LINQ 中的 select

下面通过一些例子来说明怎样使用select，

1.  可以对查询出来的结果做一些转换，下面的例子在数组中查找以"B"开头的名字，然后全部转成小写输出：

 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. 返回匿名类型，比如Linq To Sql查询数据库的时候只返回需要的信息，下面的例子是在Northwind数据库中查询Customer表，返回所有名字以"B"开头的客户的ID和名称：

 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. 对于数组，select可以对数组元素以及索引进行操作：

  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. 组合查询，可以对多个数据源进行组合条件查询（相当于使用SelectMany函数），下面的例子其实就相对于一个双重循环遍历：

    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的话，相当于进行一次子查询：

  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);