Named Constructor with required parameters and null safety

Question

The following code does not compile with sound null safety because it is possible to pass null to the constructor which initializes a non-nullable field myString.

class MyClass {
  String myString;
  MyClass({@required this.myString});
}

I would expect that adding a question mark after this.myString would solve the problem, but it doesnt and another compile error is shown.

A default value is no option for me and it seems that the only other solution is something like this:

class MyClass {
  late String myString;
  MyClass({@required myString}) {
    this.myString = myString;
  }
}

Imo this decreases readability if there are a lot of parameters. Is there a more convenient solution which suppports initialization with this. in named constructors?

Use the new keyword "required" instead of "@required". It is documented here: https://dart.dev/null-safety/understanding-null-safety#required-named-parameters — julemand101, May 24 '21 at 12:10
There is no other way. If you check the internal code of any of Flutter's widgets, you would find the same pattern. But yeah `required` instead of `@required` — Nisanth Reddy, May 24 '21 at 12:42
@NisanthReddy What do you mean by "there is no other way"? His first example would work fine if he just replace `@required` with `required`. — julemand101, May 24 '21 at 17:20
@julemand101 I meant other than using the one he mentioned, there would be no other way — Nisanth Reddy, May 24 '21 at 17:29

score 9 · Answer 1 · answered May 25 '21 at 09:37

For non-nullable types, either use required or provide a default value. You can also use late but make sure to provide a value at some later point before using it. It's like a bang operator.

Example:

class Foo {
  // All are non-nullable.
  final int a;
  final int b;
  late final int c; // I trust you that you'll provide me a value later. 
  
  Foo({
    required this.a, // <-- Mark it required  
    this.b = 0, // <-- or provide a default value. 
  });
}

score 2 · Answer 2 · answered May 24 '21 at 13:43

2

If i am not getting the idea wrong, if you want to have a String param which is nullable from the constructor, you can declare a 'String?' type.

class MyClass {
  String? myString;
  MyClass({required this.myString});
}

answered May 24 '21 at 13:43

CbL

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Named Constructor with required parameters and null safety

2 Answers2