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。
如果需要有跨进程或AppDomain的同步时，可以考虑使用Semaphore。Semaphore是取得的Windows 内核的信号量，所以在整个系统中是有效的。它主要的接口是Release和WaitOne，使用的方式和SemaphoreSlim是一致的。
信号量Semaphore是另外一个CLR中的内核同步对象。在.net中，类Semaphore封装了这个对象。与标准的排他锁对象（Monitor，Mutex，SpinLock）不同的是，它不是一个排他的锁对象，它与SemaphoreSlim，ReaderWriteLock等一样允许多个有限的线程同时访问共享内存资源。
Semaphore就好像一个栅栏，有一定的容量，当里面的线程数量到达设置的最大值时候，就没有线程可以进去。然后，如果一个线程工作完成以后出来了，那下一个线程就可以进去了。Semaphore的WaitOne或Release等操作分别将自动地递减或者递增信号量的当前计数值。当线程试图对计数值已经为0的信号量执行WaitOne操作时，线程将阻塞直到计数值大于0。在构造Semaphore时，最少需要2个参数。信号量的初始容量和最大的容量。
Semaphore的WaitOne或者Release方法的调用大约会耗费1微秒的系统时间，而优化后的SemaphoreSlim则需要大致四分之一微秒。在计算中大量频繁使用它的时候SemaphoreSlim还是优势明显，加上SemaphoreSlim还丰富了不少接口，更加方便我们进行控制，所以在4.0以后的多线程开发中，推荐使用SemaphoreSlim。SemaphoreSlim的实现如下：
public class SemaphoreSlim : IDisposable { private volatile int m_currentCount; //可用数的资源数，<=0开始阻塞 private readonly int m_maxCount; private volatile int m_waitCount; //阻塞的线程数 private object m_lockObj; private volatile ManualResetEvent m_waitHandle; private const int NO_MAXIMUM = Int32.MaxValue; //Head of list representing asynchronous waits on the semaphore. private TaskNode m_asyncHead; // Tail of list representing asynchronous waits on the semaphore. private TaskNode m_asyncTail; // A pre-completed task with Result==true private readonly static Task<bool> s_trueTask = new Task<bool>(false, true, (TaskCreationOptions)InternalTaskOptions.DoNotDispose, default(CancellationToken)); public SemaphoreSlim(int initialCount) : this(initialCount, NO_MAXIMUM){ } public SemaphoreSlim(int initialCount, int maxCount) { if (initialCount < 0 || initialCount > maxCount) { throw new ArgumentOutOfRangeException("initialCount", initialCount, GetResourceString("SemaphoreSlim_ctor_InitialCountWrong")); } if (maxCount <= 0) { throw new ArgumentOutOfRangeException("maxCount", maxCount, GetResourceString("SemaphoreSlim_ctor_MaxCountWrong")); } m_maxCount = maxCount; m_lockObj = new object(); m_currentCount = initialCount; } public void Wait(){Wait(Timeout.Infinite, new CancellationToken());} public bool Wait(int millisecondsTimeout, CancellationToken cancellationToken) { CheckDispose(); if (millisecondsTimeout < -1) { throw new ArgumentOutOfRangeException("totalMilliSeconds", millisecondsTimeout, GetResourceString("SemaphoreSlim_Wait_TimeoutWrong")); } cancellationToken.ThrowIfCancellationRequested(); uint startTime = 0; if (millisecondsTimeout != Timeout.Infinite && millisecondsTimeout > 0) { startTime = TimeoutHelper.GetTime(); } bool waitSuccessful = false; Task<bool> asyncWaitTask = null; bool lockTaken = false; CancellationTokenRegistration cancellationTokenRegistration = cancellationToken.InternalRegisterWithoutEC(s_cancellationTokenCanceledEventHandler, this); try { SpinWait spin = new SpinWait(); while (m_currentCount == 0 && !spin.NextSpinWillYield) { spin.SpinOnce(); } try { } finally { Monitor.Enter(m_lockObj, ref lockTaken); if (lockTaken) { m_waitCount++; } } // If there are any async waiters, for fairness we'll get in line behind if (m_asyncHead != null) { Contract.Assert(m_asyncTail != null, "tail should not be null if head isn't"); asyncWaitTask = WaitAsync(millisecondsTimeout, cancellationToken); } // There are no async waiters, so we can proceed with normal synchronous waiting. else { // If the count > 0 we are good to move on. // If not, then wait if we were given allowed some wait duration OperationCanceledException oce = null; if (m_currentCount == 0) { if (millisecondsTimeout == 0) { return false; } // Prepare for the main wait... // wait until the count become