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);