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avoid-non-null-assertion

added in: 1.6.0
warning
⚙️
Free+

Warns when the non-null assertion operator (! or “bang” operator) is used on a property access or method invocation.

This operator check works at runtime and it may fail and throw a runtime exception. The rule ignores the index [] operator on the Map class because it's considered the idiomatic way to access a known-present element in a map with []! according to the docs.

note

Use this rule if you want to avoid possible unexpected runtime exceptions.

⚙️ Config​

Set skip-checked-fields (default is false) to allow non-null assertions on fields that are checked to be non-null in an if statement (example).

analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-non-null-assertion:
        skip-checked-fields: false

Example​

❌ Bad:

class Test {
  String? field;

  Test? object;

  void method() {
    field!.contains('other'); // LINT: Avoid non-null assertions. Try rewriting the code without it.

    object!.field!.contains('other'); // LINT: Avoid non-null assertions. Try rewriting the code without it.

    object!.method(); // LINT: Avoid non-null assertions. Try rewriting the code without it.

    if (object case int!) { // LINT: Avoid non-null assertions. Try rewriting the code without it.
      // ...
    }
  }
}

✅ Good:

class Test {
  String? field;

  Test? object;

  void method() {
    field?.contains('other');

    object?.field?.contains('other');

    final map = {'key': 'value'};
    map['key']!.contains('other');

    object?.method();
  }
}

Example with "skip-checked-fields"​

Config
analysis_options.yaml
dart_code_metrics:
  rules:
    - avoid-non-null-assertion:
        skip-checked-fields: true

✅ Good:

class Test {
  String? field;

  void method() {
    if (field != null) {
      field!.contains('other'); // Correct, checked for non-null before
    }
  }
}

Additional Resources​