ObservableQueue?

Question

Has anyone written a version of .Net's generic Queue that implements INotifyCollectionChanged, or is there one hidden deep in the .Net framework somewhere already?

Vintner

Answer 1

A quick search didn't show any results. But the interface is simple and it would be almost trivial to extend the Queue class and add support for the interface. Just override all methods thusly:

// this isn't the best code ever; refactor as desired
protected void OnCollectionChanged( NotifyCollectionChangedEventArgs ccea){
  var temp = CollectionChanged;
  if(temp != null) temp(this, ccea);  
}

// and later in the class

public override SomeMethodThatAltersTheQueue(object something){
  // record state of collection prior to change
  base.SomeMethodThatAltersTheQueue(something)
  // create NotifyCollectionChangedEventArgs with prior state and new state
  OnCollectionChanged(ccea);
}

Aedes

Unfortunately, the .Net generic Queue class does not allow Enqueue() or Dequeue() to be overridden in a descendant class. – Vintner

I ended up making an ObservableQueue class that uses method hiding to override Enqueue() and Dequeue(). As long as the code enqueueing or dequeueing does it on the Queue As ObservableQueue, and not just polymorphicly as a regular Queue, the events will fire. – Vintner

A better one might be not to override but to create a new queue (no IQueue<T> interface! weird!) that takes a Queue<T> in a constructor and just pass method calls to the internal collection – Aedes

Answer 2

I used the same approach as Chris Wenham. Under load, performance suffers because new NotifyCollectionChangedEventArgs need to be allocated for each Enqueue/Dequeue.

Regardless, in the Enqueue, send args with NotifyCollectionChangedAction.Add, the item added, and Count-1 as the index. In the Dequeue, send args with NotifyCollectionChangedAction.Remove, the item removed, and index 0.

Deianira