[转载]LINQ 中的 select

下面通过一些例子来说明怎样使用select，参考自：LINQ Samples

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);