11. 元素操作符—【LINQ標準查詢操作符】

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>();        // 這些操作符用於實現了IEnumerable<T>介面的類型        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());        }    }