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avoid-unrelated-type-assertions

added in: 1.6.0
warning
⚙️
Pro+
preset: recommended

Warns about unrelated usages of the is operator and whereType method.

Unrelated type assertions are usually a sign of a bug (either the target variable or the used type should be different) and can lead to unexpected behavior.

⚙️ Config​

Set ignore-mixins (default is false) to exclude mixins (example).

analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-unrelated-type-assertions:
        ignore-mixins: false

Example​

❌ Bad:

class Example {
  final regularString = '';
  final myList = <int>[1, 2, 3];

  final Animal animal = Animal();

  void main() {
    final result = regularString is int; // LINT: Avoid unrelated 'is' assertion. The result is always 'false'.
    final result2 = myList is List<String>; // LINT: Avoid unrelated 'is' assertion. The result is always 'false'.

    final result3 = animal is NotAnimal; // LINT: Avoid unrelated 'is' assertion. The result is always 'false'.
  }

  void patterns() {
    if (animal case NotAnimal result) {} // LINT: Avoid unrelated 'case Type' assertion. The result is always 'false'.
    if (animal case NotAnimal()) {} // LINT: Avoid unrelated 'case Type' assertion. The result is always 'false'.
  }
}

class Animal {}

class NotAnimal {}

class HomeAnimal extends Animal {}

✅ Good:

class Example {
  final regularString = '';
  final myList = <int>[1, 2, 3];

  final Animal animal = Animal();

  void main() {
    final result = regularString is String;
    final result2 = myList is List<int>;

    final result3 = animal is Object;
  }

  void patterns() {
    final animal = Animal();

    if (animal case HomeAnimal result) {}
    if (animal case HomeAnimal()) {}
  }
}

Example with "ignore-mixins"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-unrelated-type-assertions:
        ignore-mixins: true

✅ Good:

void main() {
  final HomeAnimal homeAnimal = HomeAnimal();

  final result = homeAnimal is WithLegs; // Correct, mixins are ignored
}

class Animal with WithLegs {}

class HomeAnimal extends Animal {}

mixin WithLegs {}

Known Limitations​

The rule does not have access to all subclasses and their mixins and therefore in some cases will highlight such usages as incorrect.

class Parent {}

mixin Mixin { }

class Child with Mixin {}

void fn() {
  final list = <Parent>[]; // List of "Parents"

  final result = list.whereType<Mixin>(); // Expected
}

Additional Resources​