How to empty a BlockingCollection

Question

I have a thread adding items to a BlockingCollection .

On another thread I am using foreach (var item in myCollection.GetConsumingEnumerable())

If there is a problem I want to break out of my foreach and my method and clear whatever is left in the BlockingCollection however I can't find a way to do it.

Any ideas?

Urita

Answer 1

Possibly use the overload of GetConsumingEnumerable that takes a CancellationToken; then, if anything goes wrong from the producing side, it can cancel the consumer.

Lucianolucias

Can I call the CancellationToken.Cancel within the foreach of GetConsumingEnumerbale? – Urita

I can do it within the GetConsumingEnumerable but it throws a OperationCanceledException. A simple break will leave the foreach loop but the BlockingCollection will still have items in it – Urita

@Jon: I wasn't suggesting cancelling in the consumer, but in the producer - I thought that was where you were detecting the error. If the consumer has noticed the problem, can't it just consume everything until there's nothing left? Does it need to notify the producer to say that it doesn't want any more items? Do you even need the same blocking collection any more? Couldn't you just throw it away? – Lucianolucias

I decided upon using a break in the consumer and then when I start producing again I just create a new instance of the BlockingCollection – Urita

A note to the unwary, GetConsumingEnumerable will block once the collection is empty (waiting on more entries) unless you've previously called CompleteAdding. It's in the sample code block, but not mentioned in the method description. – Elianore

Answer 2

Just take out all remaining items:

while (collection.TryTake(out _)){}

Carmine

Didn't know that _ kan be used in an out statement. Double benefit! (Would like to upvote this answer twise :-) – Hefty

Best one! works like a charm! – Weathertight

Answer 3

I'm using this extension method:

public static void Clear<T>(this BlockingCollection<T> blockingCollection)
{
    if (blockingCollection == null)
    {
        throw new ArgumentNullException("blockingCollection");
    }

    while (blockingCollection.Count > 0)
    {
        T item;
        blockingCollection.TryTake(out item);
    }
}

I'm wondering if there's a better, less hacky, solution.

Almaalmaata

With the new out var syntax: while (blockingCollection.TryTake(out var _)){} – Lamkin

any particular reason why TryTake instead of just plain old Take? Is there any scenario where the try version would return false in this example? – Tallinn

@Tallinn There's a race condition between the while check and TryTake. Another thread, for example, could take the last element out and Take would raise an InvalidOperationException. – Almaalmaata

@PaoloMoretti Ah, I see. In my use case I only had one consumer so I didn't think of that :) – Tallinn

Similar to Kjellski's comment, but without requiring the new syntax, this could be shortened to T _; while (blockingCollection.TryTake(out _)){} – Gutturalize

Similar to @Timbo's comment (if we're going for code golf), it can be shorten to just while (blockingCollection.TryTake(out _)); – Disciplinant

@Disciplinant feel write to update the answer, if you like :-) That syntax was not supported in 2012 by the C# 4.0 compiler – Almaalmaata

Answer 4

This worked for me

while (bCollection.Count > 0)
{
    var obj = bCollection.Take();
    obj.Dispose();
}

Take() removes from the collection and you can call any clean up on your object and the loop condition does not invoke any blocking calls.

Gosselin

This answer turned up in the low quality review queue, presumably because you didn't explain the code. If you do explain it (in your answer), you are far more likely to get more upvotes—and the questioner actually learns something! – Paigepaik

This is inefficient, because depending on the underlying collection you are iterating over each element to determine the count, just to remove the first element. Also, the Dispose is unrelated to the question. – Superpatriot

Answer 5

BlockingCollection<T> yourBlockingCollection = new BlockingCollection<T>();

I assumed you mean clear your blocking collection. Jon's answer is more appropriate to your actual question I think.

Canticle

I meant I wanted to exit the foreach loop and empty whatever is in the BlockingCollection – Urita

can you please explain to me what the difference between removing those last items, and just: theBlockingCollectionUsedIntheForEachLoop = new BlockingCollection<T>(); is??? – Armourer

Answer 6

For just clearing the collection you can do:

myBlockingCollection.TakeWhile<*MyObject*>(qItem => qItem != null);

or just

myBlockingCollection.TakeWhile<*MyObject*>(qItem => true);

Desmid

The TakeWhile is a LINQ porting and does not remove from the collection. It just creates a copy – Tomkin