• 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);
    
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  • 原文地址:https://www.cnblogs.com/Strugglehard/p/4930311.html
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