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LintCode:
always_declare_return_types:
documentation: |-
#### Description
The analyzer produces this diagnostic when a method or function doesn't
have an explicit return type.
#### Example
The following code produces this diagnostic because the function `f`
doesn't have a return type:
```dart
[!f!]() {}
```
#### Common fixes
Add an explicit return type:
```dart
void f() {}
```
always_put_control_body_on_new_line:
documentation: |-
#### Description
The analyzer produces this diagnostic when the code being controlled by a
control flow statement (`if`, `for`, `while`, or `do`) is on the same line
as the control flow statement.
#### Example
The following code produces this diagnostic because the `return` statement
is on the same line as the `if` that controls whether the `return` will be
executed:
```dart
void f(bool b) {
if (b) [!return!];
}
```
#### Common fixes
Put the controlled statement onto a separate, indented, line:
```dart
void f(bool b) {
if (b)
return;
}
```
always_put_required_named_parameters_first:
documentation: |-
#### Description
The analyzer produces this diagnostic when required named parameters occur
after optional named parameters.
#### Example
The following code produces this diagnostic because the required parameter
`x` is after the optional parameter `y`:
```dart
void f({int? y, required int [!x!]}) {}
```
#### Common fixes
Reorder the parameters so that all required named parameters are before
any optional named parameters:
```dart
void f({required int x, int? y}) {}
```
always_use_package_imports:
documentation: |-
#### Description
The analyzer produces this diagnostic when an `import` in a library inside
the `lib` directory uses a relative path to import another library inside
the `lib` directory of the same package.
#### Example
Given that a file named `a.dart` and the code below are both inside the
`lib` directory of the same package, the following code produces this
diagnostic because a relative URI is used to import `a.dart`:
```dart
import [!'a.dart'!];
```
#### Common fixes
Use a package import:
```dart
import 'package:p/a.dart';
```
annotate_overrides:
documentation: |-
#### Description
The analyzer produces this diagnostic when a member overrides an inherited
member, but isn't annotated with `@override`.
#### Example
The following code produces this diagnostic because the method `m` in the
class `B` overrides the method with the same name in class `A`, but isn't
marked as an intentional override:
```dart
class A {
void m() {}
}
class B extends A {
void [!m!]() {}
}
```
#### Common fixes
If the member in the subclass is intended to override the member in the
superclass, then add an `@override` annotation:
```dart
class A {
void m() {}
}
class B extends A {
@override
void m() {}
}
```
If the member in the subclass is not intended to override the member in
the superclass, then rename one of the members:
```dart
class A {
void m() {}
}
class B extends A {
void m2() {}
}
```
avoid_empty_else:
documentation: |-
#### Description
The analyzer produces this diagnostic when the statement after an `else`
is an empty statement (a semicolon).
For more information, see the documentation for
[`avoid_empty_else`](https://dart.dev/diagnostics/avoid_empty_else).
#### Example
The following code produces this diagnostic because the statement
following the `else` is an empty statement:
```dart
void f(int x, int y) {
if (x > y)
print("1");
else [!;!]
print("2");
}
```
#### Common fixes
If the statement after the empty statement is intended to be executed only
when the condition is `false`, then remove the empty statement:
```dart
void f(int x, int y) {
if (x > y)
print("1");
else
print("2");
}
```
If there is no code that is intended to be executed only when the
condition is `false`, then remove the whole `else` clause:
```dart
void f(int x, int y) {
if (x > y)
print("1");
print("2");
}
```
avoid_function_literals_in_foreach_calls:
documentation: |-
#### Description
The analyzer produces this diagnostic when the argument to
`Iterable.forEach` is a closure.
#### Example
The following code produces this diagnostic because the argument to the
invocation of `forEach` is a closure:
```dart
void f(Iterable<String> s) {
s.[!forEach!]((e) => print(e));
}
```
#### Common fixes
If the closure can be replaced by a tear-off, then replace the closure:
```dart
void f(Iterable<String> s) {
s.forEach(print);
}
```
If the closure can't be replaced by a tear-off, then use a `for` loop to
iterate over the elements:
```dart
void f(Iterable<String> s) {
for (var e in s) {
print(e);
}
}
```
avoid_init_to_null:
documentation: |-
#### Description
The analyzer produces this diagnostic when a nullable variable is
explicitly initialized to `null`. The variable can be a local variable,
field, or top-level variable.
A variable or field that isn't explicitly initialized automatically gets
initialized to `null`. There's no concept of "uninitialized memory" in
Dart.
