/// <summary>
        /// Sorts the elements of a sequence according to a key.
        /// </summary>
        /// <exception cref="System.ArgumentNullException">source or keySelector is null.</exception>
        /// <typeparam name="TSource">The type of the elements of source.</typeparam>
        /// <typeparam name="TKey">The type of the key returned by keySelector.</typeparam>
        /// <param name="source">A sequence of values to order.</param>
        /// <param name="keySelector">A function to extract a key from an element.</param>
        /// <param name="descending">if set to <c>true</c>, sorts the elements in descending order; otherwise in ascending order.</param>
        /// <returns>An System.Linq.IOrderedEnumerable<TElement> whose elements are sorted according to a key.</returns>
        public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, bool descending)
        {
            if (descending)
                return source.OrderByDescending(keySelector);
 
            return source.OrderBy(keySelector);
        }