                Console.WriteLine("Amount to Withdraw        : -" + amount);
                balance = balance - amount;
                Console.WriteLine("Balance after Withdrawal  :  " + balance);
                return amount;
            }
            else
            {
                return 0; // transaction rejected
            }
        }
    }

    public void DoTransactions()
    {
        for (int i = 0; i < 100; i++)
        {
            Withdraw(r.Next(1, 100));
        }
    }
}

class Test
{
    static void Main()
    {
        Thread[] threads = new Thread[10];
        Account acc = new Account(1000);
        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].Start();
        }

        //block main thread until all other threads have ran to completion.
        foreach (var t in threads)
            t.Join();
    }
}

我不明白为什么使用lock 帐户余额不会变成负数；它总是以0 余额结束编译。 P.S 对不起我的英语。

Answer 1

在这个例子中，锁按预期工作，在启动多个线程时，它们都会尝试从帐户中退出，这可能会导致非常糟糕的功能。怎么样？
好吧，假设当前余额是 40，线程 1 试图取回 40，线程 2 试图取回 20，如果没有锁，他们都将成功取回，余额将是 -20，这是 Not Acceptable 。

现在您真正关心的是，为什么余额不会变为负数？简单:

lock (thisLock)
    {
        if (balance >= amount)
        {
            Console.WriteLine("Balance before Withdrawal :  " + balance);
            Console.WriteLine("Amount to Withdraw        : -" + amount);
            balance = balance - amount;
            Console.WriteLine("Balance after Withdrawal  :  " + balance);
            return amount;
        }
        else
        {
            return 0; // transaction rejected
        }
    }

锁将确保每个线程只有在余额可用时才会取款，所以条件if (balance >= amount)再加上 if (balance < 0)将确保余额不会变为负值。

如果您记录每个线程提取的金额，您可以看到详细信息:

Console.WriteLine(Withdraw(r.Next(1, 100)));

你会看到他们中的很多人会在一段时间后输出 0，因为账户不再有余额因此 return 0触发器。