using System;

using System.Collections.Generic;

using System.Linq;

​

namespace ConsoleApplication1

{

    internal class Course

    {

        public int ID { get; set; }

        public string Subject { get; set; }

    }

​

    internal class PrimaryCourse : Course

    {

        public int PrimaryCourseFees { get; set; }

    }

​

    public class Program

    {

        public static void Main(string[] args)

        {

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

            courses.Add(new Course

                {

                    ID = 1,

                    Subject = "LINQ Tutorials"

                });

​

            courses.Add(new Course

                {

                    ID = 2,

                    Subject = ".NET Threading Tutorials"

                });

​

            courses.Add(new Course

            {

                ID = 3,

                Subject = "Learn WPF"

            });

​

            courses.Add(new PrimaryCourse

            {

                ID = 4,

                Subject = "Rhino Mocks tutorials",

                PrimaryCourseFees = 55

            });

​

            var result = courses.OfType<PrimaryCourse>().Select(w => w.Subject);

​

            foreach (var subject in result)

            {

                Console.WriteLine(subject);

            }

        }

    }

}

​