undefined_extension_method

The method '{0}' isn't defined for the extension '{1}'.

The analyzer produces this diagnostic when an extension override is used to invoke a method, but the method isn't defined by the specified extension. The analyzer also produces this diagnostic when a static method is referenced but isn't defined by the specified extension.

The following code produces this diagnostic because the extension E doesn't declare an instance method named b:

extension E on String {
  String a() => 'a';
}

extension F on String {
  String b() => 'b';
}

void f() {
  E('c').b();
}

The following code produces this diagnostic because the extension E doesn't declare a static method named a:

extension E on String {}

var x = E.a();

If the name of the method is incorrect, then change it to the name of an existing method:

extension E on String {
  String a() => 'a';
}

extension F on String {
  String b() => 'b';
}

void f() {
  E('c').a();
}

If the name of the method is correct, but the name of the extension is wrong, then change the name of the extension to the correct name:

extension E on String {
  String a() => 'a';
}

extension F on String {
  String b() => 'b';
}

void f() {
  F('c').b();
}

If the name of the method and extension are both correct, but the method isn't defined, then define the method:

extension E on String {
  String a() => 'a';
  String b() => 'z';
}

extension F on String {
  String b() => 'b';
}

void f() {
  E('c').b();
}

Unless stated otherwise, the documentation on this site reflects Dart 3.7.3. Page last updated on 2025-05-08. View source or report an issue.