)         {             if (source == null)             {                 throw new ArgumentNullException(nameof(source));             }              if (source is List<T>)             {                 ((List<T>)source).RemoveRange(index, count);             }             else if (source is IExtendedList<T>)             {                 ((IExtendedList<T>)source).RemoveRange(index, count);             }             else             {                 throw new NotSupportedException(