C#的LINQ to Object

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Text.RegularExpressions;
using System.Linq;

namespace codeTest
{
    class Program
    {
        static void Main(string[] args)
        {
            //linq : Lauage Intergarted Query
            //继承IEnumerable的类，都可以使用linq
            //Linq to sql,Linq to xml,Linq to DataSet,Linq to objects
            //linq语句在具体调用时，才会执行 toList() Count()

            List<Customer> Customers = new List<Customer>();
            Customers.Add(new Customer() { CustomerName = "Li1", CityID =  });
            Customers.Add(new Customer() { CustomerName = "Li2", CityID =  });
            Customers.Add(new Customer() { CustomerName = "Li3", CityID =  });
            Customers.Add(new Customer() { CustomerName = "Li4", CityID =  });

            List<City> Citys = new List<City>();
            Citys.Add(new City() { CityID = , CityName = "北京" });
            Citys.Add(new City() { CityID = , CityName = "上海" });
            Citys.Add(new City() { CityID = , CityName = "广州" });
            Citys.Add(new City() { CityID = , CityName = "深圳" });

            #region 简单查询
            //1.Query systax
            var query = from c in Customers
                        where c.CityID %  ==
                        orderby c.CityID descending
                        select c;

            foreach (var item in query)
            {
                Console.WriteLine("{0},{1}", item.CustomerName, item.CityID);
            }

            Console.WriteLine();

            //2.Method systax
            var query2 = Customers.Where(c => c.CityID %  == ).OrderByDescending(c => c.CityID);
            foreach (var item in query2)
            {
                Console.WriteLine("{0},{1}", item.CustomerName, item.CityID);
            }
            #endregion

            #region Group
            Console.WriteLine("queryGroup");
            //1.Query systax
            var queryGroup = from c in Customers
                             group c by c.CityID;

            foreach (var group in queryGroup)
            {
                Console.WriteLine("{0}", group.Key);
                foreach (var item in group)
                {
                    Console.WriteLine("{0},{1}", item.CustomerName, item.CityID);
                }
            }

            Console.WriteLine();

            //2.Method systax
            var queryGroup2 = Customers.GroupBy(c => c.CityID);
            foreach (var group in queryGroup2)
            {
                Console.WriteLine("{0}", group.Key);
                foreach (var item in group)
                {
                    Console.WriteLine("{0},{1}", item.CustomerName, item.CityID);
                }
            }
            #endregion

            #region into
            Console.WriteLine("into");
            //1.Query systax
            var queryGroupinto = from c in Customers
                                 group c by c.CityID into cGroup
                                 where cGroup.Count() >
                                 select new { CityID = cGroup.Key, Cout = cGroup.Count() };

            foreach (var group in queryGroupinto)
            {
                Console.WriteLine("{0},{1}", group.CityID,group.Cout);
            }

            Console.WriteLine();

            //2.Method systax
            var queryGroupinto2 = Customers.GroupBy(c => c.CityID).
                Where(cGroup => cGroup.Count() > ).
                Select(cGroup => new { CityID = cGroup.Key, Cout = cGroup.Count() });
            foreach (var group in queryGroupinto2)
            {
                Console.WriteLine("{0},{1}", group.CityID, group.Cout);
            }
            #endregion

            #region join
            Console.WriteLine("join");
            //1.Query systax
            var queryJoin = from c in Customers
                            join ci in Citys on c.CityID equals ci.CityID
                            select new { CustomerName = c.CustomerName, c.CityID, ci.CityName };
            foreach (var item in queryJoin)
            {
                Console.WriteLine("{0},{1},{2}", item.CustomerName, item.CityID,item.CityName);
            }
            Console.WriteLine();
            //2.Method systax
            var queryJoin2 = Customers.Join(Citys,
                c => c.CityID,
                ci => ci.CityID,
                (c, ci) => new { CustomerName = c.CustomerName, c.CityID, ci.CityName });

            foreach (var item in queryJoin2)
            {
                Console.WriteLine("{0},{1},{2}", item.CustomerName, item.CityID, item.CityName);
            }
            #endregion

            Console.ReadLine();
        }

        class Customer
        {
            public string CustomerName { get; set; }
            public int CityID { get; set; }
        }

        class City
        {
            public int CityID { get; set; }
            public string CityName { get; set; }
        }
    }

}