ActionBlock有时似乎忽略了MaxDegreeOfParallelism

当发布到ActionBlock时，我们看到了意外的行为，在ActionBlock中，即使MaxDegreeOfParallelism为1，并行处理似乎也在发生。下面是场景。

发布到ActionBlock的类如下所示：

public class ByteHandler {
...
   public ByteHandler(ByteHandlingCallback delegateCallback){
       _byteHandlingActionBlock = new ActionBlock<byte[]>(bytes => delegateCallback.HandleBytes(bytes));
   }

   public void HandleBytes(byte[] bytes){
       _byteHandlingActionBlock.Post(bytes);
   }

在下游，我们将字节反序列化为对象，并根据它们的类型将这些对象（我们称之为Notifications）传递给处理程序：

public class NotificationHandler{
   private readonly Dictionary<string, AutoResetEvent> _secondNoticeReceivedEvents;
   public void HandleInitialNotification(InitialNotification notification){
       var secondNoticeReceivedEvent = new AutoResetEvent(false);
       if (!_secondNoticeReceivedEvents.TryAdd(notification.ID, secondNoticeReceivedEvent)) return;
       DoSomethingDownstream(notification);
       if (secondNoticeReceivedEvent.WaitOne(_timeout))
            DoSomethingElseDownstream();
       else
            throw new Exception("Second notification not received within timeout!");
       _secondNoticeReceivedEvents.TryRemove(notification.ID, out secondNoticeReceivedEvent);
   }
   public void HandleSecondNotification(SecondNotification notification){
       AutoResetEvent secondNoticeReceivedEvent;
       if (_secondNoticeReceivedEvents.TryRemove(notification.ID, out secondNoticeReceivedEvent))
            secondNoticeReceivedEvent.Set();
   }

此处理程序有一个致命错误：InitialNotifications在其相应的SecondNotifications之前进入，但HandleInitialNotification在退出之前等待HandleSecondNotification，因此线程永远不会到达HandleSecondNotification。

通常，我们会看到HandleInitialNotification被阻塞，直到等待HandleSecondNotification超时，然后在同一线程上继续执行挂起的SecondNotification。这是我们通常在日志中看到的：

2013-07-05 13:27:25,755 [13] INFO  Received InitialNotification for: XX
2013-07-05 13:27:35,758 [13] WARN  Second notification not not received for XX within timeout!
2013-07-05 13:27:35,761 [13] INFO  Received SecondNotification for: XX

这不是代码的工作方式，但考虑到它的编写方式，它应该总是超时等待SecondNotification。然而，我们偶尔也会看到HandleInitialNotification在超时之前完成，HandleSecondNotification在另一个线程上被及时处理：

2013-07-05 13:38:13,232 [56] INFO  Received InitialNotification for: YY
2013-07-05 13:38:13,258 [11] INFO  Received SecondNotification for: YY

由于我们使用默认的ActionBlock，MaxDegreeOfParallelism应该是1。那么，当发布到ActionBlock的原始线程正在阻塞时，第二个线程（源自ActionBlock）是如何接收SecondNotification的呢？