greater than zero or the timeout is expired try { waitSuccessful = WaitUntilCountOrTimeout(millisecondsTimeout, startTime, cancellationToken); } catch (OperationCanceledException e) { oce = e; } } Contract.Assert(!waitSuccessful || m_currentCount > 0, "If the wait was successful, there should be count available."); if (m_currentCount > 0) { waitSuccessful = true; m_currentCount--; } else if (oce != null) { throw oce; } if (m_waitHandle != null && m_currentCount == 0) { m_waitHandle.Reset(); } } } finally { // Release the lock if (lockTaken) { m_waitCount--; Monitor.Exit(m_lockObj); } // Unregister the cancellation callback. cancellationTokenRegistration.Dispose(); } return (asyncWaitTask != null) ? asyncWaitTask.GetAwaiter().GetResult() : waitSuccessful; } private bool WaitUntilCountOrTimeout(int millisecondsTimeout, uint startTime, CancellationToken cancellationToken) { int remainingWaitMilliseconds = Timeout.Infinite; //Wait on the monitor as long as the count is zero while (m_currentCount == 0) { // If cancelled, we throw. Trying to wait could lead to deadlock. cancellationToken.ThrowIfCancellationRequested(); if (millisecondsTimeout != Timeout.Infinite) { remainingWaitMilliseconds = TimeoutHelper.UpdateTimeOut(startTime, millisecondsTimeout); if (remainingWaitMilliseconds <= 0) { // The thread has expires its timeout return false; } } // ** the actual wait ** if (!Monitor.Wait(m_lockObj, remainingWaitMilliseconds)) { return false; } } return true; } public Task<bool> WaitAsync(int millisecondsTimeout, CancellationToken cancellationToken) { CheckDispose(); // Validate input if (millisecondsTimeout < -1) { throw new ArgumentOutOfRangeException("totalMilliSeconds", millisecondsTimeout, GetResourceString("SemaphoreSlim_Wait_TimeoutWrong")); } // Bail early for cancellation if (cancellationToken.IsCancellationRequested) return Task.FromCancellation<bool>(cancellationToken); lock (m_lockObj) { // If there are counts available, allow this waiter to succeed. if (m_currentCount > 0) { --m_currentCount; if (m_waitHandle != null && m_currentCount == 0) m_waitHandle.Reset(); return s_trueTask; } // If there aren't, create and return a task to the caller. // The task will be completed either when they've successfully acquired // the semaphore or when the timeout expired or cancellation was requested. else { Contract.Assert(m_currentCount == 0, "m_currentCount should never be negative"); var asyncWaiter = CreateAndAddAsyncWaiter(); return (millisecondsTimeout == Timeout.Infinite && !cancellationToken.CanBeCanceled) ? asyncWaiter : WaitUntilCountOrTimeoutAsync(asyncWaiter, millisecondsTimeout, cancellationToken); } } } /// <summary>Creates a new task and stores it into the async waiters list.</summary> /// <returns>The created task.</returns> private TaskNode CreateAndAddAsyncWaiter() { Contract.Assert(Monitor.IsEntered(m_lockObj), "Requires the lock be held"); // Create the task var task = new TaskNode(); // Add it to the linked list if (m_asyncHead == null) { Contract.Assert(m_asyncTail == null, "If head is null, so too should be tail"); m_asyncHead = task; m_asyncTail = task; } else { Contract.Assert(m_asyncTail != null, "If head is not null, neither should be tail"); m_asyncTail.Next = task; task.Prev = m_asyncTail; m_asyncTail = task; } // Hand it back return task; } private async Task<bool> WaitUntilCountOrTimeoutAsync(TaskNode asyncWaiter, int millisecondsTimeout, CancellationToken cancellationToken) { Contract.Assert(asyncWaiter != null, "Waiter should have been constructed"); Contract.Assert(Monitor.IsEntered(m_lockObj), "Requires