Avoiding the NullReferenceException

Easily, by a landslide, the most common exception I’ve run into in C# is the NullReferenceException. Second would be an InvalidOperationException. But NullReferenceException wins as most occurred exception hands down. This quick post shows how we can start avoiding the NullReferenceException.

I despise null. I despise null checks.

What if there was a way to get rid of null checks? Luckily there is.

Last year I was at a talk by Reid Evans, Getting Started with Functional Programming in F#. He touched on it and I’ve been using an implementation in C# ever since.

Option

an option type or maybe type is a polymorphic type that represents encapsulation of an optional value; e.g., it is used as the return type of functions which may or may not return a meaningful value when they are applied.

Optional Library

For this example, I’m going to use the Optional package. There are many similar type libraries available on NuGet, but this one is fairly feature rich and serves the purpose for my example.

NullReferenceException

So I’m going to walk through a trivial example of a ASP.NET Core application that is going to return some customer data. The sample code below will throw a NullReferenceException when the incoming customerId is not found in our database.

	[Route("api/[controller]")]
	public class CustomersController : Controller
	{
	private readonly CustomerRepository _customerRepository;

	public CustomersController()
	{
	_customerRepository = new CustomerRepository();
	}

	// GET api/values/5
	[HttpGet("{id}")]
	public IActionResult Get(int id)
	{
	// Returns null if customer doesn't exist.
	var customer = _customerRepository.GetById(id);

	// This could throw a NullReferenceException if customer is null
	return new JsonResult(new
	{
	CustomerId = customer.CustomerId,
	Name = customer.Name
	});
	}
	}

view raw controller.cs hosted with ❤ by GitHub

Null Check

Obvious answer is to wrap the return statement in an if statement to verify the customer returned from the _customerRepository is not null. If it is, then return a 404.

	[Route("api/[controller]")]
	public class CustomersController : Controller
	{
	private readonly CustomerRepository _customerRepository;

	public CustomersController()
	{
	_customerRepository = new CustomerRepository();
	}

	// GET api/values/5
	[HttpGet("{id}")]
	public IActionResult Get(int id)
	{
	// Returns null if customer doesn't exist.
	var customer = _customerRepository.GetById(id);

	if (customer == null) return new StatusCodeResult(404);

	// This could throw a NullReferenceException if customer is null
	return new JsonResult(new
	{
	CustomerId = customer.CustomerId,
	Name = customer.Name
	});
	}
	}

view raw controller404.cs hosted with ❤ by GitHub

Option

Our CustomerRepository, could’ve thrown a InvalidOperationException or another type of exception if the customer didn’t exist. But what if instead it returned an Option<Customer> and never returned null.

In the example below, our _customerRepository.GetOptionById(int) now returns an Option<Customer>. This now forces us to specify how to handle if there is a value, and if it is null.

	[Route("api/[controller]")]
	public class CustomersController : Controller
	{
	private readonly CustomerRepository _customerRepository;

	public CustomersController()
	{
	_customerRepository = new CustomerRepository();
	}

	// GET api/values/5
	[HttpGet("{id}")]
	public IActionResult Get(int id)
	{
	// Returns null if customer doesn't exist.
	var customer = _customerRepository.GetOptionById(id);

	return customer.Match<IActionResult>(
	some: value => new JsonResult(new
	{
	CustomerId = value.CustomerId,
	Name = value.Name
	}),
	none: () => new StatusCodeResult(404));
	}
	}

view raw controlleroption.cs hosted with ❤ by GitHub

Gateway

Hopefully this a a simple example of the Option type and how it can be a gateway to removing null and NullReferenceException from your code. Take a deeper look at the docs for the Optional library as there are many other great examples.

Are you using an implementation of Option/Maybe? Let me know in the comments or on Twitter.

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6 thoughts on “Avoiding the NullReferenceException”

Maxence October 13, 2017 at 3:18 am


I prefer to raise an exception like RecordNotFoundException from GetById. If I call several times GetById, I can wrap all these calls with a try/catch and handle the exception one time.

And I generally don’t return null from a method. If a method can return null, I prefix it with a Try (TryGetById).
1. Derek Comartin October 13, 2017 at 8:03 am
  
  
  My example was pretty contrived. I wouldn’t return null, but it was just to provide an example. Another scenario would of been if the Customer would of had a property that could of been null. Ultimately, the purpose was to demonstrate that an option type eliminates the null, null checks, exceptions and exception handling.
2. 1. Kefir October 23, 2017 at 6:51 am
    
    
    > would of
    > could of
    🙁
  2. Gareth Erskine-Jones October 27, 2017 at 4:19 am
    
    
    It’s really wrapping the null check in an abstraction rather than eliminating it. There’s no doubt that abstractions like this can be helpful, but they can also make code harder to read. Your last example, I’d suggest, is harder to read than the one before with the explicit null check.
3. Kefir October 23, 2017 at 6:50 am
  
  
  Exceptions are for exceptional situations.
  Missing record is a normal situation (usually) and should not throw an exception.
4. 1. Maxence October 23, 2017 at 6:58 am
    
    
    I code the “happy path”. Everything else is exceptional.

Avoiding the NullReferenceException

Option

NullReferenceException

Null Check

Option

Gateway

6 thoughts on “Avoiding the NullReferenceException”

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