queue.IsCompleted returns false even when the queue is empty?

Question

The following code never return. Debugging shows that the queue.IsCompleted returns false even when the queue is empty. Did I miss anything?

var workers = new Task[1];
using (var queue = new BlockingCollection<QueuePayload>(20))
{
    workers[0] = Task.Run(() => Consume(queue));
    queue.Add(new QueuePayload{....});
    Task.WaitAll(workers);
}

void Consume(BlockingCollection<QueuePayload> queue))
{
    while (!queue.IsCompleted)
    {
        var i = new QueuePayload();
        try
        {
            i = queue.Take();
        }
        catch (InvalidOperationException)
        {
            break;
        }
    ......

Instead of `while (!queue.IsCompleted) { i = queue.Take(); ...` I find it a lot easier to do just do `foreach(var i in queue.GetConsumingEnumerable()) { ...` — Scott Chamberlain, May 16 '16 at 18:13
So if you add `queue.CompleteAdding()` after the last `queue.Add(new QueuePayload{.....})` does it still behave that way? — Berin Loritsch, May 16 '16 at 18:20
I was following the examples in MSDN. Maybe they should update these examples. — ca9163d9, May 16 '16 at 18:21
@ScottChamberlain, won't that still block until you mark the queue as having all adding complete? — Berin Loritsch, May 16 '16 at 18:21
@BerinLoritsch Yes, queue.CompleteAdding() resolve the issue. — ca9163d9, May 16 '16 at 18:21
The examples are updated. It's a very easy detail to miss though. — Berin Loritsch, May 16 '16 at 18:22

score 5 · Accepted Answer · edited May 16 '16 at 18:04

5

The property for IsCompleted at https://msdn.microsoft.com/en-us/library/dd267315(v=vs.110).aspx has the following text:

Whether this collection has been marked as complete for adding and is empty.

Where do you mark the collection as completed? This is not an empty check - it is an empty check with a manual additional switch.

There is accordingly a CompleteAdding() method that marks that no more items will be added.

edited May 16 '16 at 18:04

Bohemian

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answered May 16 '16 at 17:59

TomTom

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Not sure who marked this down, but if the question is about why the `IsCompleted` flag is not returning `true` when the queue is empty it answers that question. – Berin Loritsch May 16 '16 at 18:18

score 1 · Answer 2 · answered May 16 '16 at 18:22

When you're done adding items to the collection, call queue.CompleteAdding(). That way queue.IsCompleted will return true when there are no items in the collection (after you have removed them all.) If you don't call CompleteAdding() then there's a possibility that more items could be added, so IsCompleted will always return false.

queue.IsCompleted returns false even when the queue is empty?

2 Answers2