using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var students = new List<Student> {

            new Student { id=1, name="john"},

            new Student { id=2, name="mike"},

            new Student { id=3, name="josh"},

        };

        var classes = new List<Class> {

            new Class { id=1, name="geography"},

            new Class { id=2, name="math"},

            new Class { id=3, name="history"},

            new Class { id=4, name="biology"}

        };

        var studentClasses = new List<StudentClass> {

            new StudentClass { id=1, classId=1, studentId=1 },

            new StudentClass { id=2, classId=2, studentId=2 },

            new StudentClass { id=3, classId=2, studentId=3 }

        };

​

        var query = 

            from c in classes

            join ljsc in studentClasses 

                on c.id equals ljsc.classId

                into leftjoinStudentClasses

            from sc in leftjoinStudentClasses.DefaultIfEmpty(new StudentClass())

            join ljs in students 

                on sc.studentId equals ljs.id

                into leftjoinStudents

            from s in leftjoinStudents.DefaultIfEmpty(new Student())

            group new { s,c } by c.name into grp

            select new 

            {

                classname = grp.Key, students = from ss in grp select ss.s.name

            };

        foreach (var item in query)

        {

            var studentList = item.students.Any( s => !string.IsNullOrEmpty(s) ) ? string.Join(",", item.students) : "null";

            Console.WriteLine("{0}: {1}", item.classname, studentList); 

        }

    }

}

public class Student

{

    public int id { get; set; }

    public string name { get; set; }

}

public class Class

{

    public int id { get; set; }

    public string name { get; set; }

}

public class StudentClass 

{

    public int id { get; set; }

    public int studentId { get; set;}

    public int classId {get;set;}

}

​