Is the Linq Count() faster or slower than List.Count or Array.Length?

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In general slower. LINQ’s Count in general is an O(N) operation while List.Count and Array.Length are both guaranteed to be O(1).

However it some cases LINQ will special case the IEnumerable<T> parameter by casting to certain interface types such as IList<T> or ICollection<T>. It will then use that Count method to do an actual Count() operation. So it will go back down to O(1). But you still pay the minor overhead of the cast and interface call.

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