using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        // Student collection

        IList<Student> studentList = new List<Student>() { 

                new Student() { StudentID = 1, StudentName = "John", Age = 13} ,

                new Student() { StudentID = 2, StudentName = "Moin",  Age = 21 } ,

                new Student() { StudentID = 3, StudentName = "Bill",  Age = 18 } ,

                new Student() { StudentID = 4, StudentName = "Ram" , Age = 20} ,

                new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 },

               new Student() { StudentID = 5, StudentName = "Ron" , Age = 15 },

               new Student() { StudentID = 6, StudentName = "Harry" , Age = 15 },

               new Student() { StudentID = 6, StudentName = "Harry" , Age = 15 },

            };

        // LINQ Query Method to find out teenager students

        List<Student> dupes = studentList.GroupBy(x => x.StudentID)

            .Where(g => g.Count() > 1)

            .SelectMany(g => g)

            .ToList();

        Console.WriteLine("Teen age Students:");

        foreach(Student std in dupes){  

            Console.WriteLine("{0},{1}", std.StudentID, std.StudentName);

        }

        List<int> dupedIds = dupes.Select(m => m.StudentID).Distinct().ToList();

        foreach(int x in dupedIds){ 

            object y = (object)x;

            Console.WriteLine("{0}",y);

        }

    }

}

​

public class Student{

​

    public int StudentID { get; set; }

    public string StudentName { get; set; }

    public int Age { get; set; }

}