Named Constructor with required parameters and null safety

user_7277725 0
Viewed 4485

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?

dart-null-safety dart flutter
2 Answers

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. 
  });
}
user_66186220

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});
}
user_44536270
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