c# - Grouping troubles -

hi having difficulties group these data has no aggregate function. have following data in 2 tables , join both paymentid , display 1 row of record.

question:
how display final result in 1 row groupby coursepaidformonthyear.
data payment.*, tutorcoursecommissions.* , coursepaidformonthyear column in same row displaying (september, october, november)

example:

referenceid| tutorid| coursepaidformonthyear                 |   1          | 1019   | september, october, november or 9,10,11| 

my work:

 var result = u in db.users                     join p in db.payments on u.id equals p.userid                     join tt in db.tutorcoursecommissions on p.id equals tt.paymentid gtt                         tt in gtt.defaultifempty()                                            u.id == user.id                     groupby tt.coursepaidformonthyear  ??                     select new { u, p, tt };              foreach (var r in result)             {                 payment payment = new payment();                                   payment.monthtopay = (r.tt == null) ? null : common.getmonthname(r.tt.coursepaidformonthyear.month, true);                 payment.amount = r.p.amount;                                      output.add(payment);             }             return output; 

what need group months user , payment, , perform aggregation on grouped months. in case used string.join() combine distinct months coming commission.

this give results along lines of { user = "john doe", payment = ..., months = "january, february" }

var result = u in db.users               u.userid == user.id               join p in db.payments on u.id equals p.userid               join comm in db.tutorcoursecommissions                on p.id equals comm.paymentid               group common.getmonthname(comm.coursepaidformonthyear,true)               new { user = u, payment = p } g               select new                {                  user = g.key.user,                  payment = g.key.payment,                  //...select out other properties here...                   months = string.join(", ", g.distinct())               };