undefined_extension_setter

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

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

extension E on String {
  set a(String v) {}
}

extension F on String {
  set b(String v) {}
}

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

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

extension E on String {}

void f() {
  E.a = 3;
}

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

extension E on String {
  set a(String v) {}
}

extension F on String {
  set b(String v) {}
}

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

If the name of the setter 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 {
  set a(String v) {}
}

extension F on String {
  set b(String v) {}
}

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

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

extension E on String {
  set a(String v) {}
  set b(String v) {}
}

extension F on String {
  set b(String v) {}
}

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

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.