using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace ToLookupDemo{    class Program    {        //自定义类        public class Product        {            public string Code { get; set; }            public string Description { get; set; }        }        static void Main(string[] args)        {            List<Product> products = new List<Product>            {                new Product{Code="SPM1",Description="1"},                new Product{Code="LME1",Description="3"},                new Product{Code="SUP1",Description="4"},                new Product{Code="SPM2",Description="2"},                new Product{Code="SUP2",Description="5"}            };            //转换为Lookup类型，得到的结果已经分组了            Lookup<string, string> lookup = (Lookup<string, string>)                products.ToLookup(c => c.Code.Substring(0, 3), c => c.Code + " " + c.Description);            //在结果中循环            foreach (IGrouping<string, string> group in lookup)            {                Console.WriteLine(group.Key);                foreach (string s in group)                    Console.WriteLine("   {0}", s);            }            Console.ReadLine();            //取得其中一组值            IEnumerable<string> spmGroup = lookup["SPM"];            foreach (var str in spmGroup)                Console.WriteLine(str);            Console.ReadLine();        }    }}