Сложность алгоритма в LINQ

MembershipUserCollection membershipUsers = new MembershipUserCollection();
users.ToList().ForEach(membershipUsers.Add); //what is the complexity of this line?

http://stackoverflow.com/questions/4008682/what-is-the-complexity-of-this-linq-example?rq=1

There are very, very few guarantees, but there are a few optimizations:

• Extension methods that use indexed access, such as ElementAt, Skip, Last or LastOrDefault, will check to see whether or not the underlying type implements IList<T>, so that you get O(1) access instead of O(N).
• The Count method checks for an ICollection implementation, so that this operation is O(1) instead of O(N).
• Distinct, GroupBy Join, and I believe also the set-aggregation methods (Union, Intersect and Except) use hashing, so they should be close to O(N) instead of O(N²).
• Contains checks for an ICollection implementation, so it may be O(1) if the underlying collection is also O(1), such as a HashSet<T>, but this is depends on the actual data structure and is not guaranteed. Hash sets override the Contains method, that's why they are O(1).
• OrderBy methods use a stable quicksort, so they're O(N log N) average case.

http://stackoverflow.com/questions/2799427/what-guarantees-are-there-on-the-run-time-complexity-big-o-of-linq-methods