prefer_constrained_box_over_container

v0.4.0 Warning Fix Widget Replacement

Flags Container widgets that only use the constraints parameter (plus optional key and child). When Container is used solely for constraints, the ConstrainedBox widget is a lighter, more descriptive alternative.

Points the opposite way from prefer_container

This rule replaces a single-parameter Container with the specific widget, while prefer_container collapses 3 or more nested layout widgets into a Container.

The thresholds keep them from firing on the same code, but after further edits a fix from one rule can produce code the other flags. If that churn bothers you, enable only one direction.

The SDK’s avoid_unnecessary_containers is a third, non-overlapping case: it removes a Container that has no parameters at all.

Why use this rule

Container is a convenience widget that composes many lower-level widgets internally. When you only need constraints, using ConstrainedBox directly avoids the overhead and communicates your intent more clearly. It also keeps the widget tree shallow and easier to reason about.

See also: ConstrainedBox | Container

Don’t

// Container with only constraints parameter
Container(
  constraints: BoxConstraints(maxWidth: 200),
  child: Text('Hello'),
);

// Container with only constraints, no child
Container(constraints: BoxConstraints.tightFor(width: 100));

Do

// Use ConstrainedBox directly
ConstrainedBox(
  constraints: BoxConstraints(maxWidth: 200),
  child: Text('Hello'),
);

// Container with additional properties besides constraints is fine
Container(
  constraints: BoxConstraints(maxWidth: 200),
  padding: EdgeInsets.all(8),
  child: Text('Hello'),
);

Configuration

This rule is in the opinionated preset, so it is on with preset: opinionated, or by name:

many_lints.yaml

rules:
  prefer_constrained_box_over_container: true

To turn it off again:

many_lints.yaml

rules:
  prefer_constrained_box_over_container: false

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

Related rules

• prefer_container — Opposing convention. Replace sequences of nested widgets with a single Container.
• prefer_align_over_container — Use Align instead of Container when only alignment is set.
• prefer_padding_over_container — Use Padding instead of Container when only padding or margin is set.
• prefer_transform_over_container — Use Transform instead of Container when only transform is set.