orderby 子句 (C# 參考)

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在查詢運算式中，orderby 子句可讓傳回的序列或子序列 (群組) 以遞增或遞減順序排序。 您可以指定多個索引鍵，以便執行一或多個次要排序作業。 排序是由項目類型的預設比較子執行。 預設排序順序為遞增。 您也可以指定自訂比較子。 不過，它只有在使用以方法為基礎的語法時才可用。 如需詳細資訊，請參閱排序資料。

範例 1

在下列範例中，第一個查詢會依照從 A 開始的字母順序排序字組，第二個查詢則以遞減順序排序相同的字組 (ascending 關鍵字是預設排序值，而且可以省略)。

class OrderbySample1
{
    static void Main()
    {
        // Create a delicious data source.
        string[] fruits = ["cherry", "apple", "blueberry"];

        // Query for ascending sort.
        IEnumerable<string> sortAscendingQuery =
            from fruit in fruits
            orderby fruit //"ascending" is default
            select fruit;

        // Query for descending sort.
        IEnumerable<string> sortDescendingQuery =
            from w in fruits
            orderby w descending
            select w;

        // Execute the query.
        Console.WriteLine("Ascending:");
        foreach (string s in sortAscendingQuery)
        {
            Console.WriteLine(s);
        }

        // Execute the query.
        Console.WriteLine(Environment.NewLine + "Descending:");
        foreach (string s in sortDescendingQuery)
        {
            Console.WriteLine(s);
        }
    }
}
/* Output:
Ascending:
apple
blueberry
cherry

Descending:
cherry
blueberry
apple
*/

範例 2

下列範例會對學生的姓氏執行主要排序，然後對其名字執行次要排序。

class OrderbySample2
{
    // The element type of the data source.
    public class Student
    {
        public required string First { get; init; }
        public required string Last { get; init; }
        public int ID { get; set; }
    }

    public static List<Student> GetStudents()
    {
        // Use a collection initializer to create the data source. Note that each element
        //  in the list contains an inner sequence of scores.
        List<Student> students = new List<Student>
        {
           new Student {First="Svetlana", Last="Omelchenko", ID=111},
           new Student {First="Claire", Last="O'Donnell", ID=112},
           new Student {First="Sven", Last="Mortensen", ID=113},
           new Student {First="Cesar", Last="Garcia", ID=114},
           new Student {First="Debra", Last="Garcia", ID=115}
        };

        return students;
    }
    static void Main(string[] args)
    {
        // Create the data source.
        List<Student> students = GetStudents();

        // Create the query.
        IEnumerable<Student> sortedStudents =
            from student in students
            orderby student.Last ascending, student.First ascending
            select student;

        // Execute the query.
        Console.WriteLine("sortedStudents:");
        foreach (Student student in sortedStudents)
            Console.WriteLine(student.Last + " " + student.First);

        // Now create groups and sort the groups. The query first sorts the names
        // of all students so that they will be in alphabetical order after they are
        // grouped. The second orderby sorts the group keys in alpha order.
        var sortedGroups =
            from student in students
            orderby student.Last, student.First
            group student by student.Last[0] into newGroup
            orderby newGroup.Key
            select newGroup;

        // Execute the query.
        Console.WriteLine(Environment.NewLine + "sortedGroups:");
        foreach (var studentGroup in sortedGroups)
        {
            Console.WriteLine(studentGroup.Key);
            foreach (var student in studentGroup)
            {
                Console.WriteLine("   {0}, {1}", student.Last, student.First);
            }
        }
    }
}
/* Output:
sortedStudents:
Garcia Cesar
Garcia Debra
Mortensen Sven
O'Donnell Claire
Omelchenko Svetlana

sortedGroups:
G
   Garcia, Cesar
   Garcia, Debra
M
   Mortensen, Sven
O
   O'Donnell, Claire
   Omelchenko, Svetlana
*/

備註

在編譯時期，orderby 子句會轉譯為 OrderBy 方法的呼叫。 orderby 子句中的多個索引鍵翻譯為 ThenBy 方法呼叫。

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