集合-字典（Lookup/SortedDictionary）

Lookup<TKey, TElement>非常类似于Dictionary<TKey, TValue>，但是把键映射在一个值集上。

必须调用ToLookup方法创建Lookup<TKey, TElement>对象。Lookup<TKey, TElement>对象是不可变的，无法向对象添加，移除元素。

public class User
    {
        public string Address;
        public string Name;
        public int Age; 
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<User> users = new List<User> { new User{Address="bj",Name="rxm",Age=27},
                                                  new User{Address="bj",Name="cwr",Age=24},
                                                  new User{Address="sh",Name="zcl",Age=28},
                                                  new User{Address="zz",Name="hst",Age=29}};

            Lookup<char, string> needUsers = (Lookup<char, string>)users.ToLookup(p => Convert.ToChar(p.Address.Substring(0, 1)),
                                                                              p => p.Name + "-" + p.Age.ToString());


            foreach (IGrouping<char, string> item in needUsers)
            {
                Console.WriteLine(item.Key); //b s z
                foreach (string str in item)
                {
                    Console.WriteLine("   {0}", str);
                }
            }
            //-----
            Console.WriteLine("--------------");
            IEnumerable<string> results = needUsers['b'];
            foreach (var item in results)
            {
                Console.WriteLine(item);
            }
            Console.Read();
        }
    }

SortedDictionary<Tkey,Tvalue>中的键都必须唯一。与这个Dictionary<Tkey,Tvalue>类不同的是，它排了序。

 SortedDictionary<string, string> items = new SortedDictionary<string, string>(StringComparer.CurrentCultureIgnoreCase);//忽略大小写
            items.Add("masanli", "逗你玩");
            items.Add("maji", "宇宙牌香烟");
            items.Add("liubaorui", "连升三级");
            items.Add("houbaolin", "北京话");
            foreach (KeyValuePair<string, string> item in items)
            {
                Console.WriteLine(string.Format("key:{0},value:{1}", item.Key, item.Value));
            }