c# - 计算带锁和不带锁的线程 (c#)

Question

Grant Crofton's answer对“无序线程问题”的问题有评论:

"And if you remove the lock the final count might be less than 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

这是可能没有锁的情况:

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并通过执行操作 Atomic 来解决

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