Skip to content
Many Lints

avoid_cascade_after_if_null

v0.3.0 Warning Fix Control Flow

Warns when a cascade expression (..) follows an if-null (??) operator without parentheses. The precedence is not what it looks like: the cascade applies to the entire if-null expression, not to the right-hand side of ??.

Why use this rule

Section titled “Why use this rule”

The cascade operator binds looser than ??, so a ?? B()..method() parses as (a ?? B())..method(). The cascade takes the whole if-null expression as its target.

That is the opposite of how the line reads. The natural reading is “if a is null, build a B and configure it” — but when a is non-null, the cascade runs against a itself:

final sb = StringBuffer('LHS');
StringBuffer? maybe = sb;
final out = maybe ?? StringBuffer('RHS')..write('-MUTATED');
// sb is now "LHS-MUTATED": the pre-existing buffer was mutated,
// the fresh StringBuffer('RHS') was discarded untouched,
// and `out` is identical to `sb`.

Adding explicit parentheses makes the intent clear and prevents subtle bugs.

See also: Cascade notation

Don’t

Section titled “Don’t”
void bad(Kettle? spareKettle) {
  // Unclear whether ..boil() applies to the result of ?? or just Kettle()
  final kettle = spareKettle ?? Kettle()
    ..boil();


  // Multiple cascades after if-null
  final kettle2 = spareKettle ?? Kettle()
    ..boil()
    ..litres = 5;
}

Do

Section titled “Do”
void good(Kettle? spareKettle) {
  // Cascade applies to the entire if-null expression
  final kettle = (spareKettle ?? Kettle())..boil();


  // Cascade applies only to the new instance
  final kettle2 = spareKettle ?? (Kettle()..boil());


  // No if-null involved, cascade is unambiguous
  final kettle3 = Kettle()..boil();
}

Configuration

Section titled “Configuration”

This rule is in the recommended preset, so it is on with preset: recommended or preset: opinionated. Add it to preset: core with avoid_cascade_after_if_null: true.

To turn it off:

many_lints.yaml
rules:
  avoid_cascade_after_if_null: false

To keep the rule on but skip certain paths, use per-rule exclude.

Section titled “Related rules”