The ! operator

Learn null assertion with !.

The ! operator

The ! operator performs a null assertion: you say 'I guarantee this value is not null'. name!.length treats name as non-nullable.

If name is actually null, it throws at runtime. So ! is used carefully: only when you are sure the value is non-null.

? and ! are opposites: ? makes a type nullable, ! uses a nullable as non-nullable. In practice null checks are preferred over !, but the bang is sometimes a handy shortcut.

! vs ?.

Both ! and ?. access a member on a nullable value, but they behave differently. ! asserts non-null; if wrong, it throws. ?. returns null when the value is null; no error.

Example: name!.length crashes if name is null; name?.length returns null instead. Which one to pick depends on your task.

Rule of thumb: ! when sure, ?. when not. ! is faster but risky; ?. is safe but yields a nullable result.

Exercise Length with !

Declare a String? name and assign 'Ayşe'. Print its length with print(name!.length).

Show example solution

void main() {
  String? name = 'Ayşe';
  print(name!.length);
}

Test yourself

What does name!.length do when name is null?
- Returns null
- Returns 0
- Throws an error
- Nothing
Show answer

Throws an error — ! throws on a null value.

Assertion with !

void main() {
  String? name = 'Ayşe';
  print(name____.length);
}

Show answer

void main() {
  String? name = 'Ayşe';
  print(name!.length);
}

Match the operators

!

Null assertion
?.

Safe access
??

Default value
?

Nullable type

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