Linq的简单实用

很少用linq，所以写点基本的用法，学习一下，先介绍下使用的两个类（偷懒在网上找了一个Ticket类）；

 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}
            };
        }
    }

接下来就是关于linq的实用了

 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();
        }