概要

    當(dāng)多個任務(wù)或線程并行運(yùn)行時，難以避免的對某些有限的資源進(jìn)行并發(fā)的訪問，

C# 并行編程 之 限制資源的并發(fā)訪問 使用SemaphoreS

�？梢钥紤]使用信號量來進(jìn)行這方面的控制（System.Threading.Semaphore）是表示一個Windows內(nèi)核的信號量對象。如果預(yù)計(jì)等待的時間較短，可以考慮使用SemaphoreSlim，它則帶來的開銷更小。

    .NetFrameWork中的信號量通過跟蹤進(jìn)入和離開的任務(wù)或線程來協(xié)調(diào)對資源的訪問。信號量需要知道資源的最大數(shù)量，當(dāng)一個任務(wù)進(jìn)入時，資源計(jì)數(shù)器會被減1，當(dāng)計(jì)數(shù)器為0時，如果有任務(wù)訪問資源，它會被阻塞，直到有任務(wù)離開為止。

    示例程序： 10個任務(wù)并行訪問3個資源

<code class="hljs cs">using System;using System.Text;using System.Threading;using System.Threading.Tasks;using System.Diagnostics;namespace Sample5_8_semaphoreslim{    class Program    {        private static int _TaskNum = 10;        private static Task[] _Tasks;        private const int MAX_RESOURCE = 3;        private const int RUN_LOOP = 10;        private static SemaphoreSlim m_Semaphore;        private static void Work1(int TaskID)        {            int i = 0;            var sw = Stopwatch.StartNew();            var rnd = new Random();            while (i < RUN_LOOP)            {                Thread.Sleep(rnd.Next(200, 500));                Console.WriteLine("TASK " + TaskID + " REQUESTing {");                m_Semaphore.Wait();                try                {                    Console.WriteLine("TASK " + TaskID + " WOrking  ...  ..." + i);                    sw.Restart();                    Thread.Sleep(rnd.Next(200, 500));                }                finally                {                    Console.WriteLine("TASK " + TaskID + " REQUESTing }");                    m_Semaphore.Release();                    i++;                }            }        }        static void Main(string[] args)        {            _Tasks = new Task[_TaskNum];            m_Semaphore = new SemaphoreSlim(MAX_RESOURCE);            int i = 0;            for (i = 0; i < _TaskNum; i++)            {                _Tasks[i] = Task.Factory.StartNew((num) =>                {                    var taskid = (int)num;                    Work1(taskid);                }, i);            }            var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>            {                Task.WaitAll(_Tasks);                Console.WriteLine("==========================================================");                Console.WriteLine("All Phase is completed");                Console.WriteLine("==========================================================");            });            try            {                finalTask.Wait();            }            catch (AggregateException aex)            {                Console.WriteLine("Task failed And Canceled" + aex.ToString());            }            finally            {                m_Semaphore.Dispose();            }            Console.ReadLine();        }    }}</code>

使用超時和取消

    信號量當(dāng)然不可能永久的阻塞在那里，

電腦資料

《C# 并行編程 之 限制資源的并發(fā)訪問 使用SemaphoreS》(http://www.ishadingyu.com)。信號量也提供了超時處理機(jī)制。方法是在Wait函數(shù)中傳入一個超時等待時間 - Wait(int TIMEOUT)。當(dāng)Wait返回值為false時表明它超時了。如果傳入了 -1，則表示無限期的等待。

    程序示例：注意其中的m_Semaphore.Release();已經(jīng)被注釋掉了，任務(wù)會等待1秒鐘然后超時。

<code class="hljs cs">using System;using System.Text;using System.Threading;using System.Threading.Tasks;using System.Diagnostics;namespace Sample5_8_semaphoreslim{    class Program    {        private static int _TaskNum = 10;        private static Task[] _Tasks;        private const int MAX_RESOURCE = 3;        private const int RUN_LOOP = 10;        private static SemaphoreSlim m_Semaphore;        private static void Work1(int TaskID)        {            int i = 0;            var sw = Stopwatch.StartNew();            var rnd = new Random();            while (i < RUN_LOOP)            {                Thread.Sleep(rnd.Next(200, 500));                Console.WriteLine("TASK " + TaskID + " REQUESTing {");                if (!m_Semaphore.Wait(1000))                {                    Console.WriteLine("TASK " + TaskID + " TIMEOUT!!!");                    return;                }                try                {                    Console.WriteLine("TASK " + TaskID + " WOrking  ...  ..." + i);                    sw.Restart();                    Thread.Sleep(rnd.Next(2000, 5000));                }                finally                {                    Console.WriteLine("TASK " + TaskID + " REQUESTing }");                    //m_Semaphore.Release();                    i++;                }            }        }        static void Main(string[] args)        {            _Tasks = new Task[_TaskNum];            m_Semaphore = new SemaphoreSlim(MAX_RESOURCE);            int i = 0;            for (i = 0; i < _TaskNum; i++)            {                _Tasks[i] = Task.Factory.StartNew((num) =>                {                    var taskid = (int)num;                    Work1(taskid);                }, i);            }            var finalTask = Task.Factory.ContinueWhenAll(_Tasks, (tasks) =>            {                Task.WaitAll(_Tasks);                Console.WriteLine("==========================================================");                Console.WriteLine("All Phase is completed");                Console.WriteLine("==========================================================");            });            try            {                finalTask.Wait();            }            catch (AggregateException aex)            {                Console.WriteLine("Task failed And Canceled" + aex.ToString());            }            finally            {                m_Semaphore.Dispose();            }            Console.ReadLine();        }    }}</code>

跨進(jìn)程或AppDomain的同步

    如果需要有跨進(jìn)程或AppDomain的同步時，可以考慮使用Semaphore。Semaphore是取得的Windows 內(nèi)核的信號量，所以在整個系統(tǒng)中是有效的。

    它主要的接口時 Release和WaitOne，使用的方式和SemaphoreSlim是一致的。