 the specified list         /// </summary>         /// <typeparam name="T"></typeparam>         /// <param name="source">The source.</param>         /// <param name="items">The items.</param>         /// <exception cref="System.ArgumentNullException">         /// source         /// or         /// items         /// </exception>         public static void Add<T>(this IList<T> source, IEnumerable<T> items)         {             if (source == null)             {                 throw new ArgumentNullException(nameof(source));             }              if (items == null)             {                 throw new ArgumentNullException(nameof(items));             }              items.ForEach(