Linq中的Select——投影

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();

 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
                 };

 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 });

 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);