how to access the underlying default concurrent queue of a blocking collection

Question

I have multiple producers and a single consumer. However if there is something in the queue that is not yet consumed a producer should not queue it again. (unique no duplica

Answer 1

In addition to the caveat Brian Gideon mentioned after Update, his solution suffers from these performance issues:

• O(n) operations on the queue (queue.Contains(item)) have a severe impact on performance as the queue grows
• locks limit concurrency (which he does mention)

The following code improves on Brian's solution by

• using a hash set to do O(1) lookups
• combining 2 data structures from the System.Collections.Concurrent namespace

N.B. As there is no ConcurrentHashSet, I'm using a ConcurrentDictionary, ignoring the values.

In this rare case it is luckily possible to simply compose a more complex concurrent data structure out of multiple simpler ones, without adding locks. The order of operations on the 2 concurrent data structures is important here.

public class NoDuplicatesConcurrentQueue : IProducerConsumerCollection
{
    private readonly ConcurrentDictionary existingElements = new ConcurrentDictionary();
    private readonly ConcurrentQueue queue = new ConcurrentQueue();

    public bool TryAdd(T item)
    {
        if (existingElements.TryAdd(item, false))
        {
            queue.Enqueue(item);
            return true;
        }
        return false;
    }

    public bool TryTake(out T item)
    {
        if (queue.TryDequeue(out item))
        {
            bool _;
            existingElements.TryRemove(item, out _);
            return true;
        }
        return false;
    }
    ...
}

N.B. Another way at looking at this problem: You want a set that preserves the insertion order.