Does List<T>.Last() enumerate the collection?

Question

Considering the boundaries of a List are known, does .Last() enumerate the collection?

I ask this because the documentation says that it is defined by Enumerable (in which case it would need to enumerate the collection)

If it does enumerate the collection then I can simply access the last element by index (as we know the .Count of a List<T>) but it seems silly to have to do this....

Answer 1

No, it does not enumerate it. It first checks if the enumerable can be casted to an IList<T>(which Array and List<T> implement), then the index will be used directly without enumeration.

Enumerable.Last source:

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> e = source.GetEnumerator()) {
            if (e.MoveNext()) {
                TSource result;
                do {
                    result = e.Current;
                } while (e.MoveNext());
                return result;
            }
        }
    }
    throw Error.NoElements();
}

Side-note: only single dimensional arrays implement IList<T>, which makes sense.