C#: Triggering an Event when an object is added to a Queue

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If you mean the non-generic Queue class, then you can just override Enqueue:

public override void Enqueue(object obj)
{
    base.Enqueue(obj);
    OnChanged(EventArgs.Empty);
}

However, if you mean the generic Queue<T> class, then note that there is no suitable virtual method to override. You might do better to encapsulate the queue with your own class:

(** important edit: removed base-class!!! **)

class Foo<T>
{
    private readonly Queue<T> queue = new Queue<T>();
    public event EventHandler Changed;
    protected virtual void OnChanged()
    {
        if (Changed != null) Changed(this, EventArgs.Empty);
    }
    public virtual void Enqueue(T item)
    {
        queue.Enqueue(item);
        OnChanged();
    }
    public int Count { get { return queue.Count; } }

    public virtual T Dequeue()
    {
        T item = queue.Dequeue();
        OnChanged();
        return item;        
    }
}

However, looking at your code, it seems possible that you are using multiple threads here. If that is the case, consider a threaded queue instead.

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