Why do only constant constructors in Dart, not non-constant constructor, can't assign different type of value?

Question

I'm getting error "A value of type 'Null' can't be assigned to a parameter of type 'String' in a const constructor." from const Text().

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key, required this.title, required this.subTitle});
  final String title;
  final String subTitle;
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
   return Scaffold(
      appBar: AppBar(
        title: Text(
          widget.title,
          textAlign: TextAlign.center,
        ),
      ),
      body: Column(
        children: <Widget>[
          const Text(
            widget.subTitle,
          ),
      ),
    )
  }
}

I already know I have to remove const to resolve the error. But Could anyone explain why does only const constructor have this error?

Are you certain that's the error you're getting? Is this your exact code? Using `const Text` there is not allowed, but the error shouldn't be saying anything about `Null` since `widget.subTitle` is non-nullable. — jamesdlin, Mar 01 '23 at 17:16

score 1 · Answer 1 · answered Mar 01 '23 at 16:29

1

Because widget.subTitle will get on runtime, instead of compile time. Another thing is subTitle is final , not const.

You can check What is the difference between the "const" and "final" keywords in Dart?

answered Mar 01 '23 at 16:29

Md. Yeasin Sheikh

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score 1 · Accepted Answer · answered Mar 01 '23 at 23:48

It is because const Constructor expects const parameters or must be able to be evaluated as const at compilation time.

And you are trying to pass widget with a non-constant value .And this gives you error because they cannot be evaluated at compile time.

You have two possible solution:

remove the const from the Text widget.

Text(widget.subTitle),

make tilte a const variable.

const MyHomePage({..., required final const this.subTitle});
final const String subTitle;

Why do only constant constructors in Dart, not non-constant constructor, can't assign different type of value?

2 Answers2