Is IEnumerable<T>.Last() optimized for List<T>?

Question

I have a List<T>, called L, containing N items.

Is L.Last(), the IEnumerable<T> extension method, going to run through all N items in linear-time?

Or is it internally optimized to have the constant-time performance of L[L.Count - 1]?

Answer 1

You are right, if you take a look the code how to implement Last (from Reflector):

public static TSource Last<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
    {
        int count = list.Count;
        if (count > 0)
        {
            return list[count - 1];
        }
    }
    else
    {
        using (IEnumerator<TSource> enumerator = source.GetEnumerator())
        {
            if (enumerator.MoveNext())
            {
                TSource current;
                do
                {
                    current = enumerator.Current;
                }
                while (enumerator.MoveNext());
                return current;
            }
        }
    }
    throw Error.NoElements();
}

It actually optimizes for List<T> by returning list[count - 1];