using System;

using System.Collections.Generic;

using System.Linq;

public static class Program

{

    public static void Main()

    {

        List<StudentDetails> students = new List<StudentDetails>()

        {

            new StudentDetails {StudentId = 1, SubjectId = 1, ClassId = 1},

            new StudentDetails {StudentId = 2, SubjectId = 2, ClassId = 1},

            new StudentDetails {StudentId = 3, SubjectId = 1, ClassId = 2},

            new StudentDetails {StudentId = 1, SubjectId = 3, ClassId = 2},

            new StudentDetails {StudentId = 1, SubjectId = 3, ClassId = 1}

        };

​

        List<Class> classes = students

            .GroupBy(s => s.ClassId)

            .Select(c => new Class

                    {

                        ClassId = c.Key,

                        Students = c

                            .GroupBy(s => s.StudentId)

                            .Select(s => new Student

                                    {

                                        StudentId = s.Key,

                                        Subjects = s

                                            .GroupBy(t => t.SubjectId)

                                            .Select(t => new Subject { SubjectId = t.Key })

                                            .ToList()

                                    }).ToList()

                    }).ToList();

​

        foreach (Class @class in classes)

        {

            Console.WriteLine("Class({0})", @class.ClassId);

​

            foreach (Student student in @class.Students)

            {

                Console.WriteLine("\tStudent({0})", student.StudentId);

​

                foreach (Subject subject in student.Subjects)

                {

                    Console.WriteLine("\t\tSubject({0})", subject.SubjectId);

                }

            }

        }

    }

}

​

public class StudentDetails

{

    public int SubjectId { get; set; }

    public int StudentId { get; set; }

    public int ClassId { get; set; }

}

​

public class Class

{

    public int ClassId { get; set; }

    public List<Student> Students { get; set; }

}

​

public class Student

{

    public int StudentId { get; set; }

    public List<Subject> Subjects { get; set; }

}

​

public class Subject

{

    public int SubjectId { get; set; }

}