We could do this (a rough first attempt):

    static class SortDirectionExtensions
    {
        public static string ToParameter(this SortDirection sortDirection)
        {
            var member = sortDirection.GetType().GetMember(sortDirection.ToString()).FirstOrDefault();
            if (member == null) return null;

            var attribute = member.GetCustomAttributes(typeof(ParameterAttribute), false)
                .Cast<ParameterAttribute>()
                .FirstOrDefault();

            return attribute != null ? attribute.Value : null;
        }
    }