C# Online Compiler | .NET Fiddle

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);
            }
        }
    }
}

 
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
                {

Compilation error (line 10, col 42): Unsafe code may only appear if compiling with /unsafe