Try Dequeue in ConcurrentQueue

Question

The TryDequeue in ConcurrentQueue<T> will return false if no items in queue.

If the queue is empty I need that my queue will wait until new item to be added in queue and it dequeue that new one, and the process will continues like that.

Should I use Monitor.Enter, Wait, Pulse or any better options in C# 4.0?

hmmm - it surprises me that this isn't conveniently exposed - it seems like a common use-case... — Marc Gravell, Feb 16 '11 at 08:31
I think with 4.0 a BCL class should expose this, but if nothing else appears, perhaps look [at this answer](http://stackoverflow.com/questions/530211/creating-a-blocking-queuet-in-net/530228#530228), in which TryDequeue will block until *either* there is an item (returning `true`) *or* the queue is empty and explicitly shut down (returning `false`) — Marc Gravell, Feb 16 '11 at 08:32
If the Queue is empty I need that my Thread will wait until new item to be added in queue and it dequeue that new one, and the process will continues like that. — C-va, Feb 16 '11 at 08:34
@Marc : I need to resume my thread to dequeue atonce the new item is added. How can i achieve this. — C-va, Feb 16 '11 at 08:40
the above does that, but it looks like "Damien_The_Unbeliever" has the answer here — Marc Gravell, Feb 16 '11 at 09:04
Old question, but people coming here may also want to check out [this](http://stackoverflow.com/a/7868264/4289902) answer that suggests using a [BufferBlock](https://msdn.microsoft.com/en-us/library/mt604455(v=vs.110).aspx), which allows async waiting. — Jono Job, Sep 22 '16 at 04:03

score 55 · Accepted Answer · answered Feb 16 '11 at 08:45

55

Isn't this what BlockingCollection is designed for?

As I understand it, you can wrap your ConcurrentQueue with one of these, and then call Take.

answered Feb 16 '11 at 08:45

Damien_The_Unbeliever

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score 2 · Answer 2 · answered Aug 14 '18 at 22:51

You can use BlockingCollection.

Do something like that:

private BlockingCollection<string> rowsQueue;
private void ProcessFiles() {
   this.rowsQueue = new BlockingCollection<string>(new ConcurrentBag<string>(), 1000);
   ReadFiles(new List<string>() { "file1.txt", "file2.txt" });


   while (!this.rowsQueue.IsCompleted || this.rowsQueue.Count > 0)
   {
       string line = this.rowsQueue.Take();

       // Do something
   }
}

private Task ReadFiles(List<string> fileNames)
{
    Task task = new Task(() =>
    {
        Parallel.ForEach(
        fileNames,
        new ParallelOptions
        {
            MaxDegreeOfParallelism = 10
        },
            (fileName) =>
            {
                using (StreamReader sr = File.OpenText(fileName))
                {
                    string line = String.Empty;
                    while ((line = sr.ReadLine()) != null)
                    {
                           this.rowsQueue.Add(line);
                    }
                }
            });

        this.rowsQueue.CompleteAdding();
    });

    task.Start();

    return task;
}

score -2 · Answer 3 · answered Feb 16 '11 at 08:41

-2

You could periodically check the number of elements in the queue and when the number of elements is greater than zero, you give a signal using e.g. ManualResetEvent to the thread which dequeues the element(s) until the queue is empty.

Here is the pseudo code for this:

Check Thread:

while(true)
{
  int QueueLength = 0;
  lock(Queue)
  {
    queueLength = Queue.Length;
  }

  if (Queue.Length > 0)
  {
    manualResetEvent.Set();
  }
  else
  {
    Thread.Sleep(...);
  }       
}

Dequeue Thread:

while(true)
{
  if(manualResetEvent.WaitOne(timeout))
  {
    DequeueUntilQueueEmpty();
  }
}

Consider using lock in the DequeueUntilQueueEmpty, too.

answered Feb 16 '11 at 08:41

Tomas Walek

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1

Actually im using Monitor.enter,wait,pulseall to make the thread wait and signal. Is ManualResetEvent a better option? – C-va Feb 16 '11 at 09:36
Actually im using Monitor.enter,wait,pulseall to make the thread wait and signal. Is ManualResetEvent a better option? – C-va Feb 16 '11 at 09:45
ManualResetEvent is lightweight option, Monitor is a heavy one. – Tomas Walek Feb 17 '11 at 09:53

Try Dequeue in ConcurrentQueue

3 Answers3