type 'List<List<dynamic>>' is not a subtype of type 'List<List<num>>' in type cast

[Sacchi]

What is correct way to cast List<List<dynamic>> to List<List<num>> ?

As, if I cast using .cast(), I am getting following CastError -

type List<dynamic> is not a subtype of type List<num> in type cast

And if I also use as List<List<num>>, I am getting following CastError -

type List<List<dynamic>> is not a subtype of type List<List<num>> in type cast

---

Details -

I have function List<Point> toListOfPoint(List<List<num>> list_of_list){...} from import 'package:poly/poly.dart';
So, In order to pass dynamic list I'm casting List<List<dynamic>> to List<List<num>>
But, I'm getting following error(s)-

1. type List<dynamic> is not a subtype of type List<num> in type cast

with code -

import 'package:poly/poly.dart';
main(){
List correctDynamicList = [1, 2];
// type `List<dynamic>` is not a subtype of type `List<num>` in type cast
  try {
    var l = [correctDynamicList];
  // `l.runtimetype` = `List<List<dynamic>>`
    print("${l.runtimeType}");
    List<Point> wrongListOfList = toListOfPoint(l.cast());
  } on CastError catch (e) {
    print(
        "${++exception_count}. ${e.runtimeType} Exception handled : \n \t \u2022 ${e}");
  }
}

---

2. If I use as List<List<num>> I get following error -

type List<List<dynamic>> is not a subtype of type List<List<num>> in type cast

with following code -

import 'package:poly/poly.dart';
main(){
List correctDynamicList = [1, 2];
// type `List<dynamic>` is not a subtype of type `List<num>` in type cast
  try {
    var l = [correctDynamicList] as List<List<num>>;
    List<Point> wrongListOfList = toListOfPoint(l.cast());
  } on CastError catch (e) {
    print(
        "${++exception_count}. ${e.runtimeType} Exception handled : \n \t \u2022 ${e}");
  }
}

---

• Dart SDK Version (2.2.0)
• using Windows

[Erik Ernst]

One thing you need to take into account is that List<List<dynamic>> is not a subtype of List<List<num>>. So if you have a List<List<dynamic>> (not just an object o such that o is List<List<dynamic>> is true, but such that List<List<dynamic>> is the most specific type that o has) then there is no cast that will allow you to assign that object to a variable of type List<List<num>>. Just like there is no cast which will allow you to assign a String to a variable of type bool (you can make it a run-time error rather than a compile-time error by inserting a cast to dynamic, but it will fail).

You can obtain a view on a List<List<dynamic>> of type List<List<num>> by calling cast<List<num>>() on it. This will dynamically check that it actually contains instances of List<num>, and throw if any of them isn't. So you may then need to do something similar for each of the nested lists, if they are actually of type List<dynamic>.

The best approach would usually be to create your lists with the required type from the very beginning. Clients who don't care can then work on it using a type like List<List<dynamic>>, and clients who really insist on getting a List<List<num>> would get what they need.

[Leaf Petersen]

What is correct way to cast List<List<dynamic>> to List<List<num>> ?

If you want to convert from one to other (and can't arrange for the original to be of the right type), the following will do what you want:

  List<List<dynamic>> l = [[3]];
  // Map over the list, casting each element list.
  List<List<num>> ln = l.map((x) => x.cast<num>()).toList();