CODEWITHSUNDEEP
c#blockingcollectionconcurrent-collections
sakura-bloom
sakura-bloom

Reputation: 4594

BlockingCollection - making consumer wait

Using the second example from Microsoft Docs, when I have a non-blocking consumer, what is the preferred approach to make consumer wait when there are no items in a BlockingCollection? The example from the docs is as follows.

static void NonBlockingConsumer(BlockingCollection<int> bc, CancellationToken ct)
{
    // IsCompleted == (IsAddingCompleted && Count == 0)
    while (!bc.IsCompleted)
    {
        int nextItem = 0;
        try
        {
            if (!bc.TryTake(out nextItem, 0, ct))
            {
                Console.WriteLine(" Take Blocked");
            }
            else
                Console.WriteLine(" Take:{0}", nextItem);
        }

        catch (OperationCanceledException)
        {
            Console.WriteLine("Taking canceled.");
            break;
        }

        // Slow down consumer just a little to cause
        // collection to fill up faster, and lead to "AddBlocked"
        Thread.SpinWait(500000);
    }

    Console.WriteLine("\r\nNo more items to take.");
}

The above example uses SpinWait to pause the consumer.

Simply using the following would probably keep the CPU very busy.

if (!bc.TryTake(out var item))
{
    continue;
}

What is the preferred approach here to make consumer wait? I am planning of using several BlockingCollections and looking for the most optimal way of using it.

Upvotes: 1

Views: 2074

Answers (1)

mjwills
mjwills

Reputation: 23898

I would suggest using Take rather than TryTake.

A call to Take may block until an item is available to be removed.

The link you mentioned in your question has a good (blocking) example:

while (!dataItems.IsCompleted)
{

    Data data = null;
    // Blocks if number.Count == 0
    // IOE means that Take() was called on a completed collection.
    // Some other thread can call CompleteAdding after we pass the
    // IsCompleted check but before we call Take. 
    // In this example, we can simply catch the exception since the 
    // loop will break on the next iteration.
    try
    {
        data = dataItems.Take();
    }
    catch (InvalidOperationException) { }

    if (data != null)
    {
        Process(data);
    }
}
Console.WriteLine("\r\nNo more items to take.");

Upvotes: 3

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