How would you obtain the first and last items in a Queue?

Question

Say I have a rolling collection of values where I specify the size of the collection and any time a new value is added, any old values beyond this specified size are dropped off. Obviously (and I've tested this) the best type of collection to use for this behavior is a Queue:

myQueue.Enqueue(newValue)
If myQueue.Count > specifiedSize Then myQueue.Dequeue()

However, what if I want to calculate the difference between the first and last items in the Queue? Obviously I can't access the items by index. But to switch from a Queue to something implementing IList seems like overkill, as does writing a new Queue-like class. Right now I've got:

Dim firstValue As Integer = myQueue.Peek()
Dim lastValue As Integer = myQueue.ToArray()(myQueue.Count - 1)
Dim diff As Integer = lastValue - firstValue

That call to ToArray() bothers me, but a superior alternative isn't coming to me. Any suggestions?

Answer 1

One thing you could do is have a temporary variable that stores the value that was just enqueued because that will be the last value and so the variable can be accessed to get that value.

Answer 2

public class LastQ<T> : Queue<T>
{
    public T Last { get; private set; }

    public new void Enqueue(T item)
    {
         Last = item;
         base.Enqueue(item);
    }
}

Edit: Obviously this basic class should be more robust to do things like protect the Last property on an empty queue. But this should be enough for the basic idea.