public void RemoveAwardFromUser(Guid userId, Guid awardId) { Award award = awards.Find(x => x.id == awardId); if (award == null) { throw new ArgumentOutOfRangeException($"No award with {awardId} found"); } User user = users.Find(x => x.id == userId); if (user == null) { throw new ArgumentOutOfRangeException($"No user with {userId} found"); } award.RemoveOwner(userId); user.RemoveAward(awardId); }