null_check_on_nullable_type_parameter

Group: style

Maturity: experimental

Dart SDK: >= 2.11.0-182.0.dev • (Linter v0.1.120)

Since info is static, may be stale

View all Lint Rules

Using the Linter

Don't use null check on a potentially nullable type parameter.

Given a generic type parameter T which has a nullable bound (e.g. the default bound of Object?), it is very easy to introduce erroneous null checks when working with a variable of type T?. Specifically, it is not uncommon to have T? x; and want to assert that x has been set to a valid value of type T. A common mistake is to do so using x!. This is almost always incorrect, since if T is a nullable type, x may validly hold null as a value of type T.

BAD:

T run<T>(T callback()) {
  T? result;
   (() { result = callback(); })();
  return result!;
}

GOOD:

T run<T>(T callback()) {
  T? result;
   (() { result = callback(); })();
  return result as T;
}