C# 多級排序

    處理了帶多個屬性的對象后，就要考慮另一種情形了：按一個字段給查詢結果排序是不夠的，需要查詢顧客，并按照區(qū)域使結果以字母順序排列，再按區(qū)域中的國家或城市名稱以字母順序排序。使用LINQ，可方便地完成這個任務，如下面的示例所示。

試—試多級掃后序：BeginningCSharp7_22_9_MultiLevelOrdering\Program.cs

按照下面的步驟在Visual Studio 2017中創(chuàng)建示例：

(1)修改前面的示例 BeginningCSharp7_22_8_SelectDistinclQueryr 或在 C:\BeginnmgGSharp7\Chapter22目錄中創(chuàng)建一個新的控制臺應用程序 BegirmingCSharp7_22_9_MultiLevelOrdering。

(2)如 BeginningCSharp7_22_8_SelectDistinctQuery 示例所示，創(chuàng)建 Customer 類并初始化 customers 列表 (List<Customer> customers)，這些代碼與前面示例中的代碼完全相同。

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

        var queryResults =

            from c in customers

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

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

          ;

(4)結果處理循環(huán)和Main()方法的其余代碼與前面例子中的相同。

(5)編譯并執(zhí)行程序，從所有顧客中選擇出來的屬性將先按區(qū)域排序，再按國家排序，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:

示例說明

    Customer類和customers列表的初始化與前面例子中的相同。因為要查看所有的顧客，這個査詢中沒有where子句，但按順序列出了要排序的字段，它們放在orderby子句的一個用逗號分開的列表中：

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

    這很容易，但不太直觀，這個簡單的字段列表允許放在orderby子句中，但不能放在select子句中，不過這就是LINQ的工作方式。如果知道select子句會創(chuàng)建一個新對象，而根據(jù)定義，ordeAy子句會逐字段執(zhí)行，這樣就不會覺得這個字段列表難以理解了。

    可給列出的任意字段添加descending關鍵字，反轉該字段的排序順序。例如，要對查詢結果按區(qū)域升序排序，再按國家降序排序，只需要在列表中的Coimtry后面加上descending關鍵字即可，如下所示：

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

添加了descending關鍵字后，結果如下：

{ 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，即使國家的順序被反轉了，印度和中國的城市仍按升序排序。