How to effectively implement DAL with sorting?

private static IQueryable<vw_gui_User> GetUsersOrderedQuery(IQueryable<vw_gui_User> query, string sortExpression, bool asc)
        {
            switch (sortExpression)
            {
                case "Username":
                    {
                        query = asc ? query.OrderBy(users => users.Username) :
                            query.OrderByDescending(users => users.Username);
                        break;
                    }
                case "email":
                    {
                        query = asc ? query.OrderBy(users => users.email) :
                            query.OrderByDescending(users => users.email);
                        break;
                    }
                case "Total":
                    {
                        query = asc ? query.OrderBy(users => users.Total) :
                            query.OrderByDescending(users => users.Total);
                        break;
                    }
                case "RealBalance":
                    {
                        query = asc ? query.OrderBy(users => users.RealBalance) :
                            query.OrderByDescending(users => users.RealBalance);
                        break;
                    }
                case "BonusMoney":
                    {
                        query = asc ? query.OrderBy(users => users.BonusMoney) :
                            query.OrderByDescending(users => users.BonusMoney);
                        break;
                    }
                case "PartnerProgram":
                    {
                        query = asc ? query.OrderBy(users => users.PartnerProgram) :
                            query.OrderByDescending(users => users.PartnerProgram);
                        break;
                    }
                case null:
                default:
                    sortExpression = null;
                    break;
            }

            if (sortExpression != null)
            {
                var ordered = ((IOrderedQueryable<vw_gui_User>)query);
                query = ordered.ThenBy(users => users.id);
            }
            else
                query = query.OrderBy(users => users.id);
            return query;
        }

public List<Artefact> Select(int StartIndex, int PageSize,
           string SortExpression, bool SortDirection)
       {
           using (ZADataContext db = new ZADataContext())
           {
               ///How to implement this method without cases to process SortExpression?
           }
       }