如何在Linq中分组？

using System;
using System.Collections.Generic;
using System.Linq;

public class Student
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Class { get; set; }
}

public class Program
{
    public static void Main()
    {
        List<Student> students = new List<Student>
        {
            new Student { Name = "Alice", Age = 20, Class = "A" },
            new Student { Name = "Bob", Age = 22, Class = "B" },
            new Student { Name = "Charlie", Age = 21, Class = "A" },
            new Student { Name = "David", Age = 23, Class = "B" }
        };

        var groupedStudents = students.GroupBy(s => s.Class);

        foreach (var group in groupedStudents)
        {
            Console.WriteLine($"Class: {group.Key}");
            foreach (var student in group)
            {
                Console.WriteLine($"  {student.Name}, Age: {student.Age}");
            }
        }
    }
}

Class: A
  Alice, Age: 20
  Charlie, Age: 21
Class: B
  Bob, Age: 22
  David, Age: 23

var groupedStudentsSorted = students.GroupBy(s => s.Class).OrderBy(g => g.Key);

var groupedStudentsNonNullClass = students.Where(s => s.Class != null).GroupBy(s => s.Class);