GroupJoin操作：

 

private void GroupJoinQuery()

     {    ///构建数据源

         List<UserInfo> users = new List<UserInfo>();

         List<RoleInfo> roles = new List<RoleInfo>();

         for (int i = 1; i < 10; i++)

         {

              users.Add(new UserInfo(i,"User0" + i.ToString(),"User0" + i.ToString() + "@web.com",i * 2));

              roles.Add(new RoleInfo(i,"RoleName0" + i.ToString()));

         }

         ///查询ID值小于9，且角色包含在roles中的用户

         var value = from u in users

                       where u.ID < 9

                       join r in roles on u.RoleID equals r.ID into g

                       select new

                       {

                            ID = u.ID,

                            Username = u.Username,

                            Email = u.Email,

                            RoleID = u.RoleID,

                            Roles = g.ToList()//用ToList()将查询结果转化为List集合

                       };                    

         ///显示查询结果

         foreach (var v in value)

          {

              Response.Write(v.Username + "," + (v.Roles.Count > 0 ? v.Roles[0].RoleName : string.Empty) + "</br>");

         }

     }