// A MemoChecker ...
class MemoChecker
{
    IQueryable<Memo> _memos;
    IMemoDueNotifier _notifier;

    // Construct a memo checker with the store of memos and the notifier
    // that will be used to display overdue memos.
    public MemoChecker(IQueryable<Memo> memos, IMemoDueNotifier notifier)
    {
        _memos = memos;
        _notifier = notifier;
    }

    // Check for overdue memos and alert the notifier of any that are found.
    public void CheckNow()
    {
        var overdueMemos = _memos.Where(memo => memo.DueAt < DateTime.Now);

        foreach (var memo in overdueMemos)
            _notifier.MemoIsDue(memo);
    }
}