dart-coding

Dart Coding

Match the project's existing conventions. When uncertain, read 2-3 existing files to infer the local style. Check pubspec.yaml for SDK constraints, analysis_options.yaml for lint rules, and any .dart_tool/ config for code generation. These defaults apply only when the project has no established convention.

Never rules

These are unconditional. They prevent bugs and crashes regardless of project style.

• Never rely on a field null-check for promotion unless the field qualifies — flow analysis promotes locals and, since Dart 3.2, private final fields. Public final fields and private mutable fields are never promoted — the value could change between the test and the use. Copy to a local first.

// Wrong: _temp is mutable; not promoted to non-null
if (_temp != null) {
  print(_temp.length); // compile error or stale value
}

// Correct: local is promoted
final t = _temp;
if (t != null) {
  print(t.length);
}

• Never use ! to "guarantee" non-null without prior verification — ! is a runtime cast that throws TypeError when the value is null. It silently turns a static type error into a production crash. Use ??, if-checks, or pattern matching first.
• Never mix late with ? — late String? confuses two distinct states (uninitialized vs explicitly null) and defeats the purpose of late. Pick one: late String (deferred init, never null) or String? (may be null at any time).
• Never index a Map without a null-check or ! for known-present keys — map['k'] returns V?. Calling .length on the result without handling null is a compile error in null-safe code; using ! blindly turns missing keys into runtime crashes.

// Wrong: subscript returns null on missing key
final n = map['k'].length;

// Correct: handle missing key explicitly
final v = map['k'];
final n = v?.length ?? 0;

// Correct: assert presence with rationale
final n = map['k']!.length; // key inserted at line 12, guaranteed present