关于CountdownEvent网上的介绍比较少，因为它是实现和使用都很简单，先看看网上的一些评论吧：
CountDownEvent调用成员函数Wait()将阻塞，直至成员函数Signal() 被调用达特定的次数，这时CountDownEvent称作就绪态，对于处于就绪态的CountDownEvent，调用Wait()函数将不会再阻塞，只有手动调用Reset()函数后，调用Wait()函数将再次阻塞。CountDownEvent可以通过TryAddCount()和AddCount()函数来增加函数Signal() 需被调用的次数，但只有当CountDownEvent处于未就绪态时才会成功。否则根据调用函数的不同，将有可能抛出异常
当有新的需要同步的任务产生时，就调用AddCount增加它的计数，当有任务到达同步点是，就调用Signal函数减小它的计数，当CountdownEvent的计数为零时，就表示所有需要同步的任务已经完成，可以开始下一步任务了。
我们来看看CountdownEvent的实现：
public class CountdownEvent : IDisposable { private int m_initialCount; // The original # of signals the latch was instantiated with. private volatile int m_currentCount; // The # of outstanding signals before the latch transitions to a signaled state. private ManualResetEventSlim m_event; // An event used to manage blocking and signaling. private volatile bool m_disposed; // Whether the latch has been disposed. public CountdownEvent(int initialCount) { if (initialCount < 0) { throw new ArgumentOutOfRangeException("initialCount"); } m_initialCount = initialCount; m_currentCount = initialCount; // Allocate a thin event, which internally defers creation of an actual Win32 event. m_event = new ManualResetEventSlim(); // If the latch was created with a count of 0, then it's already in the signaled state. if (initialCount == 0) { m_event.Set(); } } public int CurrentCount { get { int observedCount = m_currentCount; return observedCount < 0 ? 0 : observedCount; } } public bool IsSet { get { return (m_currentCount <= 0); } } //<returns>true if the signal caused the count to reach zero and the event was set; otherwise, false. public bool Signal() { ThrowIfDisposed(); Contract.Assert(m_event != null); if (m_currentCount <= 0) { throw new InvalidOperationException(Environment.GetResourceString("CountdownEvent_Decrement_BelowZero")); } int newCount = Interlocked.Decrement(ref m_currentCount); if (newCount == 0) { m_event.Set(); return true; } else if (newCount < 0) { throw new InvalidOperationException(Environment.GetResourceString("CountdownEvent_Decrement_BelowZero")); } return false; } //<returns>true if the signals caused the count to reach zero and the event was set; otherwise, false. public bool Signal(int signalCount) { if (signalCount <= 0) { throw new ArgumentOutOfRangeException("signalCount"); } ThrowIfDisposed(); Contract.Assert(m_event != null); int observedCount; SpinWait spin = new SpinWait(); while (true) { observedCount = m_currentCount; if (observedCount < signalCount) { throw new InvalidOperationException(Environment.GetResourceString("CountdownEvent_Decrement_BelowZero")); } if (Interlocked.CompareExchange(ref m_currentCount, observedCount - signalCount, observedCount) == observedCount) { break; } spin.SpinOnce(); } if (observedCount == signalCount) { m_event.Set(); return true; } Contract.Assert(m_currentCount >= 0, "latch was decremented below zero"); return false; } public void AddCount(int signalCount) { if (!TryAddCount(signalCount)) { throw new InvalidOperationException(Environment.GetResourceString("CountdownEvent_Increment_AlreadyZero")); } } //<returns> true if the increment succeeded; otherwise, false public bool TryAddCount(int signalCount) { if (signalCount <= 0) { throw new ArgumentOutOfRangeException("signalCount"); } ThrowIfDisposed(); int observedCount; SpinWait spin = new SpinWait(); while (true) { observedCount = m_currentCount; if (observedCount <= 0) { return false; } else if (observedCount > (Int32.MaxValue - signalCount)) { throw new InvalidOperationException(Environment.GetResourceString("CountdownEvent_Increment_AlreadyMax")); } if (Interlocked.CompareExchange(ref m_currentCount, observedCount + signalCount, observedCount) == observedCount) { break; } spin.SpinOnce(); } return true; } public void Reset() { Reset(m_initialCount); } public void Reset(int count) { ThrowIfDisposed(); if (count < 0) { throw new ArgumentOutOfRangeException("count"); } m_currentCount = count; m_initialCount = count; if (count == 0) { m_event.Set(); } else { m_event.Reset(); } } //Blocks the current thread until the is set public void Wait() { Wait(Timeout.Infinite, new CancellationToken()); } public bool Wait(int millisecondsTimeout, CancellationToken cancellationToken) { if (millisecondsTimeout < -1) { throw new ArgumentOutOfRangeException("millisecondsTimeout"); } ThrowIfDisposed(); cancellationToken.ThrowIfCancellationRequested(); bool returnValue = IsSet; // If not completed yet, wait on the event. if (!returnValue) { // ** the actual wait returnValue = m_event.Wait(millisecondsTimeout, cancellationToken); //the Wait will throw OCE itself if the token is canceled. } return returnValue; } }
代码非常简单，CountdownEvent内部是实现还是依赖于ManualResetEventSlim实例，int initialCount参数为0是就调用ManualResetEventSlim的set方法，这样Wait的对象就不被阻塞了。IsSet是一个主要属性【return (m_currentCount <= 0)】，Signal()和Signal(int signalCount)方法就是减少m_currentCount的，主要采用原子操作【Interlocked.Decrement(ref m_currentCount)】 和【Interlocked.CompareExchange(ref m_currentCount, observedCount - signalCount, observedCount) == observedCount】，如果减少后m_currentCount==0 就调用set方法，为Wait的线程放行；注意这里面有使用SpinWait的自旋，那么AddCount、TryAddCount和Signal方法相反，主要是增加m_currentCount，实现方式一样，采用原子操作+自旋；Reset还原为初始值，Reset(int count)还原为制定的值，Wait方法主要看是否是IsSet【是则是调用了Set方法的】，则直接返回【当前m_currentCount==0】，否者就调用ManualResetEventSlim的Wai方法。