As .NET developers, we’ve all seen this exception hundreds of times: “System.NullReferenceException – Object reference not set to an instance of an object”. In .NET, this exception occurs when trying to access a reference variable with a null value. A null value means the variable does not hold a reference to any object on the heap. It is one of the most frustrating and prolific errors that we programmers encounter. But it needn’t be this way! We can prevent this error by following a few simple rules. But first, a little history…
The null reference was invented by Tony Hoare, inventor of QuickSort, one of the world’s most widely used sorting algorithms. In this introduction to his talk at QCon 2009, Tony describes the impact the null reference has had on software:
I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years. In recent years, a number of program analysers like PREfix and PREfast in Microsoft have been used to check references, and give warnings if there is a risk they may be non-null. More recent programming languages like Spec# have introduced declarations for non-null references. This is the solution, which I rejected in 1965.
So obviously null references have caused quite a lot of damage. But neither Tony or null references are to blame. It’s the careless use of null references that has made them as damaging and prolific as they are.
I can’t think of a single reason why you would need to use null references as part of your system design. Here are some tips for preventing null references in your system.
Never use null references as part of the design
Business logic should not be based around testing for null references. If an object requires an empty state, be explicit about it by creating an empty representation of the object. You can then check if the object is in an empty state by comparing the current instance to an empty instance.
Here is an example of some code that uses a generic interface called ICanBeEmpty to support an empty representation of a Customer object. An extension method called HasValue() allows us to check if an object represents an empty instance.
Guard statements are often used to check for null references in methods. If you design your system not to pass nulls, you won’t need guards to check for null in your methods. But when you can’t guarantee input to your public methods, then you need to be defensive about null references.
A call to a method or property should never return a null reference. Instead, return an empty representation of an object, or throw an exception if a non-empty value is expected.
Fail fast if a null reference is detected
Design-by-contract technologies, such as Spec#, have declarations that can check for null references at compile time. You can also use an aspect-oriented programming (AOP) solution, such as PostSharp, to create custom attributes that ensures an exception is thrown if any null references are passed in, or returned by a method at runtime. By throwing an exception as soon as a null reference is detected, we can avoid hunting through code to find the source of a null reference.
If you are using a third-party service or component where you might receive a null reference, then wrap the call in a method that handles any null references to ensure they don’t leak into the rest of the system.
Always ensure object members are properly instantiated
All object members should be instantiated when an object is created. Be careful with strings in C#, as these are actually reference types. Always set string variables to a default value, such as string.Empty.
Nullable value types are ok
The nullable value types introduced in C# 2.0, such as int? and DateTime?, are better at handling null references as you have to explicitly cast them to a non-null value before accessing them. Be careful with using the Value property on a nullable type without first checking if the variable has a non-null value using the HasValue property. You can use GetValueOrDefault to return a default value if the variable is null.
By limiting the use of null references and not letting them leak into other parts of the system, we can prevent the troublesome NullReferenceException from ruining our day.