c#使用布尔值作为键将结果分组

下面的示例演示使用布尔值作为键将结果划分成两个组。 请注意，该值由 group 子句中的子表达式生成。

class GroupSample1
{
    // The element type of the data source.
    public class Student
    {
        public string First { get; set; }
        public string Last { get; set; }
        public int ID { get; set; }
        public List<int> Scores;
    }

    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, Scores= new List<int> {97, 72, 81, 60}},
           new Student {First="Claire", Last="O'Donnell", ID=112, Scores= new List<int> {75, 84, 91, 39}},
           new Student {First="Sven", Last="Mortensen", ID=113, Scores= new List<int> {99, 89, 91, 95}},
           new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List<int> {72, 81, 65, 84}},
           new Student {First="Debra", Last="Garcia", ID=115, Scores= new List<int> {97, 89, 85, 82}}
        };

        return students;
    }

    static void Main()
    {
        // Obtain the data source.
        List<Student> students = GetStudents();

        // Group by true or false.
        // Query variable is an IEnumerable<IGrouping<bool, Student>>
        var booleanGroupQuery =
            from student in students
            group student by student.Scores.Average() >= 80; //pass or fail!

        // Execute the query and access items in each group
        foreach (var studentGroup in booleanGroupQuery)
        {
            Console.WriteLine(studentGroup.Key == true ? "High averages" : "Low averages");
            foreach (var student in studentGroup)
            {
                Console.WriteLine("   {0}, {1}:{2}", student.Last, student.First, student.Scores.Average());
            }
        }

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}

1. /* Output:
2. Low averages
3. Omelchenko, Svetlana:77.5
4. O'Donnell, Claire:72.25
5. Garcia, Cesar:75.5
6. High averages
7. Mortensen, Sven:93.5
8. Garcia, Debra:88.25
9. */

group 子句 - C# 参考 | Microsoft Learn