dart-use-primary-constructors

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha bb74402d114f · 29 Oct 2026Report an issue or request a re-scan

• Publisherflutter GitHub organizationOwner of the repository this skill is read from
• Source folderflutter/agent-plugins/skills/dart-use-primary-constructorsThe exact folder we audited
• LicenceBSD-3-ClauseAs the repository states it
• Commit pinned for this verdictbb74402d114fA new audit runs when the source changes
• Repository statusNot archivedLast push 2 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Help users write syntactically and semantically correct primary constructors in Dart, and migrate/use the new constructor syntax, empty-body semicolon syntax, in-body initializer list syntax, and abbreviated concise…

---
name: dart-use-primary-constructors
description: >
  Help users write syntactically and semantically correct primary constructors in Dart, and migrate/use the new constructor syntax, empty-body semicolon syntax, in-body initializer list syntax, and abbreviated concise constructor syntax.
metadata:
  model: models/gemini-3.1-pro-preview
  last_modified: Thu, 09 Jul 2026 23:13:25 GMT
---

# Dart Primary Constructors & New Constructor Syntax Skill

Use this skill when helping users write, refactor, or debug code using Dart's **Primary Constructors** feature.

### Dart Version Requirements
*   **Dart 3.13 and above**: Primary constructors are enabled by default.
*   **Dart 3.12**: The feature is available but experimental. Users must explicitly enable the experiment flag `primary-constructors` via `--enable-experiment=primary-constructors` or in `analysis_options.yaml`:
```yaml
analyzer:
  enable-experiment:
    - primary-constructors
```
*   **Dart 3.11 and earlier**: Primary constructors are not supported.

---

## 1. Overview
Primary Constructors allow developers to declare a non-redirecting generative constructor as well as a set of instance variables directly in the class header. This significantly reduces boilerplate and improves code readability.

### Key Benefits
- Combines field declaration, parameter declaration, and initialization into a single declaration known as a declaring parameter declaration.
- Enables safe reference to constructor parameters in non-late field initializers (Primary Initializer Scope).
- Allows empty declaration bodies to be represented concisely with a semicolon (`;`).
- Introduces abbreviated concise syntax for in-body constructors.

---

## 2. Syntax Reference

### 2.1 Basic Class Header Syntax
To declare a primary constructor, place a parameter list immediately after the type name (and optional type parameters):

```dart
// Declares fields x and y, and a generative constructor Point(this.x, this.y)
class Point(var int x, var int y);

// Declares final fields
class PointFinal(final int x, final int y);
```

### 2.2 Declaring, Initializing, and Plain Parameters
A primary constructor parameter list distinguishes between three types of parameters:
1. **Declaring Parameters**: Indicated by the `var` or `final` modifier (e.g., `final int x`). They implicitly create a corresponding instance field in the class.
2. **Initializing Parameters**: Indicated by the `this.` or `super.` prefix (e.g., `this.x` or `super.x`). They initialize an existing field or a super constructor parameter, respectively.
3. **Regular Parameters**: Declared without modifiers (e.g., `int y`). They do not become fields and are only available during initialization (e.g., in field initializers or the `this :` initializer list in the class body).

```dart
// `x` is a field and a parameter because it has the keyword `final`. In particular, we can use the name `x` in the initializer list in the in-body part of the primary constructor. 'y' is a only parameter because it has neither of the keywords `final` or `var`, but `y` is passed to the super constructor via the `this :` initializer list.
class C(final int x, int y) extends Base {
  this : super(y);
}
```

Declaring parameters and initializing parameters are two ways of achieving the same goal: declaring a class with instance fields which are set in the constructor. Regular parameters are different in that their values are not automatically routed to an instance field.

### 2.3 Constant Primary Constructors
To make a primary constructor `const`, place the `const` keyword before the class/type name in the declaration header:

```dart
class const Point(final int x, final int y);
extension type const Ext(int x);
enum const MyEnum(final int x) {
  entry(1);
}
```

### 2.4 Extension Types
Extension types **must** use primary constructors.
- The single parameter in the header is the representation field.
- The representation variable cannot use the `var` modifier (using `var` triggers the `representation_field_modifier` error).
- The representation variable can optionally use the `final` modifier. If `final` is not present then it is inferred; that is, the parameter is declaring whether or not it's explicitly `final`.

### 2.5 Empty Body Semicolon Shorthand (`;`)
When a class, mixin class, mixin, extension or extension type has an empty body, the `{}` braces can be replaced by a semicolon (`;`):

```dart
class C(int x);
mixin class MC;
extension type ET(int x);
mixin M;
extension Ext on C;
```

### 2.6 The In-Body Part of a Primary Constructor (`this ...`)
If a primary constructor requires assertions or custom field initializations, they can be declared in the body using the `this :` syntax:

```dart
class Point(var int x, var int y) {
  // Initializer list in class body
  this : assert(x >= 0), y = y * 2;
}
```

You can also write a constructor body with this syntax (`this {...}`).

### 2.7 Abbreviated Concise Constructor Syntax
For constructors declared within the class body, the class name can be omitted and replaced with the `new` or `factory` keywords:

| Traditional Syntax | Abbreviated Concise Syntax |
| :--- | :--- |
| `MyClass() {}` | `new() {}` |
| `MyClass.name() {}` | `new name() {}` |
| `const MyClass();` | `const new();` |
| `const MyClass.name();` | `const new name();` |
| `factory MyClass() => ...` | `factory() => ...` |
| `factory MyClass.name() => ...` | `factory name() => ...` |

---

## 3. Semantics & Scoping Rules

### 3.1 Primary Initializer Scope
When a primary constructor is declared, its formal parameters are introduced into the **Primary Initializer Scope**. This scope is the current scope for non-late field initializers in the class body and the primary constructor's initializer list (after `this :`).
This allows non-late fields to reference constructor parameters directly during declaration:
  ```dart
  class DeltaPoint(final int x, int delta) {
    // 'x' and 'delta' are in scope here
    final int y = x + delta;
  

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/flutter/agent-plugins
git -C agent-plugins checkout bb74402d114f

Works in

1. 2026-10-29
   ✓SAFE B 89 / 100

   first audit · source bb74402d114f

Questions about dart-use-primary-constructors

• github.com/flutter/agent-plugins/tree/main/skills/dart-use-prima — the audited source at commit bb74402d114f and every finding aboveObserved · 29 Oct 2026

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