Interlocked.Increment(ref currentIndex);

appreciatewo 2008-08-21 07:07:05

Interlocked.Increment(ref currentIndex);
这个是什么意思,这么久了从来没有一个真正的回答出这个问题,这个功能具体是什么?

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dylenr 2010-03-29

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嗯，懂了，谢谢

zlkingdom 2008-08-22

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2楼的例子够生动的，其实使用Interlocked.Increment就是为了实现线程安全

宝_爸 2008-08-22

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让++成为原子操作。
什么叫原子操作呢。就是不会被别人打断，因为C#中的一个语句，编译成机器代码后会变成多个语句。
在多线程环境中，线程切换有可能会发生在这多个语句中间。使用Interlocked.Increment可以避免被打断。

gomoku 2008-08-22

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Try this first:



    class Program

    {

        static void Main(string[] args)

        {

            for (int loop = 0; loop < 20; loop++)

            {

                sum = 0;

                Thread t1 = new Thread(Thread1);

                Thread t2 = new Thread(Thread2);

                t1.Start();

                t2.Start();



                t1.Join();

                t2.Join();

                Console.WriteLine("sum = " + sum);         // sum = 200000 ?

            }

        }



        static int sum;

        static void Thread1()

        {

            for (int i = 0; i < 100000; i++) sum++;

        }

        static void Thread2()

        {

            for (int i = 0; i < 100000; i++) sum++;

        }

    }









/*

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 192361

sum = 175155

sum = 200000

sum = 176024

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 200000

sum = 176322

*/

Why the sum is not always 200000?
The reason is that sum++ is not thread safe (see the possible problem).
That is the reason we need Interlocked.Increment(), which guarantees the sum is always 200000.

Possible Problem:



Thread1 (sum++)                   Thread2 (sum++)

--------------------------------------------------------------------

mov   EAX, dword ptr sum          .

inc   EAX                         .

.                                 mov   EAX, dword ptr sum           // load sum into a register

.                                 inc   EAX                          // increase it

.                                 mov   dword ptr sum, EAX           // save back

mov   dword ptr sum, EAX

--------------------------------------------------------------------



problem: two sum++ are called in different thread,

but the sum is incremented only once.