BlockingCollection实现了生产者/消费者模式，是对IProducerConsumerCollection<T>接口的实现。与其他Concurrent集合一样，每次Add或Take元素，都会导致对集合的lock。只有当确定需要在内存中创建一个生产者，消费者模式时，再考虑这个类。


MSDN中的示例用法：

using (BlockingCollection<int> bc = new BlockingCollection<int>())
    {
       Task.Factory.StartNew(() =>
       {
           for (int i = 0; i < 1000; i++)
           {
               bc.Add(i);
               Thread.Sleep(50); 
           }


           // Need to do this to keep foreach below from hanging
           bc.CompleteAdding();
       });


       // Now consume the blocking collection with foreach.
       // Use bc.GetConsumingEnumerable() instead of just bc because the
       // former will block waiting for completion and the latter will
       // simply take a snapshot of the current state of the underlying collection.
       foreach (var item in bc.GetConsumingEnumerable())
       {
           Console.WriteLine(item);
       }
   }

重点在于迭代时使用bc.GetConsumingEnumerable()，而不是直接对bc迭代。


可以在这里查看GetConsumingEnumerable的实现源码：
https://github.com/dotnet/corefx/blob/master/src/System.Collections.Concurrent/src/System/Collections/Concurrent/BlockingCollection.cs

public IEnumerable<T> GetConsumingEnumerable(CancellationToken cancellationToken)
        {
            CancellationTokenSource linkedTokenSource = null;
            try
            {
                linkedTokenSource = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _consumersCancellationTokenSource.Token);
                while (!IsCompleted)
                {
                    T item;
                    if (TryTakeWithNoTimeValidation(out item, Timeout.Infinite, cancellationToken, linkedTokenSource))
                    {
                        yield return item;
                    }
                }
            }
            finally
            {
                if (linkedTokenSource != null)
                {
                    linkedTokenSource.Dispose();
                }
            }
        }

会不断的check IsCompleted属性：

public bool IsCompleted
        {
            get
            {
                CheckDisposed();
                return (IsAddingCompleted && (_occupiedNodes.CurrentCount == 0));
            }
        }

会判断生产者线程是否已经完成添加并且没有元素被消费。



参考链接：
https://msdn.microsoft.com/en-us/library/dd267312(v=vs.110).aspx