static void AddOrUpdateInstructor(string courseTitle, string instructorName) { var course = Courses.SingleOrDefault(c => c.Title == courseTitle); var inst = course.Instructors.SingleOrDefault(i => i.LastName == instructorName); if (inst == null) { var instructor = Instructors.Single(i => i.LastName == instructorName); course.Instructors.Add(instructor); } }