the lock be held"); using (var cts = cancellationToken.CanBeCanceled ? CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, default(CancellationToken)) : new CancellationTokenSource()) { var waitCompleted = Task.WhenAny(asyncWaiter, Task.Delay(millisecondsTimeout, cts.Token)); if (asyncWaiter == await waitCompleted.ConfigureAwait(false)) { cts.Cancel(); // ensure that the Task.Delay task is cleaned up return true; // successfully acquired } } // If we get here, the wait has timed out or been canceled. // If the await completed synchronously, we still hold the lock. If it didn't, // we no longer hold the lock. As such, acquire it. lock (m_lockObj) { // Remove the task from the list. If we're successful in doing so, // we know that no one else has tried to complete this waiter yet, // so we can safely cancel or timeout. if (RemoveAsyncWaiter(asyncWaiter)) { cancellationToken.ThrowIfCancellationRequested(); // cancellation occurred return false; // timeout occurred } } // The waiter had already been removed, which means it's already completed or is about to // complete, so let it, and don't return until it does. return await asyncWaiter.ConfigureAwait(false) await asyncWaiter.ConfigureAwait(false); } public int Release(){ return Release(1);} public int Release(int releaseCount) { CheckDispose(); // Validate input if (releaseCount < 1) { throw new ArgumentOutOfRangeException( "releaseCount", releaseCount, GetResourceString("SemaphoreSlim_Release_CountWrong")); } int returnCount; lock (m_lockObj) { // Read the m_currentCount into a local variable to avoid unnecessary volatile accesses inside the lock. int currentCount = m_currentCount; returnCount = currentCount; // If the release count would result exceeding the maximum count, throw SemaphoreFullException. if (m_maxCount - currentCount < releaseCount) { throw new SemaphoreFullException(); } // Increment the count by the actual release count currentCount += releaseCount; // Signal to any synchronous waiters int waitCount = m_waitCount; if (currentCount == 1 || waitCount == 1) { Monitor.Pulse(m_lockObj); } else if (waitCount > 1) { Monitor.PulseAll(m_lockObj); } // Now signal to any asynchronous waiters, if there are any. While we've already // signaled the synchronous waiters, we still hold the lock, and thus // they won't have had an opportunity to acquire this yet. So, when releasing // asynchronous waiters, we assume that all synchronous waiters will eventually // acquire the semaphore. That could be a faulty assumption if those synchronous // waits are canceled, but the wait code path will handle that. if (m_asyncHead != null) { Contract.Assert(m_asyncTail != null, "tail should not be null if head isn't null"); int maxAsyncToRelease = currentCount - waitCount; while (maxAsyncToRelease > 0 && m_asyncHead != null) { --currentCount; --maxAsyncToRelease; // Get the next async waiter to release and queue it to be completed var waiterTask = m_asyncHead; RemoveAsyncWaiter(waiterTask); // ensures waiterTask.Next/Prev are null QueueWaiterTask(waiterTask); } } m_currentCount = currentCount; // Exposing wait handle if it is not null if (m_waitHandle != null && returnCount == 0 && currentCount > 0) { m_waitHandle.Set(); } } // And return the count return returnCount; } ///Removes the waiter task from the linked list.