DataSet ds = new DataSet();
            ds.ReadXml(@"D:\foo.xml");
            DataTable tbl =ds.Tables["cj"];

            tbl.PrimaryKey = new DataColumn[] { tbl.Columns["a"] };
            DataView view = tbl.DefaultView;

            view.Sort = "a ASC";

            DataTable sortedBy_a = view.ToTable();

            //remove all the CJ tables -- they're currently unsorted
            ds.Tables.Remove("cj");

            //add in all the sorted CJ tables
            ds.Tables.Add(sortedBy_a);

            StringWriter sw = new StringWriter();

           ds.WriteXml(sw);
           Console.WriteLine(sw.ToString());
           Console.ReadLine();