10. 转换操作符—【LINQ标准查询操作符】

public class AsEnumerable_LINQ
    {
        static string ContextString = System.Configuration.ConfigurationSettings.AppSettings["ContextString"].ToString();
        static DataContext context = new DataContext(ContextString);
        static Table<Contact> contacts = context.GetTable<Contact>();

        public static void AsEnumerable_Print()
        {
            IEnumerable<Contact> query = contacts.AsEnumerable().Where(con => con.FirstName.StartsWith("K"));
            foreach (var item in query)
            {
                Console.WriteLine(item.FirstName);
            }
        }

        public static void Cast_Print()
        {
            /*
             * 利用Cast将IEnumerable集合中的元素转换为某种指定的类型，这样做的好处是通过提供必要的类型信息 
             * 可以将标准查询操作符应用于非泛型集合之上
             */
            ArrayList names = new ArrayList();
            names.Add("Alex");
            names.Add("Jack");
            names.Add("Miles");
            names.Add("Hugo");
            names.Add("Ben");

            IEnumerable<string> query = names.Cast<string>().Select(name => name);
            foreach (var item in query)
            {
                Console.WriteLine(item.ToString());
            }
        }

        public static void OfType_Print()
        {
            ArrayList names = new ArrayList();
            names.Add("Alex");
            names.Add("Jack");
            names.Add("Miles");
            names.Add("Hugo");
            names.Add("Ben");
            names.Add(12);
            names.Add(3);
            names.Add(20);
            names.Add(83);

            IEnumerable<int> query = names.OfType<int>().Select(name => name);
            foreach (int item in query)
            {
                Console.WriteLine(item.ToString());
            }
        }

        public static void ToArray_Print()
        {
            var query = contacts.Select(con => con.FirstName).ToArray();
            foreach (string s in query)
            {
                Console.WriteLine(s.ToString());
            }
        }

        public static void ToDictionary_Print()
        {
            Dictionary<string, Contact> dic = contacts.ToDictionary(con => con.EmailAddress);
            foreach (KeyValuePair<string,Contact> item in dic)
            {
                Console.WriteLine(item.Value.FirstName + " " + item.Value.LastName + " " + item.Key);
            }
        }

        public static void ToList_Print()
        {
            var query = (from c in contacts
                         select c.FirstName).ToList();
            foreach (var item in query.Take(10))
            {
                Console.WriteLine(item.ToString());
            }
        }

        public static void ToLookUp_Print()
        {
            ILookup<string, string> lookUp = contacts.ToLookup(con => con.FirstName, con => con.MiddleName + " " + con.LastName);

            foreach (IGrouping<string,string> item in lookUp)
            {
                Console.WriteLine(item.Key);
                foreach (var i in item)
                {
                    Console.WriteLine(" " + i.ToString());
                }
            }
        }
    }