You will have to implement your own IProducerConsumerCollection<T> that behaves like a set (e.g. no duplicates allowed). Here is a simplistic version that use a critical section (C# lock) to make it thread-safe. For high concurrency scenarios you may be able to improve performance by using a class like SpinWait the same way as ConcurrentQueue<T> does.
public class ProducerConsumerSet<T> : IProducerConsumerCollection<T> {
readonly object gate = new object();
readonly Queue<T> queue = new Queue<T>();
readonly HashSet<T> hashSet = new HashSet<T>();
public void CopyTo(T[] array, int index) {
if (array == null)
throw new ArgumentNullException("array");
if (index < 0)
throw new ArgumentOutOfRangeException("index");
lock (gate)
queue.CopyTo(array, index);
}
public bool TryAdd(T item) {
lock (gate) {
if (hashSet.Contains(item))
return false;
queue.Enqueue(item);
hashSet.Add(item);
return true;
}
}
public bool TryTake(out T item) {
lock (gate) {
if (queue.Count == 0) {
item = default(T);
return false;
}
item = queue.Dequeue();
hashSet.Remove(item);
return true;
}
}
public T[] ToArray() {
lock (gate)
return queue.ToArray();
}
public void CopyTo(Array array, int index) {
if (array == null)
throw new ArgumentNullException("array");
lock (gate)
((ICollection) queue).CopyTo(array, index);
}
public int Count {
get { return queue.Count; }
}
public object SyncRoot {
get { return gate; }
}
public bool IsSynchronized {
get { return true; }
}
public IEnumerator<T> GetEnumerator() {
List<T> list = null;
lock (gate)
list = queue.ToList();
return list.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator() {
return GetEnumerator();
}
}
If required you can elaborate on this class to customize equality by supplying an optional IEqualityComparer<T> that then is used to initialize the HashSet<T>.
The IProducerConsumerCollection<T>.Add methods returns false when there is an attempt to insert a duplicate item. This results in an InvalidOperationException thrown by the BlockingCollection<T>.Add method so you will probably have to wrap the code to add an item into something like this:
bool AddItem<T>(BlockingCollection<T> blockingCollection, T item) {
try {
blockingCollection.Add(item);
return true;
}
catch (InvalidOperationException) {
return false;
}
}
Note that if you add items to a collection that has been completed you will also get an InvalidOperationException and you will have to examine the exception message to determine the underlying reason for the exception.
