当多个任务或线程并行运行时，难以避免的对某些有限的资源进行并发的访问。可以考虑使用信号量来进行这方面的控制（System.Threading.Semaphore）是表示一个Windows内核的信号量对象。如果预计等待的时间较短，，可以考虑使用SemaphoreSlim，它则带来的开销更小。

.NetFrameWork中的信号量通过跟踪进入和离开的任务或线程来协调对资源的访问。信号量需要知道资源的最大数量，当一个任务进入时，资源计数器会被减1，当计数器为0时，如果有任务访问资源，它会被阻塞，直到有任务离开为止。

示例程序： 10个任务并行访问3个资源

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(); } } } 使用超时和取消

信号量当然不可能永久的阻塞在那里。信号量也提供了超时处理机制。方法是在Wait函数中传入一个超时等待时间 - Wait(int TIMEOUT)。当Wait返回值为false时表明它超时了。如果传入了 -1，则表示无限期的等待。

程序示例：注意其中的m_Semaphore.Release();已经被注释掉了，任务会等待1秒钟然后超时。

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(); } } } 跨进程或AppDomain的同步