Not able to assign a `List` to extended List in Dart

Question

I need to extend List to be able to overload [] operator.

I tried all the approaches from here: How do I extend a List in Dart?. But all of them give me the same error A value of type 'List<dynamic>' can't be assigned to a variable of type 'MyCustomList<int>'. when I assign usual List to my extended CustomList:

import 'package:quiver/collection.dart';

class MyCustomList<E> extends DelegatingList<E> {
  final List<E> _l = [];
  List<E> get delegate => _l;
}

void main() {
  MyCustomList<int> l = []; // << error here
}

Is it possible to extend so that this error disappeared?

I know that I can change to:

MyCustomList<int> l = [] as MyCustomList<int>;

But I would like to avoid it because of it would require massive changes in my real code.

Even the casting solution will not work, because you're trying to cast an instance of a superclass to a subclass. The literal `[]` always creates an instance of `List`, which has no member `delegate`, and therefore cannot be casted to `MyCustomList` — Michael Horn, Apr 02 '23 at 14:35

score 1 · Accepted Answer · answered Apr 02 '23 at 14:35

The issue with your code is that you're trying to assign a List<int> to a variable of type MyCustomList<int>.

You can create a constructor and initialize the private list _l.

class MyCustomList<E> extends DelegatingList<E> {
  MyCustomList(List<E> list) {
    _l.addAll(list);
  }

  final List<E> _l = [];

  List<E> get delegate => _l;
}

void main() {
  MyCustomList<int> l = MyCustomList([1, 2, 3]);
}

Note: seems the [] operator overload is for accessing and assigning value to an element and you can't use it for instantiate list.

@override
void operator []=(int index, E value) => _l[index] = value;

@override
E operator [](int index) => _l[index];

Not able to assign a `List` to extended List in Dart

1 Answers1