---
name: dart-language-d794bc9ac950
description: "Dart 3.x language feature standards: null safety, records, sealed classes, switch pattern matching, extensions, and async/await. Use when using !, ?., ??, late, sealed classes, record types, switch expressions, or async patterns — and before introducing any new Dart 3.x construct to confirm the modern idiomatic approach."
metadata:
  triggers:
    files:
      - "**/*.dart"
    keywords:
      - sealed
      - record
      - switch
      - pattern
      - "!"
      - late
      - async
      - extension
---

# Dart Language Patterns

## **Priority: P0 (CRITICAL)**

## Implementation Guidelines

- **Null Safety**: Avoid `!`. Use `?.`, `??`, or short-circuiting. Use `late` only if necessary.
- **Immutability**: Use `final` for all variables. Use `@freezed` for data classes.
- **Pattern Matching (3.x)**: Use `switch (value)` with patterns and destructuring.
- **Records**: Use a Record (e.g., `(String, int)`) for multiple values; use named fields when meaning matters. Destructure at call sites: `final (error, data) = fetchUser();`.
- **Sealed Classes**: Use `sealed class` for exhaustive state handling in domain logic.
- **Extensions**: Use `extension` to add utility methods to third-party types.
- **Wildcards (3.7+)**: Use `_` for unused variables in declarations and patterns.
- **Tear-offs**: Prefer using tear-offs (e.g., `list.forEach(print)`) over anonymous lambdas (e.g., `list.forEach((e) => print(e))`).
- **Asynchrony**: Prefer `async/await` over raw `Future.then`. Use `unawaited` for fire-and-forget logic if necessary.
- **Encapsulation**: Use `_` prefix for library-private members. Prefer `final` properties.
- **Collections**: Use `collection-if`, `collection-for`, and spread operators `...`.
- **Safe List Access**: Prefer `.firstOrNull`, `.lastOrNull`, or `.elementAtOrNull(i)`.
- **No dynamic**: Use `Object` or generics instead of `dynamic`.
- **Type Aliases**: Use `typedef` for complex IDs or callbacks.

## Anti-Patterns

- **No ! Operator**: not use bang operator `!` unless you can prove value non-null via `if` or `assert`.
- **No var for members**: not use `var` for class members; use `final` or explicit types.
- **No logic in constructors**: not perform complex calculations or async work inside constructors.
- **No zero-arg methods for pure computations**: Use a getter. `int get invoiceType =>` not `int toInvoiceType()`.
- **No generic conversion names**: Name value-object converters for their target context: `get apiFilterType` not `get invoiceType`.

## Code

```dart
// Sealed class and Switch expression
sealed class Result {}
class Success extends Result { final String data; Success(this.data); }
class Failure extends Result {}

String message(Result r) => switch (r) {
  Success(data: var d) => "Got $d",
  Failure() => "Error",
};
```

## References

- feature-based-clean-architecture | tooling

## Null-safety checklist

- Replace `!` with `?`-aware access, explicit narrowing, or a guard clause; do not silence nullability with a force unwrap.

## Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:
- Replace ! with ?
- destructure,destructuring,final (error,result.error


---
Source attribution: HoangNguyen0403
Original project: https://github.com/HoangNguyen0403/agent-skills-standard
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2026 Gabriel Moreira

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: HoangNguyen0403
Original project: https://github.com/HoangNguyen0403/agent-skills-standard
Attribution basis: Upstream repository explicitly recorded in package mirror path

Fields of work: Development & Engineering, Product Management
Roles: Backend Developer, Frontend Developer, Full-Stack Developer, Mobile Developer, Product Manager
Specialties: Backend, Cross Platform, Frontend, Mobile
Tools: Dart
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 72ac67c04a19a0b95b44d2378db65b76d2802c43316b68ec87a973848605e874