#### Example
The following code produces this diagnostic because the variable `f` is
explicitly initialized to `null`:
```dart
class C {
int? [!f = null!];
void m() {
if (f != null) {
print(f);
}
}
}
```
#### Common fixes
Remove the unnecessary initialization:
```dart
class C {
int? f;
void m() {
if (f != null) {
print(f);
}
}
}
```
avoid_print:
documentation: |-
#### Description
The analyzer produces this diagnostic when the function `print` is invoked
in production code.
#### Example
The following code produces this diagnostic because the function `print`
can't be invoked in production:
```dart
void f(int x) {
[!print!]('x = $x');
}
```
#### Common fixes
If you're writing code that uses Flutter, then use the function
[`debugPrint`][debugPrint], guarded by a test
using [`kDebugMode`][kDebugMode]:
```dart
import 'package:flutter/foundation.dart';
void f(int x) {
if (kDebugMode) {
debugPrint('x = $x');
}
}
```
If you're writing code that doesn't use Flutter, then use a logging
service, such as [`package:logging`][package-logging], to write the
information.
avoid_relative_lib_imports:
documentation: |-
#### Description
The analyzer produces this diagnostic when the URI in an `import`
directive has `lib` in the path.
#### Example
Assuming that there is a file named `a.dart` in the `lib` directory:
```dart
%uri="lib/a.dart"
class A {}
```
The following code produces this diagnostic because the import contains a
path that includes `lib`:
```dart
import [!'../lib/a.dart'!];
```
#### Common fixes
Rewrite the import to not include `lib` in the URI:
```dart
import 'a.dart';
```
avoid_renaming_method_parameters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a method that overrides a
method from a superclass changes the names of the parameters.
#### Example
The following code produces this diagnostic because the parameter of the
method `m` in `B` is named `b`, which is different from the name of the
overridden method's parameter in `A`:
```dart
class A {
void m(int a) {}
}
class B extends A {
@override
void m(int [!b!]) {}
}
```
#### Common fixes
Rename one of the parameters so that they are the same:
```dart
class A {
void m(int a) {}
}
class B extends A {
@override
void m(int a) {}
}
```
avoid_return_types_on_setters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a setter has an explicit return
type.
Setters never return a value, so declaring the return type of one is
redundant.
#### Example
The following code produces this diagnostic because the setter `s` has an
explicit return type (`void`):
```dart
[!void!] set s(int p) {}
```
#### Common fixes
Remove the return type:
```dart
set s(int p) {}
```
avoid_returning_null_for_void:
documentation: |-
#### Description
The analyzer produces this diagnostic when a function that has a return
type of `void` explicitly returns `null`.
#### Example
The following code produces this diagnostic because there is an explicit
return of `null` in a `void` function:
```dart
void f() {
[!return null;!]
}
```
#### Common fixes
Remove the unnecessary explicit `null`:
```dart
void f() {
return;
}
```
avoid_shadowing_type_parameters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a type parameter shadows a type
parameter from an enclosing declaration.
Shadowing a type parameter with a different type parameter can lead to
subtle bugs that are difficult to debug.
#### Example
The following code produces this diagnostic because the type parameter `T`
defined by the method `m` shadows the type parameter `T` defined by the
class `C`:
```dart
class C<T> {
void m<[!T!]>() {}
}
```
#### Common fixes
Rename one of the type parameters:
```dart
class C<T> {
void m<S>() {}
}
```
avoid_single_cascade_in_expression_statements:
documentation: |-
#### Description
The analyzer produces this diagnostic when a single cascade operator is
used and the value of the expression isn't being used for anything (such
as being assigned to a variable or being passed as an argument).
#### Example
The following code produces this diagnostic because the value of the
cascade expression `s..length` isn't being used:
```dart
void f(String s) {
[!s..length!];
}
```
#### Common fixes
Replace the cascade operator with a simple access operator:
```dart
void f(String s) {
s.length;
}
```
avoid_slow_async_io:
documentation: |-
#### Description
The analyzer produces this diagnostic when an asynchronous file I/O method
with a synchronous equivalent is used.
