11. 元素操作符—【LINQ标准查询操作符】

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

        // 这些操作符用于实现了IEnumerable<T>接口的类型
        public static void DefaultIfEmpty_Print()
        {
            IEnumerable<string> query = from c in contact
                        where c.FirstName.StartsWith("Z")
                        select c.FirstName;
            foreach (string item in query.DefaultIfEmpty())
            {
                Console.WriteLine(item.ToString());
            }

            var query2 = from c in contact
                        where c.FirstName.StartsWith("ZZ")
                        select c.FirstName;
            foreach (string item in query2.AsEnumerable().DefaultIfEmpty("none"))
            {
                Console.WriteLine(item.ToString());
            }
        }

        public static void ElementAt_Print()
        {
            IEnumerable<string> query = from c in contact
                        where c.FirstName.StartsWith("Z")
                        select c.FirstName;
            Console.WriteLine(query.ElementAt(0));
        }

        public static void ElementAtOrDefault_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.FirstName.StartsWith("Z")
                                        select c.FirstName;
            Console.WriteLine(query.ElementAtOrDefault(50000));
        }

        public static void First_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.FirstName.StartsWith("Z")
                                        select c.FirstName;
            Console.WriteLine(query.First());
        }

        public static void Last_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.FirstName.StartsWith("Z")
                                        select c.FirstName;
            Console.WriteLine(query.Last());
        }

        public static void FirstOrDefault_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.FirstName.StartsWith("Zs")
                                        select c.FirstName;
            Console.WriteLine(query.FirstOrDefault());
        }

        public static void LastOrDefault_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.FirstName.StartsWith("Za")
                                        select c.FirstName;
            Console.WriteLine(query.LastOrDefault());
        }

        public static void Single_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.ContactID.Equals(20)
                                        select c.FirstName;
            Console.WriteLine(query.Single());

            IEnumerable<int> query2 = from c in contact
                                         select c.ContactID;
            Console.WriteLine(query2.Single(c=>c.Equals(10)));
        }

        public static void SingleOrDefault_Print()
        {
            IEnumerable<string> query = from c in contact
                                        where c.LastName.Equals("Kobylinski")
                                        select c.FirstName;
            Console.WriteLine(query.SingleOrDefault());
        }
    }