public class User    {        public int UserID { get; set; }        public string UserName { get; set; }        public static List<User> GetList()        {            return new List<User>()            {               new User() { UserID=1,UserName="James"},                new User() { UserID=2,UserName="Wade"},                 new User() { UserID=3,UserName="Paul"},                  new User() { UserID=4,UserName="Curry"},                   new User() { UserID=5,UserName="Ivan"}            };        }    }

public class Ticket    {        /// <summary>        /// 票号        /// </summary>        public string TicketNo { get; set; }        /// <summary>        /// 订单号        /// </summary>        public int OrderID { get; set; }        /// <summary>        /// 备注        /// </summary>        public string Description { get; set; }        public int UserID { get; set; }        public static List<Ticket> GetList()        {            return new List<Ticket>()            {                 new Ticket(){ TicketNo="999-12311",OrderID=79121281,Description="改签",UserID=1},                 new Ticket(){ TicketNo="999-24572",OrderID=29321289,Description="退票",UserID=2},                 new Ticket(){ TicketNo="999-68904",OrderID=19321289,Description="成交",UserID=3},                 new Ticket(){ TicketNo="999-24172",OrderID=64321212,Description="未使用",UserID=4},                 new Ticket(){ TicketNo="999-24579",OrderID=19321289,Description="退票",UserID=5},                 new Ticket(){ TicketNo="999-21522",OrderID=79121281,Description="未使用",UserID=1},                 new Ticket(){ TicketNo="999-24902",OrderID=79121281,Description="退票",UserID=2},                 new Ticket(){ TicketNo="999-04571",OrderID=29321289,Description="改签",UserID=5},                 new Ticket(){ TicketNo="999-23572",OrderID=96576289,Description="改签",UserID=2},                 new Ticket(){ TicketNo="999-24971",OrderID=99321289,Description="成交",UserID=5}            };        }    }

 static void Main(string[] args)        {            var Userlist = User.GetList();            var Ticketlist = Ticket.GetList();            var list1 = from a in Userlist                        orderby a.UserID descending                        select a;            var list2= from a in Userlist                       where a.UserID==1                       select a;            //左联            var list3 = from user in Userlist                        join ticekt in Ticketlist                        on user.UserID equals ticekt.UserID into temp                        from ticekt in temp.DefaultIfEmpty()                        select new { UserName = user.UserName, TicketNo = ticekt!=null?ticekt.TicketNo:null };            //右联            var list4=from user in Userlist                      join ticekt in Ticketlist                      on user.UserID equals ticekt.UserID into temp                      from ticekt in temp.DefaultIfEmpty()                      select new { UserName =user!=null? user.UserName:null, TicketNo = ticekt.Description };            //内联            var list5 = from user in Userlist                        join ticekt in Ticketlist                        on user.UserID equals ticekt.UserID                        select user;            //group by            var list6 = from ticekt in Ticketlist                        group ticekt by ticekt.UserID into g                        select new { g.Key, Count = g.Count() };                      Console.Read();        }