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prefer-correct-json-casts

added in: 1.8.0
warning
Pro+
preset: recommended

Warns when a JSON object type cast is done in an unsafe way that will throw at runtime.

Casting a JSON object to a Map or List with generic types specified as anything except Object? / Object / dynamic can lead to a runtime exception. Instead, cast first to Object? and then to the desired type.

Example​

❌ Bad:

void main() {
  final jsonRaw = '{"items": {"hello": 1}}';
  final items = json['items'] as Map<String, int>; // LINT: Using 'Map' as generic type will throw at runtime. Replace with 'Object?' and cast separately.

  final jsonRaw = '{"items": [1, 2, 3]}';
  final items = json['items'] as List<int>; // LINT: Using 'List' as generic type will throw at runtime. Replace with 'Object?' and cast separately.

  if (json case Object() as List<String>) {} // LINT: Using 'List' as generic type will throw at runtime. Replace with 'Object?' and cast separately.
}

✅ Good:

import 'dart:convert';

void main() {
  final jsonRaw = '{"items": {"hello": 1}}';
  final json = jsonDecode(jsonRaw) as Map<String, Object?>;
  final items = (json['items'] as Map<String, Object?>).cast<String, int>();

  final jsonRaw = '{"items": [1, 2, 3]}';
  final json = jsonDecode(jsonRaw) as Map<String, Object?>;
  final items = (json['items'] as List<Object?>).cast<int>().toList();
}

Additional Resources​