</summary> private bool RemoveAsyncWaiter(TaskNode task) { Contract.Requires(task != null, "Expected non-null task"); Contract.Assert(Monitor.IsEntered(m_lockObj), "Requires the lock be held"); // Is the task in the list? To be in the list, either it's the head or it has a predecessor that's in the list. bool wasInList = m_asyncHead == task || task.Prev != null; // Remove it from the linked list if (task.Next != null) task.Next.Prev = task.Prev; if (task.Prev != null) task.Prev.Next = task.Next; if (m_asyncHead == task) m_asyncHead = task.Next; if (m_asyncTail == task) m_asyncTail = task.Prev; Contract.Assert((m_asyncHead == null) == (m_asyncTail == null), "Head is null iff tail is null"); // Make sure not to leak task.Next = task.Prev = null; // Return whether the task was in the list return wasInList; } private static void QueueWaiterTask(TaskNode waiterTask) { ThreadPool.UnsafeQueueCustomWorkItem(waiterTask, forceGlobal: false); } public int CurrentCount { get { return m_currentCount; } } public WaitHandle AvailableWaitHandle { get { CheckDispose(); if (m_waitHandle != null) return m_waitHandle; lock (m_lockObj) { if (m_waitHandle == null) { m_waitHandle = new ManualResetEvent(m_currentCount != 0); } } return m_waitHandle; } } private sealed class TaskNode : Task<bool>, IThreadPoolWorkItem { internal TaskNode Prev, Next; internal TaskNode() : base() {} [SecurityCritical] void IThreadPoolWorkItem.ExecuteWorkItem() { bool setSuccessfully = TrySetResult(true); Contract.Assert(setSuccessfully, "Should have been able to complete task"); } [SecurityCritical] void IThreadPoolWorkItem.MarkAborted(ThreadAbortException tae) { /* nop */ } } }
SemaphoreSlim类有几个私有字段很重要，m_currentCount表示可用资源，如果m_currentCount>0每次调用Wait都会减1，当m_currentCount<=0时再次调用Wait方法就会阻塞。每次调用Release方法m_currentCount都会加1.m_maxCount表示最大可用资源数，是在构造函数中指定的。m_waitCount表示当前阻塞的线程数。TaskNode m_asyncHead，m_asyncTail这2个变量主要用于异步方法。
我们首先来看看Wait方法，这里还有它的异步版本WaitAsync。在Wait方法中首先检查m_currentCount是否为0，如果是我们用SpinWait自旋10次；任意一次Wait都需要锁住m_lockObj对象，m_asyncHead != null表示当前已经存在异步的对象，所以我们调用WaitAsync方法，如果没有那么我们调用WaitUntilCountOrTimeout方法，该方法在m_currentCount==0会阻塞到到m_currentCount不为0或者超时；看到WaitUntilCountOrTimeout方法中【if (!Monitor.Wait(m_lockObj, remainingWaitMilliseconds))】，就很明了Wait方法中【CancellationTokenRegistration cancellationTokenRegistration = cancellationToken.InternalRegisterWithoutEC(s_cancellationTokenCanceledEventHandler, this)】存在的原因了，确实很巧妙【这里和ManualResetEventSlim相似】。现在我们回到WaitAsync方法，该方法也是首先检查m_currentCount是否大于0，大于直接返回。否者调用CreateAndAddAsyncWaiter创建一个Task<bool>【Task<bool>是一个链表结构】，如果没有取消且超时大于-1，那么就调用WaitUntilCountOrTimeoutAsync方法，该方法首先包装一个Task【var waitCompleted = Task.WhenAny(asyncWaiter, Task.Delay(millisecondsTimeout, cts.Token))】然后等待线程【await waitCompleted.ConfigureAwait(false)】返回的是asyncWaiter或者另一个Delay的Task。如果返回的不是asyncWaiter说明已经超时需要调用RemoveAsyncWaiter，然后返回 await asyncWaiter.ConfigureAwait(false)，如果返回的是asyncWaiter，那么就调用Cancel方法。那么这里的asyncWaiter.ConfigureAwait(false)什么时候退出了【或者说不阻塞】，这就要看Release中的QueueWaiterTask方法了。
QueueWaiterTask方法或调用TaskNode的ExecuteWorkItem方法。
那现在我们来看看Release方法，该方法会把currentCount加1，然后把等待线程转为就绪线程【Monitor.Pulse(m_lockObj)或 Monitor.PulseAll(m_lockObj)】，如果存在异步的话，看看还可以释放几个异步task【 int maxAsyncToRelease = currentCount - waitCount】，这里Release的注释很重要，只是没怎么明白，现释同步的waiters，然后在释放异步的waiters，但是释放同步后锁的资源没有释放，在释放异步的waiters时候是把currentCount减1，这样感觉异步waiters优先获取资源。也不知道我的理解是否正确？
1）当ConfigureAwait(true)，代码由同步执行进入异步执行时，当前同步执行的线程上下文信息（比如HttpConext.Current，Thread.CurrentThread.CurrentCulture）就会被捕获并保存至SynchronizationContext中，供异步执行中使用，并且供异步执行完成之后（await之后的代码）的同步执行中使用（虽然await之后是同步执行的，但是发生了线程切换，会在另外一个线程中执行「ASP.NET场景」）。这个捕获当然是有代价的，当时我们误以为性能问题是这个地方的开销引起，但实际上这个开销很小，在我们的应用场景不至于会带来性能问题。
2）当Configurewait(flase)，则不进行线程上下文信息的捕获，async方法中与await之后的代码执行时就无法获取await之前的线程的上下文信息，在ASP.NET中最直接的影响就是HttpConext.Current的值为null。