avoid_unnecessary_constructor

v1.0.0 Warning Code Organization

Flags an empty unnamed constructor that writes out exactly what Dart supplies when no constructor is declared at all.

class A { A(); } has no parameters, no initializers, no body and no documentation. The line adds nothing a reader can act on, and it invites the assumption that construction does something.

This rule is in the opinionated preset, so it is on with preset: opinionated or preset: pedantic.

Don’t

class UserRepository {
  UserRepository();

  Future<void> refresh() async {}
}

Do

class UserRepository {
  Future<void> refresh() async {}
}

Examples

const is a real difference

const A(); lets callers write const A(), which the implicit constructor does not allow. It is never reported:

// Accepted — the const-ness is the point
class EmptyState {
  const EmptyState();
}

void render() {
  const state = EmptyState();
}

A second constructor makes the empty one load-bearing

Dart supplies the unnamed constructor only when no constructor is written. Once a named one exists, deleting A() makes A() illegal:

// Accepted — removing `Duration()` would break every plain `Duration()` call
class Timeout {
  Timeout();

  Timeout.seconds(this.value);

  int value = 0;
}

Documentation and annotations carry information

An empty constructor that is documented or annotated says something the implicit one cannot, so it stays:

// Accepted
class Analytics {
  /// Constructing this is cheap; the transport connects lazily.
  Analytics();
}

// Accepted
class Legacy {
  @Deprecated('Use Legacy.fromConfig instead.')
  Legacy();
}

Named and private constructors are a different construct

Only the unnamed one duplicates a default:

// Accepted — there is no implicit `Cache._()`
class Cache {
  Cache._();

  static final instance = Cache._();
}

Known limitations

No quick fix. Deleting the line is safe in the reported cases, but the constructor is often the anchor a diff or a code review is hanging on, and removing it silently would hide that.

Turning this rule off

To disable this rule:

many_lints.yaml

rules:
  avoid_unnecessary_constructor: false

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

Related rules

• arguments_ordering — Keep named arguments in a configured order.
• avoid_duplicate_mixins — Flag a mixin applied twice in one with clause.
• avoid_generics_shadowing — Avoid generic type parameters that shadow top-level declarations.
• avoid_unnecessary_extends — Remove an explicit extends Object.