Dart Lesson 38 of 102 3 min read

Null-Aware Operators in Dart: ?., ??, ??= and !

Learn Dart's null-aware operators with examples: safe access with ?., default values with ??, assign-if-null with ??= and the ! assertion.

Checking for null with if works, but it gets wordy. Dart has a set of small operators that handle the common cases in a few characters.

?? gives a default #

a ?? b means “use a, but if it is null use b”.

void main() {
  String? nickname;
  var display = nickname ?? 'Guest';
  print(display);

  nickname = 'Sunny';
  print(nickname ?? 'Guest');
}

Guest
Sunny

The result of ?? is non-nullable when the right-hand side is, so display is a plain String.

?. reaches in safely #

a?.b means “if a is not null, get b; otherwise the whole thing is null”.

void main() {
  String? email;
  print(email?.length);
  print(email?.toUpperCase());

  email = 'hi@example.com';
  print(email?.length);
}

null
null
14

Chains stop at the first null, so user?.address?.city never crashes. Combine it with ?? to finish with a real value:

void main() {
  String? email;
  var length = email?.length ?? 0;
  print(length);
}

0

??= assigns only if null #

void main() {
  int? retries;

  retries ??= 3; // retries was null, so it becomes 3
  retries ??= 9; // already has a value, nothing happens

  print(retries);
}

3

It is handy for filling in a value the first time it is needed, as in cache[key] ??= compute(key).

! insists the value is not null #

The “bang” operator converts a nullable value to a non-nullable one. If the value is in fact null, the program throws.

void main() {
  String? token = 'abc123';
  String sure = token!;
  print(sure.length);

  String? missing;
  // print(missing!.length); // would throw: Null check operator used on a null value
  print(missing?.length);
}

6
null

Every ! is a promise the compiler cannot verify. Use it only when you know something the compiler does not, and prefer ??, ?. or an if check whenever you can. A codebase sprinkled with ! has given up the benefit of null safety.

Other null-aware forms #

void main() {
  List<int>? numbers;
  Map<String, int>? ages;

  print(numbers?[0]);        // null-aware index
  print(ages?['Ram']);
  print([1, 2, ...?numbers]); // null-aware spread

  StringBuffer? buffer;
  buffer?..write('a')..write('b'); // null-aware cascade
  print(buffer);
}

null
null
[1, 2]
null

Choosing the right one #

A realistic example #

class User {
  final String name;
  final String? phone;
  User(this.name, [this.phone]);
}

String contactLine(User? user) {
  final name = user?.name ?? 'Unknown';
  final phone = user?.phone ?? 'no phone';
  return '$name ($phone)';
}

void main() {
  print(contactLine(User('Maya', '9800000000')));
  print(contactLine(User('Raj')));
  print(contactLine(null));
}

Maya (9800000000)
Raj (no phone)
Unknown (no phone)

Try it yourself #

Given Map<String, String?> settings = {'theme': null, 'lang': 'ne'};, print the theme in upper case, falling back to LIGHT, in a single expression. Then use ??= to set a default theme in the map.