public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int index)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }

    IList<TSource> list = source as IList<TSource>;

    if (list != null)
    {
        return list[index];
    }

    if (index < 0)
    {
        throw Error.ArgumentOutOfRange("index");
    }

    IEnumerator<TSource> enumerator = source.GetEnumerator();

    using (enumerator)
    {
        while (enumerator.MoveNext())
        {
            if (index == 0)
            {
                return enumerator.Current;
            }

            index--;
        }

        throw Error.ArgumentOutOfRange("index");
    }
}