Is the BlockingCollection.TakeFromAny method suitable for building a blocking priority queue?

Question

I need to build a blocking priority queue and my hunch is that TakeFromAny may be the secret ingredient, however the documentation on that method is sparse. What is its purpose / appropriate use?

My requirement is that multiple threads will add to either a high priority or low priority queue. One thread will consume these two queues always taking from the high priority queue before the low priority queue.

It's quite possible that the neither the BlockingCollection or the TakeFromAny method will be of any use to me. If so, then a pointer in the right direction would be appreciated.

Latonia

Answer 1

You are right. The documentation is rather sparse. However, I took a look at the implemenation via Reflector and I believe you can use the BlockingCollection.TakeFromAny method to simulate the priority bias you desire. The reason is because the implementation uses the WaitHandle.WaitAny method which returns the smallest index of all signaled objects. That means if you have two or more queues with items available then the queue appearing first in the array will always be chosen.

The following code should always output "high".

var low = new BlockingCollection<object> { "low" };
var high = new BlockingCollection<object> { "high" };
var array = new BlockingCollection<object>[] { high, low };
object item;
int index = BlockingCollection<object>.TakeFromAny(array, out item);
Console.WriteLine(item);

Karakoram

Thanks brian for this very timely response. Your answer agrees with what I was observing. My concern now is that since this behaviour is not documented, how much of a risk am I taking using trusting that it won't change from one version to the next? – Latonia

@Ralph: I understand your concern. I have a suspicion that this is the exact behavior Microsoft was after so my hunch is that it will always behave this way. It would definitely be nice to get that critical piece of information added to the documentation for confirmation though huh? – Karakoram

Wouldn't it be called TakeFromFirstAvailable if that was the behavior MS was after? – Delivery

@Delivery --- i think you're right, TakeFromAny suggests that any collection is as good as another, and so the behaviour I'm depending on could be "fixed" in a future version. Wonder what the best way is to get MS to clarify their intention for this method. – Latonia

@Gabe: Good point. I guess you could make a similar argument for the WaitHandle.WaitAny method as well. – Karakoram