using System;

using System.Collections.Generic;

using System.Linq;

​

namespace ConsoleApplication1

{

    abstract class Course

    {

        public int ID { get; set; }

        public string Subject { get; set; }

        public int Rank { get; set; }

    }

​

    internal class FreeCourse : Course

    {

​

    }

​

    internal class PaidCourse : Course

    {

        public decimal Fees { get; set; }

    }

​

    public class Program

    {

        public static void Main(string[] args)

        {

            List<Course> courses = new List<Course>();

            courses.Add(new FreeCourse

            {

                ID = 1,

                Subject = "LINQ Tutorials",

                Rank = 5

            });

​

            courses.Add(new FreeCourse

            {

                ID = 2,

                Subject = ".NET Threading Tutorials",

                Rank = 4

            });

​

            courses.Add(new PaidCourse

            {

                ID = 3,

                Subject = "Learn WPF",

                Rank = 3

            });

​

            courses.Add(new PaidCourse

                {

                    ID = 4,

                    Subject = "Datagrid Tutorials",

                    Rank = 3

                });

​

            var paidCourses = from course in courses.OfType<PaidCourse>()

                              select course;

​

            var freeCourses = from course in courses.OfType<FreeCourse>()

                              select course;

​

            Console.WriteLine("Paid Courses");

            foreach (var course in paidCourses)

            {

                Console.WriteLine("\t" + course.Subject);

            }

​

            Console.WriteLine("Free Courses");

            foreach (var course in freeCourses)

            {

                Console.WriteLine("\t" + course.Subject);

            }

        }

    }

}

​