Sound null safety

Why null safety?

null means "no value". In many older languages any variable could be null, and using it caused crashes such as "Null check operator used on a null value" or "NoSuchMethodError: The getter 'length' was called on null". Since Dart 3, null safety is always on and sound: if the type says a value is not null, it really is never null at run time. The compiler checks this for you.

Nullable and non-nullable types

void main() {
  String name = 'Asha';
  // String bad = null;        // error: a value of type 'Null' can't be assigned
  String? nickname;            // nullable, starts as null

  print(name.length);          // 4
  // print(nickname.length);   // error: the property 'length' can't be accessed on 'String?'
  print(nickname?.length);     // null
}

The null-aware operators

void main() {
  String? referral;
  final code = referral?.trim().toUpperCase() ?? 'NONE';
  print(code);                 // NONE

  int? retries;
  retries ??= 3;
  print(retries);              // 3
}

Flow analysis (type promotion)

int lengthOrZero(String? text) {
  if (text == null) {
    return 0;
  }
  return text.length;          // text is promoted to String here
}

Promotion works on local variables and parameters. It does not work on class fields that could change between the check and the use; copy the field into a local variable first (final t = this.title; if (t != null) ...).

late and required

class Profile {
  final String name;
  final String? bio;
  late final String initials;      // set later, but before use

  Profile({required this.name, this.bio}) {
    initials = name.split(' ').map((w) => w[0]).join();
  }
}

If you read a late variable before it is set, Dart throws a LateInitializationError. Use late only when you are sure about the order, for example values set in initState in Flutter (module 4).

Common errors and fixes

• "The argument type 'String?' can't be assigned to the parameter type 'String'": handle the null first (?? default, or a null check), do not just add !.
• "Non-nullable instance field must be initialized": give it a value in the constructor, make it nullable, or (carefully) late.
• "Null check operator used on a null value" at run time: an ! was wrong. Replace it with proper handling.
• Making everything nullable "to be safe": then every use needs checks. Use non-null types wherever a value must exist.