Ling之select

select用法：

1、Dictionary<string, string>转json

Newtonsoft.Json.JsonConvert.SerializeObject(dicSubject.Select(l => new { label=l.Key,value=l.Value}))

2、Select可以修改数组、集合中的值

arrSub.Select(l => subjects.First(m => m.Name.Equals(l)).Id);

3. 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);

4. 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);

5. 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);

6. 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 = { , , , , , ,  };

   int[] numbersB = { , , , ,  };

    var pairs =

        from a in numbersA

        from 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);

7.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);

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Ling之select

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