Flutter - The return type 'Null' isn't a 'City', as required by the closure's context

Question

I got an error when I tried to use List.firstWhere and set orElse to return null

Error shows: The return type 'Null' isn't a 'City', as required by the closure's context

Sample code below

/// city.dart

class City {
  final int id;
  final String no;
  final String name;
  final String website;
  final bool status;

  City(this.id, this.no, this.name, this.website, this.status);

  City.fromJson(Map<String, dynamic> json)
      : id = json['id'],
        no = json['no'],
        name = json['name'],
        website = json['website'],
        status = json['status'];

  Map<String, dynamic> toJson() =>
      {'id': id, 'no': no, 'name': name, 'website': website, 'status': status};
}

/// main.dart

/// declare a list variable
List<City> _cities = [];

...


_cities.firstWhere((element) => element.id == 1, orElse: () => null); // error here

Though I can use firstWhereOrNull method in package:collection and won't get any error, I want to figure out how to use firstWhere in a correct way.

Thanks for help!

Answer 1

its throw error because you declare the list is non-null value.

if we look into the function we can see the different between them.

• firsWhereOrNull ( package:collection )

as you can see, the T? is nullable by default. so its will return null

T? firstWhereOrNull(bool Function(T element) test) {
    for (var element in this) {
      if (test(element)) return element;
    }
    return null;
  }

---

• firstWhere its non-null value. data type of orElse is following based on our declared value. since you declare List<City> this is non-null value,

then when you set function orElse: () => null it will throw IterableElementError.noElement();

 E firstWhere(bool test(E element), {E orElse()?}) {
    for (E element in this) {
      if (test(element)) return element;
    }
    if (orElse != null) return orElse();
    throw IterableElementError.noElement();
  }

but if you declare List<City?> the E in firstWhere now is nullable value. then it will no error.