gistfile1.cs

public class CoursePlan
{
  private readonly IEnumerable<Course> courses;
  private IEnumerable<Registrant> registrants;
  private readonly int instructorCount;

  public CoursePlan(IEnumerable<Course> courses, IEnumerable<Registrant> registrants, int instructorCount)
  {
    this.courses = courses;
    this.registrants = registrants;
    this.instructorCount = instructorCount;
  }

  public Solution Solve()
  {
    registrants.ToList().ForEach(SolveFor);
    return new Solution(courses.Where(course => course.IsMorning), courses.Where(course => !course.IsMorning));
  }

  private void SolveFor(Registrant registrant)
  {
    SolveMorning(registrant);
    SolveAfternoon(registrant);
  }

  private void SolveMorning(Registrant registrant)
  {
    HalfDaySolution(registrant, true);
  }

  private void SolveAfternoon(Registrant registrant)
  {
    HalfDaySolution(registrant, false);
  }

  private void HalfDaySolution(Registrant registrant, bool isMorning)
  {
    if (!OutOfInstructors())
      registrant
        .CourseWishes
        .Where(course => course.IsMorning == isMorning)
        .Where(course => !course.IsFull)
        .FirstOrDefault()
        .Register(registrant);
  }

  private bool OutOfInstructors()
  {
    return courses.Count(course => course.IsEmpty) == instructorCount;
  }
}