计算带锁和不带锁的线程（c#）

Question

Grant Crofton's answer对问题“无序线程问题”有评论：

“如果你移除锁，最终计数可能会少于 100。”

为什么？

这是上下文的代码

class Program
{
    static object locker = new object();
    static int count=0;

    static void Main(string[] args)
    {
        for (int j = 0; j < 100; j++)
        {
            (new Thread(new ParameterizedThreadStart(dostuff))).Start(j);
        }
        Console.ReadKey();
    }

    static void dostuff(object Id)
    {
        lock (locker)
        {
            count++;
            Console.WriteLine("Thread {0}: Count is {1}", Id, count);
        }
    }
}

Answer 1

这是可能在没有锁的情况下发生的：

Count = 0

ThreadA Reads Count As 0 
ThreadB Reads Count As 0
ThreadA Writes New Incremented Count As 0 + 1
ThreadB Writes New Incremented Count As 0 + 1

Count = 1

这些被称为Race Conditions 并通过使操作Atomic 解决

Count = 0

ThreadB Wins Race To Lock()
ThreadB Reads Count As 0
ThreadB Writes New Incremented Count As 0 + 1
ThreadB Unlocks

ThreadA Next for Lock()
ThreadA Reads Count As 1
ThreadA Writes New Incremented Count As 1 + 1
ThreadA Unlocks

Count = 2

Answer 2

因为锁可以保护count 变量免受多个线程试图一次递增它的影响。如果发生这种情况，可能会给您带来奇怪的结果。

锁确保线程一次只能进入您的函数一个，因此只能一次执行一个内部操作。

如果你删除它，那么线程可以同时进入，在任何这些操作的中间被中断，给你奇怪的结果，乱序的结果，等等。