A simple domain that can book rooms if a lesson matches the given requirements.

BookingDomain.cs

class UnableToBookRoomException : Exception
{
	public UnableToBookRoomException()
		: base("Unable to book the room")
	{
	}
}

class Room
{
	public IEnumerable<Requirement> Requirements
	{
		get { return _requirements; }
	}

	public void Book(Lesson lesson, DateTime startsOn, DateTime endsOn)
	{
		var canBook = _requirements.All(x => x.Valid(lesson));

		if (!canBook)
		{
			throw new UnableToBookRoomException();
		}

		// Do more stuff.
	}
}

abstract class Requirement
{
	public bool Valid(Lesson lesson);
}

class NationalityRequirement : Requirement
{
	public int Amount { get; private set; }

	public string Nationality { get; private set; }

	public override bool Valid(Lesson lesson)
	{
		var valid = lesson.Students.Count(x => x.Nationality == Nationality) >= Amount;

		return valid;
	}
}

class Lesson
{
	private readonly HashSet<Student> _students = new HashSet<Student>();

	public Lecturer Lecturer { get; private set; }

	public IEnumerable<Student> Students
	{
		get { return _students; }
	}

	public void Enlist(Student student)
	{
		if (_students.Contains(student))
		{
			throw new ArgumentException("Student is already enlisted.");
		}
	}
}

class Lecturer
{
}

class Student
{
	public string Nationality { get; private set; }
}