C# 多级排序

    处理了带多个属性的对象后，就要考虑另一种情形了：按一个字段给查询结果排序是不够的，需要查询顾客，并按照区域使结果以字母顺序排列，再按区域中的国家或城市名称以字母顺序排序。使用LINQ，可方便地完成这个任务，如下面的示例所示。

试—试多级扫后序：BeginningCSharp7_22_9_MultiLevelOrdering\Program.cs

按照下面的步骤在Visual Studio 2017中创建示例：

(1)修改前面的示例 BeginningCSharp7_22_8_SelectDistinclQueryr 或在 C:\BeginnmgGSharp7\Chapter22目录中创建一个新的控制台应用程序 BegirmingCSharp7_22_9_MultiLevelOrdering。

(2)如 BeginningCSharp7_22_8_SelectDistinctQuery 示例所示，创建 Customer 类并初始化 customers 列表 (List<Customer> customers)，这些代码与前面示例中的代码完全相同。

(3)在Main()方法的customers列表初始化之后，输入如下所示的查询：

        var queryResults =

            from c in customers

            orderby. c.Region, c.Country, c.City

            select new { c.ID, c.Region, c.Country, c,City }

          ;

(4)结果处理循环和Main()方法的其余代码与前面例子中的相同。

(5)编译并执行程序，从所有顾客中选择出来的属性将先按区域排序，再按国家排序，最后按城市排序，如下所示：

{ ID = 0, Region = Africa, Country = Egypt, City = Cairo }

{ ID = J, Region = Africa, Country = Nigeria, City = Lagos }

{ ID = R, Region = Asia, Country = China, City = Beijing }

{ ID = I, Region = Asia, Country = China, City = Shanghai }

{ ID = D, Region = Asia, Country = India, City = Delhi }

{ ID = B, Region = Asia, Country = India, City = Mumbai }

{ ID = L, Region = Asia, Country = Indonesia, City = Jakarta }

{ ID = P, Region = Asia, Country = Iran, City = Tehran }

{ ID = Mr Region = Asia, Country = Japan, City = Tokyo }

{ ID = G, Region = Asia, Country = Korea, City = Seoul }

{ ID = C, Region = Asia, Country = Pakistan, City = Karachi }

{ ID = H, Region = Asia# Country = Turkey, City = Istanbul }

{ ID = F, Region = Europe, Country = Russia, City = Moscow }

{ ID = Q, Region = Europe, Country = UK^ City = London }

{ ID = K, Region = North America, Country = Mexico, City = Mexico City }

{ ID = N, Region = North America, Country = USA, City = Los Angeles }

{ ID = A, Region = North America, Country = USA, City = New York }

{ ID = E, Region = South America, Country = Brazil, City = Sao Paulo }

{ ID = S, Region = South America, Country = Coloinbiar City = Bogota }

{ ID = T, Region = South America, Country = Peru, City = Lima }

Program finished, press Enter/Return to continue:

示例说明

    Customer类和customers列表的初始化与前面例子中的相同。因为要查看所有的顾客，这个査询中没有where子句，但按顺序列出了要排序的字段，它们放在orderby子句的一个用逗号分开的列表中：

        orderby c.Region, c,Country, c.City

    这很容易，但不太直观，这个简单的字段列表允许放在orderby子句中，但不能放在select子句中，不过这就是LINQ的工作方式。如果知道select子句会创建一个新对象，而根据定义，ordeAy子句会逐字段执行，这样就不会觉得这个字段列表难以理解了。

    可给列出的任意字段添加descending关键字，反转该字段的排序顺序。例如，要对查询结果按区域升序排序，再按国家降序排序，只需要在列表中的Coimtry后面加上descending关键字即可，如下所示：

    orderby c.Region, c.Country descending/ c.City

添加了descending关键字后，结果如下：

{ ID = J, Region = Africa, Country = Nigeria, City = Lagos }

{ ID = 0, Region = Africa, Country = Egypt, City = Cairo }

{ ID = H, Region = Asia, Country = Turkey, City = Istanbul }

{ ID = C, Region = Asia, Country = Pakistan, City = Karachi }

{ ID = G, Region = Asia, Country = Korea, City = Seoul }

{ ID = M, Region = Asia, Country = Japan, City = Tokyo }

{ ID = P, Region = Asia, Country = Iran, City = Tehran }

{ ID = L, Region = Asia, Country = Indonesia, City = Jakarta }

{ ID = D, Region = Asia, Country = India, City = Delhi }

{ ID = B, Region = Asia, Country = India, City = Mumbai }

{ ID = R, Region = Asia, Country = China, City = Beijing }

{ ID = I, Region = Asia, Country = China, City = Shanghai }

{ ID = Q, Region = Europe, Country = UK, City = London }

{ ID = F, Region = Europe, Country = Russia, City = Moscow }

{ ID = N, Region = North America, Country = USA, City = Los Angeles }

{ ID = A, Region = North America, Country = USA, City = New York }

{ ID = K, Region = North America, Country = Mexico, City = Mexico City }

{ ID = T, Region = South America, Country = Peru, City = Lima }

{ ID = S, Region = South America, Country = Colombiaf City = Bogota }

{ ID = E, Region = South America, Country = Brazil, City = Sao Paulo }

Program finished, press Enter/Return to continue:

注意，即使国家的顺序被反转了，印度和中国的城市仍按升序排序。