Sound null safety

Contents

Introduction through examples
Null safety principles
Dart 3 and null safety
Dart 2.x and null safety
Migrating existing code
Where to learn more

The Dart language enforces sound null safety.

Null safety prevents errors that result from unintentional access of variables set to null.

For example, if a method expects an integer but receives null, your app causes a runtime error. This type of error, a null dereference error, can be difficult to debug.

With sound null safety, all variables require a value. This means Dart considers all variables non-nullable. You can assign values of the declared type only, like int i=42. You can never assign a value of null to default variable types. To specify that a variable type can have a null value, add a ? after the type annotation: int? i. These specific types can contain either a null or a value of the defined type.

Sound null safety changes potential runtime errors into edit-time analysis errors. With null safety, the Dart analyzer and compilers flag if a non-nullable variable has either:

Not been initialized with a non-null value
Been assigned a null value. These checks allows you to fix these errors before deploying your app.

Introduction through examples

With null safety, none of the variables in the following code can be null:

// With null safety, none of these can ever be null.
var i = 42; // Inferred to be an int.
String name = getFileName();
final b = Foo();

To indicate that a variable might have the value null, just add ? to its type declaration:

int? aNullableInt = null;

To try an interactive example, see the null safety codelab.
To learn more about this topic, see Understanding null safety.

Null safety principles

Dart supports null safety using the following two core design principles:

Non-nullable by default. Unless you explicitly tell Dart that a variable can be null, it’s considered non-nullable. This default was chosen after research found that non-null was by far the most common choice in APIs.
Fully sound. Dart’s null safety is sound, which enables compiler optimizations. If the type system determines that something isn’t null, then that thing can never be null. Once you migrate your whole project and its dependencies to null safety, you reap the full benefits of soundness—not only fewer bugs, but smaller binaries and faster execution.

Dart 3 and null safety

Dart 3 has built-in sound null safety. Dart 3 prevents code without it from running.

To learn how to migrate to Dart 3, check out the Dart 3 migration guide. Packages developed without null safety support cause issues when resolving dependencies:

$ dart pub get

Because pkg1 doesn't support null safety, version solving failed.
The lower bound of "sdk: '>=2.9.0 <3.0.0'" must be 2.12.0 or higher to enable null safety.

Libraries incompatible with Dart 3 cause analysis or compilation errors.

$ dart analyze .
Analyzing ....                         0.6s

  error • lib/pkg1.dart:1:1 • The language version must be >=2.12.0. 
  Try removing the language version override and migrating the code.
  • illegal_language_version_override

$ dart run bin/my_app.dart
../pkg1/lib/pkg1.dart:1:1: Error: Library doesn't support null safety.
// @dart=2.9
^^^^^^^^^^^^

To resolve these issues:

Check for null safe versions of any packages you installed from pub.dev
migrate all of your source code to use sound null safety.

Dart 3 can be found in the stable channels for Dart and Flutter. To learn more, check out the download page for details. To test your code for Dart 3 compatibility, use Dart 3 or later.

$ dart --version                     # make sure this reports 3.0.0-417.1.beta or higher
$ dart pub get / flutter pub get     # this should resolve without issues
$ dart analyze / flutter analyze     # this should pass without errors

If the pub get step fails, check the status of the dependencies.

If the analyze step fails, update your code to resolve the issues listed by the analyzer.

Dart 2.x and null safety

From Dart 2.12 to 2.19, you need to enable null safety. You cannot use null safety in SDK versions earlier than Dart 2.12.

To enable sound null safety, set the SDK constraint lower-bound to a language version of 2.12 or later. For example, your pubspec.yaml file might have the following constraints:

environment:
  sdk: '>=2.12.0 <3.0.0'

Migrating existing code

Dart code written without null safety support can be migrated to use null safety. We recommend using the dart migrate tool, included in the Dart SDK versions 2.12 to 2.19.

$ cd my_app
$ dart migrate

To learn how to migrate your code to null safety, check out the migration guide.

Where to learn more

To learn more about null safety, check out the following resources: