Null Safety Migration Tooling

Note: the null safety migration tooling is in an early state and may have bugs and other issues.

For best results, use SDK version or higher.

How Migration Works

The migration uses a new interactive algorithm designed specifically for Dart null safety.

Typical code migration tools are designed to be run once, handle most cases, and let the developer do manual cleanup on the result. This does not work well for null safety and attempting this workflow will result in a lot more manual work. Similarly, after your migration has been applied, the migration cannot be rerun without first reverting it.

Why does the interactive approach save so much time?

Remember that Dart already has nullable types. Every type in old Dart code is nullable! What old Dart lacks is non-null types.

And like most migrations, our tool tries to preserve your code's current behavior. In the case of null safety, we may mark a lot of your types as nullable -- because they really were nullable before.

Nulls are traced through your program as far as they can go, and types are marked nullable in this process. If the tool makes a single mistake or choice you disagree with, it can lead many excess nullable types.

Interactive feedback to the tool

Unintentional null is the top cause of crashes in Dart programs. By marking your intention with comments like /*?*/ and /*!*/, we can stop these unintentional nulls from spreading through your program in your migrated code. Adding a small number of these hints will have a huge impact on migration quality.

The high level workflow of the tool is therefore driven through an interactive web UI. After running dart migrate, open the resulting URL in a browser. Scan through the changes, use the "nullability trace" feature to find the best place to add a nullability hint (adding a hint in the best place can prevent dozens of types from being made nullable). Rerun the migration and repeat, committing the hints as you go. When the output is correct and acceptable, apply the migration and publish your null-safe code!

For example,

List<int> ints = const [0, null];
int zero = ints[0];
int one = zero + 1;
List<int> zeroOne = [zero, one];

The default migration will be backwards compatible, but not ideal.

List<int?> ints = const [0, null];
int? zero = ints[0];
int one = zero! + 1;
List<int?> zeroOne = <int?>[zero, one];

zero should not be marked nullable, but it is. We then have cascading quality issues, such as null-checking a value that shouldn't have been marked null, and marking other variables as null due to deep null tracing. We can fix this all by adding a single /*!*/ hint.

List<int?> ints = const [0, null];
int/*?*/ zero = ints[0]!; // Just add /*?*/ here, the migration tool does the rest!
int one = zero + 1;
List<int> zeroOne = <int>[zero, one];

If you add one hint before migrating, you have done the equivalent of making five manual edits after migrating. To find the best place to put your hints, use the preview tool's nullability trace feature. This lets you trace back up to the root cause of any type's inferred nullability. Add hints as close to the original source of null as possible to have the biggest impact to the migration.

Note: The migration tool cannot be rerun on a migrated codebase. At that point in time, every nullable and non-nullable type is indistinguishable from an intentionally nullable or non-nullable type. The opportunity to change large numbers of types for you at once without also accidentally changing your intent has been lost. A long migration effort (such as one on a large project) can be done incrementally, by committing these hints over time.

Migrating a package

  1. Select a package to work on, and open a command terminal in the top-level of the package directory.

  2. Run pub get in order to make available all dependencies of the package.

  3. Run the migration tool from the top-level of the package directory:

    dart migrate

The migration tool will display a URL for the web interface. Open that URL in a browser to view, analyze, and improve the proposed null-safe migration.

Using the tool

  1. Run the tool (see above).
  2. Once analysis and migration is complete, open the indicated URL in a browser.
  3. Start with an important or interesting file in your package on the left side by clicking on it.
  4. Look at the proposed edits in the upper right, and click on them in turn.
  5. If you see an edit that looks wrong:
    1. Use the "trace view" in the bottom right to find the root cause
    2. Either click on an "add hint" button to correct it at the root, or open your editor and make the change manually.
      1. Some changes are as simple as adding a /*!*/ hint on a type. The tool has buttons to do this for you.
      2. Others may require larger refactors. These changes can be made in your editor, and may often be committed and published immediately.
    3. Periodically rerun the migration and repeat.
  6. Once you are satisfied with the proposed migration:
    1. Save your work using git or other means. Applying the migration will overwrite the existing files on disk.
      • Note: In addition to making edits to the Dart source code in the package, applying the migration edits the package's pubspec.yaml file, in order to change the Dart SDK version constraints, under the environment field, and the "Package Config" file, located in the package's .dart_tool directory, named package_config.json.
    2. Apply the migration by clicking the Apply Migration button in the interface.
    3. Tip: leaving the web UI open may help you if you later have test failures or analysis errors.
  7. Rerun pub get and test your package.
    1. If a test fails, you may still use the preview to help you figure out what went wrong.
    2. If large changes are required, revert the migration, and go back to step one.
  8. Commit and/or publish your migrated null-safe code.

Providing feedback

Please file issues at, and reference the analyzer-nnbd-migration label (you may not be able to apply the label yourself).