LINQ 中的 select

下面通過一些例子來說明怎樣使用select，參考自：LINQ Samples

1.  可以對查詢出來的結果做一些轉換，下面的例子在數組中尋找以"B"開頭的名字，然後全部轉成小寫輸出：

 string[] names = { "Jack", "Bob", "Bill", "Catty", "Willam" }; var rs = from n in names          where n.StartsWith("B")          select n.ToLower(); foreach (var r in rs)     Console.WriteLine(r);

2. 返回匿名型別，比如Linq To Sql查詢資料庫的時候只返回需要的資訊，下面的例子是在Northwind資料庫中查詢Customer表，返回所有名字以"B"開頭的客戶的ID和名稱：

 NorthwindDataContext dc = new NorthwindDataContext(); var cs = from c in dc.Customers          where c.ContactName.StartsWith("B")          select new                 {                     CustomerID = c.CustomerID,                     CustomerName = c.ContactTitle + " " + c.ContactName                 }; foreach (var c in cs)     Console.WriteLine(c);

3. 對於數組，select可以對數組元素以及索引進行操作：

  string[] names = { "Jack", "Bob", "Bill", "Catty", "Willam" };  var rs = names.Select((name, index) => new { Name = name, Index = index });  foreach (var r in rs)      Console.WriteLine(r);

4. 組合查詢，可以對多個資料來源進行組合條件查詢（相當於使用SelectMany函數），下面的例子其實就相對於一個雙重迴圈遍曆：

    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };    int[] numbersB = { 1, 3, 5, 7, 8 };    var pairs =        from a in numbersA,                b in numbersB        where a < b        select new {a, b};    Console.WriteLine("Pairs where a < b:");    foreach (var pair in pairs)        Console.WriteLine("{0} is less than {1}", pair.a, pair.b); 

  而用Linq To Sql的話，相當於進行一次子查詢：

  NorthwindDataContext dc = new NorthwindDataContext();  var rs = from c in dc.Customers           from o in c.Orders           where o.ShipCity.StartsWith("B")           select new { CustomerName = c.ContactName, OrderID = o.OrderID };  foreach (var r in rs)      Console.WriteLine(r);