Packet escrever de volta todas as somas

  var test = new List <MesStock> { nova MesStock {Cuid = " 1 " , CEDD = " 1 " , a PNO = " 1 " , Sno = " 1 " , TotalNums = 1 },
                 nova MesStock {Cuid = " 2 " , CEDD = " 1 " , a PNO = " 1 " , Sno = " 1 " , TotalNums = 1 },
                 nova MesStock {Cuid = "3 ", CEDD = " 2 " , Pno = " 2 " , Sno = " 2 " , TotalNums = 2 },
                 novo MesStock {Cuid = " 4 " , CEDD = " 2 " , Pno = " 2 " , Sno = " 2 " , TotalNums = 2 },
                 novo MesStock {Cuid = " 5 " , CEDD = " 3 " , Pno = " 3 " ,Sno = " 3 ", TotalNums = 3 }, 
            }; 
            Lista <MesStock> res = new List <MesStock> (); 
            test.GroupBy (x => + x.Deid x.Pno + x.Sno, (x, y) => 
             { 
                 var Total = y.Sum (a => a.TotalNums);
                  retorno y.Select (t => 
                 { 
                     t.TotalNums = total; 
                     res.Add (t); 
                     retorno t; 
                 }) ToList ();. 
             }) ToList ().; 
            foreach ( var produto em res)
            {
                Console.WriteLine (item.Cuid + " | " + item.TotalNums); 
            }