List<tblAssociation> assoList = dataContext.tblAssociations.Join( // source

                        dataContext.tblUser, // target

                        a => a.Id,  // FK

                        b => b.Id,  //PK

                        (a, b) => new {a,b}  // result

                        ).Where(x=>x.b.IdRole==idRolet).Select(x => x.a).ToList();