Software Design Blog

Software solutions become overcomplicated with dozens of long and ugly checks for null. It is like a pothole and you will eventually fall in when you’re not looking carefully.

The problem is that methods returns null instead of real objects and every client must check for null to avoid the application from blowing up due to a NullReferenceExcetion (Object reference not set to an instance of an object).

The solution is to return a null object that exhibits default behaviour. This is called the Null Object design pattern.

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Example 1: Debug Mode

Here is an example where additional tasks are performed such as logging verbose messages when the application is in debug mode.

The cyclomatic complexity of the application has increased due to the number checks as shown below.

Null Checking

            if (isDebugMode)
            {
                logger.Log("Saving - Step 1");
            }

            // Save Step 1

            if (isDebugMode)
            {
                logger.Log("Saving - Step 2");
            }

            // Save Step 2

Null Object Design Pattern

            
public class NullLogger : ILogger
{
    public void Log(string message)
    {
        // Purposefully provides no behaviour
    }
}

logger.Log("Saving - Step 1");
// Save Step 1
logger.Log("Saving - Step 2");
// Save Step 2

Example 2: Registration System

The use case is to create a service to confirm reservations and to send optional confirmation notices. The service is also responsible for retrieving reservations that can be filtered based on the confirmation status.

The following interfaces will be used in the illustration.

    public interface INotification<in T>
    {
        void Notifiy(T message);
    }

    public interface IReservationRepository
    {
        void Save(Confirmation request);
        IEnumerable<Reservation> GetReservations();
    }

Null Checking

Here is the complicated example that performs null checks.

    public class ReservationServiceV1 : IReservationService
    {
        private readonly IReservationRepository _repository;
        private readonly INotification<Confirmation> _notifier;

        public ReservationServiceV1(IReservationRepository repository, 
                                    INotification<Confirmation> notifier)
        {
            if (repository == null) throw new ArgumentNullException("repository");
            _repository = repository;
            _notifier = notifier;
        }

        public void Confirm(Confirmation request)
        {
            _repository.Save(request);
            if (_notifier != null) _notifier.Notifiy(request);
        }

        public IEnumerable<Reservation> GetReservations(bool confirmed)
        {
            var reservations = _repository.GetReservations();
            return reservations == null ? null : 
                   reservations.Where(reservation => reservation.Confirmed == confirmed);
        }
    }

Null Object Design Pattern

Here is the simplified version that works with default behaviour.

 
public class NullNotification<T> : INotification<T>
{
    public void Notifiy(T message)
    {
        // Purposefully provides no behaviour
    }
}
   
public class ReservationServiceV2 : IReservationService
{
    private readonly IReservationRepository _repository;
    private readonly INotification<Confirmation> _notifier;

    public ReservationServiceV2(IReservationRepository repository, 
                                INotification<Confirmation> notifier)
    {
        if (repository == null) throw new ArgumentNullException("repository");
        if (notifier == null) throw new ArgumentNullException("notifier");
        _repository = repository;
        _notifier = notifier;
    }

    public void Confirm(Confirmation request)
    {
        _repository.Save(request);
        _notifier.Notifiy(request);
    }

    public IEnumerable<Reservation> GetReservations(bool confirmed)
    {
        return _repository.GetReservations()
                          .Where(reservation => reservation.Confirmed == confirmed);
    }
}

Summary

Avoid returning null and return default behaviour instead; such as an empty list. The Null Object design pattern will simplify code and reduce potential slip-ups causing unexpected failure.