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Sound null safety

The Dart language now supports sound null safety!

When you opt into null safety, types in your code are non-nullable by default, meaning that variables can’t contain null unless you say they can. With null safety, your runtime null-dereference errors turn into edit-time analysis errors.

With null safety, all of the variables in the following code are non-nullable:

// In null-safe Dart, 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;

You can use null safety in your normal development environment, migrate existing code to use null safety, enable null safety in DartPad, or learn about null safety using DartPad with Null Safety (shown in the following screenshot).

Screenshot of DartPad in null-safe mode

For an interactive, example-driven introduction to null safety language features, see the null safety codelab. For an in-depth discussion, see Understanding null safety.

Null safety principles

Dart null safety support is based on the following three 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.

  • Incrementally adoptable. You choose what to migrate to null safety, and when. You can migrate incrementally, mixing null-safe and non-null-safe code in the same project. We provide tools to help you with the migration.

  • 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.

Enabling null safety

Sound null safety is available in Dart 2.12 and Flutter 2.

Migrating an existing package or app

For instructions on how to migrate your code to null safety, see the migration guide.

Behind the scenes: SDK constraints

To make Dart treat your code as null safe, the SDK constraints must require a language version that has null safety support. For example, your pubspec.yaml file might have the following constraints:

environment:
  sdk: ">=2.12.0 <3.0.0"

Known issues

Some parts of the Dart ecosystem still need additional work to migrate to null safety.

The Dart team is currently aware of the following issues:

  • Migration of the pub.dev packages owned by the Dart team is nearly complete, but a few are still missing. See pub.dev for null-safe packages from the Dart team.
  • Packages that have unit tests typically have a dev dependency on the test package. A few packages extend the framework in package:test, and might have a direct dependency on it. When publishing these packages you might get pub warnings about the test package not being fully migrated. As long as your package doesn’t ignore any of the import_of_legacy_library_into_null_safe or export_legacy_symbol analyzer warnings, then you can ignore this warning and safely publish your package.

Where to learn more

For more information about null safety, see the following resources: