用锁定和不用（c＃）计算线程(Counting threads with lock and without (c#))

用锁定和不用（c＃）计算线程(Counting threads with lock and without (c#))

Grant Crofton对“无序线程问题”问题的回答有一个评论：

“如果你取消锁定，最终计数可能会小于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); } } }

Grant Crofton's answer to the question "Unordered threads problem" has a comment:

"And if you remove the lock the final count might be less than 100."

Why?

Here is the code for context

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); } } }

最满意答案

这是没有锁的情况下 会发生的情况 ：

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

这些称为种族条件 ，并通过使原子操作来解决

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

This is what could happen without a lock:

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

These are called Race Conditions and are solved by making the operation Atomic

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