Dart 3.12 Ships Primary Constructors Behind an Experiment Flag

Dart 3.12 (released 2026-05-20) shipped one of the language’s most-requested features as an experimental preview: primary constructors. If you have ever written the same field, plus this.field constructor parameter, plus assignment three times for a simple data class, this is the syntax that kills that pattern. It is gated behind --enable-experiment=primary-constructors for now, but it is worth wiring into a branch today because it changes how a lot of everyday Dart code reads.

This follows the other Dart 3.12 boilerplate cut, private named parameters as initializing formals. Primary constructors go further: they move the whole declaration into the class header.

One line instead of four

Here is the data class everyone writes, the part the compiler should have been generating all along:

class Point {
  final int x;
  final int y;
  Point(this.x, this.y);
}

With a primary constructor, the field declarations and the constructor collapse into the header. An empty class body becomes a semicolon:

class Point(final int x, final int y);

The rule is simple: a parameter marked final or var in the header becomes an instance field. Drop the modifier and it stays a plain constructor parameter, not a field. So class User(String name); takes name as an argument without storing it, while class User(final String name); stores it.

Fields can depend on header parameters

Header parameters are in scope inside the class body, so you can initialize other non-late fields from them without an initializer list:

class DeltaPoint(final int x, int delta) {
  final int y = x + delta;
}

Here delta is a constructor parameter (no final, so not a field) and y is computed from it.

Adding validation with a body

When you need an assert or some setup, you write a constructor body introduced by this. An initializer-list-only form ends in a semicolon:

class Point(var int x, var int y) {
  this : assert(x >= 0 && y >= 0) {
    print('Point initialized at ($x, $y)');
  }
}

Named constructors get a tighter form too, using new in the body:

class Pet {
  String name;

  new() : name = 'Fluffy';
  new withName(this.name);
}

Turning it on

The feature is experimental, so you opt in per run:

dart run --enable-experiment=primary-constructors bin/main.dart

Because it is experimental, treat it as a preview: the syntax can still shift before it stabilizes, and final and var now carry special meaning in a parameter list, so do not roll it into shared production code yet. But for a side branch, primary constructors make Flutter widget models, value objects, and config holders dramatically shorter. The full spec, including super parameters and the named-constructor rules, lives in the Dart primary constructors docs.