LINQ Expression GroupBy首先选择

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

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

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

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

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