avoid_collection_methods_with_unrelated_types

v0.3.0 Warning Collection & Type

Calling collection methods like contains(), remove(), or containsKey() with an argument whose type is unrelated to the collection’s type parameter will always return null, false, or -1. This indicates a logical error since Dart’s type system allows it due to these methods accepting Object?.

Why use this rule

Methods like List.contains() and Map.containsKey() accept Object? for historical reasons, so the compiler won’t catch type mismatches. Passing a String to List<int>.contains() compiles fine but always returns false, hiding a bug.

See also: Dart generics

Don’t

void example() {
  final list = <int>[1, 2, 3];

  // String argument on int list
  list.contains('a');
  list.remove('a');

  final set = <int>{1, 2, 3};

  // String argument on int set
  set.contains('a');
  set.lookup('a');

  final map = <int, String>{};

  // String key on int-keyed map
  map.containsKey('a');

  // int value on String-valued map
  map.containsValue(42);

  // String key on int-keyed map
  final value = map['a'];
  map.remove('a');
}

Do

void example() {
  final list = <int>[1, 2, 3];
  list.contains(1);
  list.remove(2);
  list.indexOf(3);

  final set = <int>{1, 2, 3};
  set.contains(1);

  final map = <int, String>{};
  map.containsKey(1);
  map.containsValue('hello');
  final value = map[1];
  map.remove(1);

  // Subtypes are fine
  final numList = <num>[1, 2, 3];
  numList.contains(42); // int is subtype of num

  // Dynamic is allowed (type not statically known)
  dynamic unknown = 42;
  list.contains(unknown);
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      avoid_collection_methods_with_unrelated_types: false