Dart 3.12 Drops the Initializer List for Private Fields

Dart 3.12 shipped at Google I/O 2026 alongside Flutter 3.44, and buried in the release is a small language change that removes a chunk of boilerplate every Dart developer has written a hundred times. You can now use private named parameters as initializing formals. In plain terms: a constructor can take a named argument and assign it straight to a private field, no initializer list required.

The underscore problem Dart carried for years

Dart marks a field private by prefixing it with an underscore. That convention collided with named constructor parameters, because the language refused to let a named parameter start with _. The result was a familiar dance: accept a public-named parameter, then manually copy it into the private field in the initializer list.

class Hummingbird {
  final String _petName;
  final int _wingbeatsPerSecond;

  Hummingbird({required String petName, required int wingbeatsPerSecond})
    : _petName = petName,
      _wingbeatsPerSecond = wingbeatsPerSecond;
}

Every private field meant a parameter declaration plus an initializer entry. For a class with eight private fields, that is sixteen lines of pure plumbing. Initializing formals (this.field) solved this for public fields years ago, but private fields were stuck on the slow path.

What 3.12 actually changes

Dart 3.12 makes the obvious thing work. Write this._field as a named parameter and the compiler handles the rest:

class Hummingbird {
  final String _petName;
  final int _wingbeatsPerSecond;

  Hummingbird({required this._petName, required this._wingbeatsPerSecond});
}

The field stays private, but the parameter name exposed to callers is the public version with the underscore stripped off. So the call site reads exactly as before:

void main() {
  print(Hummingbird(petName: 'Dash', wingbeatsPerSecond: 75));
}

This is purely a constructor-side change. The public API is identical, callers do not touch anything, and the private fields stay private. You are deleting the initializer list, nothing more.

The constraints worth knowing

This exception applies only to initializing formals. A regular named parameter still cannot start with an underscore, because that would leak a private name into the public signature with no public counterpart to fall back on. The underscore name also has to map to a legal public identifier: this._ and this._2x are rejected, because stripping the underscore leaves “ and 2x, neither of which is a valid parameter name.

The feature also pairs with primary constructors, which landed as an experiment in the same cycle and require a language version of at least 3.13. With both in play you can declare a private field directly in the constructor header and let the compiler publish the public-facing name automatically.

If you are already on Flutter 3.44, you have Dart 3.12 and can use this today. Bump your pubspec.yaml SDK constraint to ^3.12.0, run dart fix, and start deleting initializer lists. See the Dart 3.12 announcement for the full release notes.