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collection_methods_unrelated_type

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Lint rule

The argument type '{0}' isn't related to '{1}'.

Description

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The analyzer produces this diagnostic when any one of several methods in the core libraries are invoked with arguments of an inappropriate type. These methods are ones that don't provide a specific enough type for the parameter to allow the normal type checking to catch the error.

The arguments that are checked are:

  • an argument to Iterable<E>.contains should be related to E
  • an argument to List<E>.remove should be related to E
  • an argument to Map<K, V>.containsKey should be related to K
  • an argument to Map<K, V>.containsValue should be related to V
  • an argument to Map<K, V>.remove should be related to K
  • an argument to Map<K, V>.[] should be related to K
  • an argument to Queue<E>.remove should be related to E
  • an argument to Set<E>.lookup should be related to E
  • an argument to Set<E>.remove should be related to E

Example

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The following code produces this diagnostic because the argument to contains is a String, which isn't assignable to int, the element type of the list l:

dart
bool f(List<int> l)  => l.contains('1');

Common fixes

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If the element type is correct, then change the argument to have the same type:

dart
bool f(List<int> l)  => l.contains(1);

If the argument type is correct, then change the element type:

dart
bool f(List<String> l)  => l.contains('1');

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.