Queue vs List

Question

I\'m currently using a List as a queue (use lst[0] then lst.removeAt(0)) to hold objects. There\'s about 20 items max at a g

Answer 1

Short answer:
Queue is faster than List when it's used like a queue. List is faster than Queue when used like a list.

Long answer:
A Queue is faster for dequeuing operation, which is an O(1) operation. The entire block of subsequent items of the array is not moved up. This is possible because a Queue need not facilitate removal from random positions, but only from the top. So it maintains a head (from which the item is pulled upon Dequeue) and tail position (to which the item is added upon Enqueue). On the other hand removing from the top of a List requires itself to shift positions of every subsequent item one up. This is O(n) - worst case if you're removing from the top, which is what a dequeue operation is. The speed advantage can be noticeable if you're dequeuing in a loop.

A List is more performant if you need indexed access, random retrieval etc. A Queue will have to enumerate fully to find the appropriate index position (it doesn't expose IList).

That said, a Stack vs List is much closer, there is no performance difference in pushing and popping operations. They both push to end and remove from end of array structures (both of which are O(1)).

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Of course you should use the correct structure that reveals the intent. In most cases they will perform better as well since they are tailor-made for the purpose. I believe had there been no performance difference at all, Microsoft wouldn't have included Queue and Stack in the framework for merely different semantics. It would have been simply easily extensible if that was the case. Think about SortedDictionary and SortedList, both of which do exactly the same but is differentiated by only performance characteristic; they find a place in BCL.