Dart

// ❌ Manual null check
String display;
if (user.name != null) {
  display = user.name!;
} else {
  display = 'Unknown';
}

// ✅
final display = user.name ?? 'Unknown';

// ❌ Nested null checks
if (user != null && user.address != null && user.address!.city != null) {
  print(user.address!.city!);
}

// ✅
final city = user?.address?.city;
if (city != null) print(city);

// ❌ Late field when nullable is correct
late String name; // crashes if accessed before assignment

// ✅ — use late only when you guarantee initialization before access
String? name; // honestly nullable
// late is fine for: late final _controller = TextEditingController();

// ❌ Bang operator (!) everywhere
final name = user.name!;
final city = user.address!.city!;

// ✅ — promote through null checks
final name = user.name;
if (name == null) return;
// name is now non-null (promoted)

// ❌ Imperative accumulation
final result = <String>[];
for (final item in items) {
  if (item.isActive) result.add(item.name.toUpperCase());
}

// ✅
final result = items
    .where((i) => i.isActive)
    .map((i) => i.name.toUpperCase())
    .toList();

// ❌ Manual map construction
final map = <String, List<Item>>{};
for (final item in items) {
  map.putIfAbsent(item.category, () => []).add(item);
}

// ✅ (using collection-if/for in a different way — but groupBy isn't built-in)
// The loop above is actually idiomatic Dart. Use package:collection for groupBy:
import 'package:collection/collection.dart';
final map = groupBy(items, (Item i) => i.category);

// ❌ Building list with add() calls
final widgets = <Widget>[];
widgets.add(Header());
if (showSubtitle) widgets.add(Subtitle());
widgets.add(Body());

// ✅ — collection-if
final widgets = [
  Header(),
  if (showSubtitle) Subtitle(),
  Body(),
];

// ❌ Spreading manually
final all = <int>[];
all.addAll(listA);
all.addAll(listB);

// ✅
final all = [...listA, ...listB];

// ❌ Manual data class boilerplate
class Point {
  final int x, y;
  const Point(this.x, this.y);
  @override bool operator ==(Object other) => ...
  @override int get hashCode => ...
  @override String toString() => 'Point($x, $y)';
}

// ✅ (Dart 3.0+)
typedef Point = ({int x, int y});
// or for named class semantics:
class Point {
  final int x, y;
  const Point(this.x, this.y);
}
// Use package:equatable or Dart records for equality

// ❌ Verbose constructor
class User {
  final String name;
  final int age;
  User(String name, int age) : name = name, age = age;
}

// ✅ — initializing formals
class User {
  final String name;
  final int age;
  const User(this.name, this.age);
}

// ❌ Mutable fields on an immutable object
class Config {
  String host;
  int port;
  Config(this.host, this.port);
}

// ✅
class Config {
  final String host;
  final int port;
  const Config(this.host, this.port);
}

// ❌ Switch on type with if-else chain
if (shape is Circle) {
  return (shape as Circle).radius * pi;
} else if (shape is Rectangle) { ... }

// ✅ (Dart 3.0+)
return switch (shape) {
  Circle(:final radius) => radius * radius * pi,
  Rectangle(:final width, :final height) => width * height,
};

// ❌ .then() chains
fetchUser()
    .then((user) => fetchPosts(user))
    .then((posts) => display(posts))
    .catchError((e) => log(e));

// ✅
try {
  final user = await fetchUser();
  final posts = await fetchPosts(user);
  display(posts);
} catch (e) {
  log(e);
}

// ❌ Sequential await for independent work
final a = await fetchA();
final b = await fetchB();

// ✅
final results = await Future.wait([fetchA(), fetchB()]);
// or with typed destructuring:
final (a, b) = await (fetchA(), fetchB()).wait; // Dart 3.0+ record

// ❌ StreamBuilder doing too much in build
StreamBuilder(
  stream: stream,
  builder: (ctx, snap) {
    if (snap.hasError) return Error();
    if (!snap.hasData) return Loading();
    final data = snap.data!;
    // 50 lines of widget tree...
  },
)

// ✅ — extract widget, or use listen + setState for simple cases

// ❌ Catching Exception (too broad)
try { process(); }
on Exception catch (e) { print(e); }

// ✅ — catch specific types
try {
  process();
} on FormatException catch (e) {
  throw AppException('Invalid format', cause: e);
} on HttpException catch (e) {
  throw AppException('Network error', cause: e);
}

// ❌ Returning null for errors
Future<User?> fetchUser() async {
  try { return await api.getUser(); }
  catch (_) { return null; } // caller doesn't know why
}

// ✅ — let exceptions propagate, or use sealed Result type
sealed class Result<T> {}
class Success<T> extends Result<T> { final T value; Success(this.value); }
class Failure<T> extends Result<T> { final Object error; Failure(this.error); }

// ❌ Rebuilding entire tree on state change
setState(() {
  // changes a single value, but the build() method builds 200 widgets
});

// ✅ — extract subtrees into separate widgets or use ValueListenableBuilder
ValueListenableBuilder<int>(
  valueListenable: counter,
  builder: (_, value, __) => Text('$value'),
)

// ❌ const-able widget without const
Container(color: Colors.blue)

// ✅
const ColoredBox(color: Colors.blue)
// Mark constructors const when possible; use `const` keyword at call site

// ❌ Navigator.push with MaterialPageRoute everywhere
Navigator.push(context, MaterialPageRoute(builder: (_) => DetailPage()));

// ✅ — named routes or GoRouter
context.go('/detail');