C# Online Compiler | .NET Fiddle

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);
            }
        }
    }
}

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

Cached Result

Paid Courses
    Learn WPF
    Datagrid Tutorials
Free Courses
    LINQ Tutorials
    .NET Threading Tutorials