Nullable reference types in C# 8.0

Nullable reference types are a new feature in C# 8.0. They allow you to spot places where you’re unintentionally dereferencing a null value (or not checking it.) You may have seen these types of checks being performed before C# 8.0 in ReSharper’s Value and Nullability Analysis checks.

These are potential sources for bugs, and can cause application crashes and NullReferenceExceptions. C# 8.0’s compiler supports nullable types, and can warn you when you are dereferencing a null value without first checking if it is null if the type ends with a “?”; consequently, any type without a “?” added to the end is a non-nullable reference type. For everything else, it’ll use flow analysis. In this article, I will explain how you can use nullable reference types to help make your code less prone to NullReferenceExceptions, and to make it more consumable by other APIs.

Null attributes

There are also a few attributes you can use to declare the arguments and return values for null-related code. These attributes extend the nullable types and allows the compiler to make more judgements:

  • AllowNull, the argument could be null, even if the type doesn’t allow it. For example, we are setting a string inside of a getter/setter to null. In C# 8.0, strings are known as a nullable “string!”, and so the AllowNull annotation allows setting it to null, even though the string that we return isn’t null (for example, we do a comparison check and set it to a default value if null.) This is a precondition.
  • DisallowNull, the argument isn’t null, even if the type allows it. This is a precondition.
  • MaybeNull, the output might be null. So, the callers have to check if the output is null. This is a postcondition.
  • NotNull, which means that the input wasn’t null when the call returns, even if the type allows it to be null. This is a postcondition.
  • NotNullWhen, which is a post condition that asserts the argument isn’t null depending on the boolean value of the return of the method. For example, say my method is bool MethodA([MaybeNullWhen(false) out string outVal], and it returns true. Then outVal isn’t null. If it returns false, then outVal could be null.
  • MaybeNullWhen, “signifies that a parameter could be null even if the type disallows it, conditional on the bool returned value of the method.” This means that if I were to annotate an argument with [MaybeNullWhen(false)], then the output (signified through the “out” keyword) could be null if the method returns false. This is a postcondition.
  • NotNullIfNotNull, “signifies that any output value is non-null conditional on the nullability of a given parameter whose name is specified”. What this means is that if I pass in a “string?” the output’s nullability is true, and vice-versa. This is a postcondition.

There are some other conditions, such as:

Why are these checks important?

These checks help ensure the safety of the code you write, and also allows other consumers of your library to know when to use null checks (and where to omit them.) While it is possible to null check every call that is null-ambiguous, it can be error-prone because:

  • Too many null checks clutter the code and wastes time trying to create error handlers to safely stop program execution.
  • You might forget to write a null check, and because there are null checks for everything else, it is unclear which method is missing a null check.

Here is an excellent example from Microsoft’s docs:

string? userInput = GetUserInput();
if (!string.IsNullOrEmpty(userInput))
  int messageLength = userInput.Length; // no null check needed.
// null check needed on userInput here.

In this case, they annotated the string.IsNullOrEmpty method with [NotNullWhen(false)], which means that if the method returns false, then no null checks are needed. The annotation can be read as “it’s not null when the output is false”. These higher-level logical statements help the compiler make inferences about the code. While this sounds like a trivial comparison to do through the compiler only without annotations, it’s actually a very complex research topic.

Microsoft Pex, a “White-Box test generation for .NET” is a program that analyses every possible path through your program symbolically to discover edge cases and missing conditionals that can cause NullReferenceExceptions (and more.) While it is extremely interesting, it’s a bit outside the scope of this post.

How do I use them?

If you are upgrading a legacy project, Microsoft recommends that you don’t turn it on for everything at once, but there might be a lot of warnings and it could be overwhelming. This is especially true if your team treats warnings as errors (a compiler option), as development would have to cease for several days to fix the null warnings. This isn’t a great strategy, and so incrementally enabling null checks helps prevent an explosion of warnings that could go ignored if not addressed promptly.

There are a few ways you can prioritize adding null annotations for the first set of checks. One of the ways is to start with the very small, straightforward methods. If the method is easy to reason about, then adding the null annotations can be easier to do, and if the small method is used throughout the code many times, then it can help infer what null checks are and are not needed in larger methods. While there are potentially infinite many ways to prioritize null checks, this approach can be helpful if you are not familiar with null checking.


Null reference types can help make your code more maintenance friendly and easier to spot bugs, as nulls can cause unexpected problems and application crashes. While nullable reference types aren’t a panacea (as it is possible to ignore the warnings), they can help provide the compiler with extra information. This extra information can be used to deduce errors and find logic errors. Gradually implementing nullable reference types helps find potential errors without overwhelming the developers with warnings. If there are too many warnings, they could be disregarded, further causing more problems down the line.