The following are the specific flagged asynchronous methods:
- `Directory.exists`
- `Directory.stat`
- `File.lastModified`
- `File.exists`
- `File.stat`
- `FileSystemEntity.isDirectory`
- `FileSystemEntity.isFile`
- `FileSystemEntity.isLink`
- `FileSystemEntity.type`
#### Example
The following code produces this diagnostic because the async method
`exists` is invoked:
```dart
import 'dart:io';
Future<void> g(File f) async {
await [!f.exists()!];
}
```
#### Common fixes
Use the synchronous version of the method:
```dart
import 'dart:io';
void g(File f) {
f.existsSync();
}
```
avoid_type_to_string:
documentation: |-
#### Description
The analyzer produces this diagnostic when the method `toString` is
invoked on a value whose static type is `Type`.
#### Example
The following code produces this diagnostic because the method `toString`
is invoked on the `Type` returned by `runtimeType`:
```dart
bool isC(Object o) => o.runtimeType.[!toString!]() == 'C';
class C {}
```
#### Common fixes
If it's essential that the type is exactly the same, then use an explicit
comparison:
```dart
bool isC(Object o) => o.runtimeType == C;
class C {}
```
If it's alright for instances of subtypes of the type to return `true`,
then use a type check:
```dart
bool isC(Object o) => o is C;
class C {}
```
avoid_types_as_parameter_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a parameter in a
parameter list is the same as a visible type (a type whose name is in
scope).
This often indicates that the intended name of the parameter is missing,
causing the name of the type to be used as the name of the parameter
rather than the type of the parameter. Even when that's not the case (the
name of the parameter is intentional), the name of the parameter will
shadow the existing type, which can lead to bugs that are difficult to
diagnose.
#### Example
The following code produces this diagnostic because the function `f` has a
parameter named `int`, which shadows the type `int` from `dart:core`:
```dart
void f([!int!]) {}
```
#### Common fixes
If the parameter name is missing, then add a name for the parameter:
```dart
void f(int x) {}
```
If the parameter is intended to have an implicit type of `dynamic`, then
rename the parameter so that it doesn't shadow the name of any visible type:
```dart
void f(int_) {}
```
avoid_unnecessary_containers:
documentation: |-
#### Description
The analyzer produces this diagnostic when a widget tree contains an
instance of `Container` and the only argument to the constructor is
`child:`.
#### Example
The following code produces this diagnostic because the invocation of the
`Container` constructor only has a `child:` argument:
```dart
import 'package:flutter/material.dart';
Widget buildRow() {
return [!Container!](
child: Row(
children: [
Text('a'),
Text('b'),
],
)
);
}
```
#### Common fixes
If you intended to provide other arguments to the constructor, then add
them:
```dart
import 'package:flutter/material.dart';
Widget buildRow() {
return Container(
color: Colors.red.shade100,
child: Row(
children: [
Text('a'),
Text('b'),
],
)
);
}
```
If no other arguments are needed, then unwrap the child widget:
```dart
import 'package:flutter/material.dart';
Widget buildRow() {
return Row(
children: [
Text('a'),
Text('b'),
],
);
}
```
avoid_web_libraries_in_flutter:
documentation: |-
#### Description
The analyzer produces this diagnostic when a library in a package that
isn't a web plugin contains an import of a web-only library:
- `dart:html`
- `dart:js`
- `dart:js_util`
- `dart:js_interop`
- `dart:js_interop_unsafe`
- `package:js`
- `package:web`
#### Example
When found in a package that isn't a web plugin, the following code
produces this diagnostic because it imports `dart:html`:
```dart
import [!'dart:html'!];
import 'package:flutter/material.dart';
class C {}
```
#### Common fixes
If the package isn't intended to be a web plugin, then remove the import:
```dart
import 'package:flutter/material.dart';
class C {}
```
If the package is intended to be a web plugin, then add the following
lines to the `pubspec.yaml` file of the package:
```yaml
flutter:
plugin:
platforms:
web:
pluginClass: HelloPlugin
fileName: hello_web.dart
```
See [Developing packages & plugins](https://flutter.dev/to/develop-packages)
for more information.
await_only_futures:
documentation: |-
#### Description
The analyzer produces this diagnostic when the expression after `await`
has any type other than `Future<T>`, `FutureOr<T>`, `Future<T>?`,
`FutureOr<T>?` or `dynamic`.
An exception is made for the expression `await null` because it is a
common way to introduce a microtask delay.
Unless the expression can produce a `Future`, the `await` is unnecessary
and can cause a reader to assume a level of asynchrony that doesn't exist.
#### Example
The following code produces this diagnostic because the expression after
`await` has the type `int`:
```dart
void f() async {
[!await!] 23;
}
```
#### Common fixes
Remove the `await`:
```dart
void f() async {
23;
}
```
camel_case_extensions:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of an extension
doesn't use the 'UpperCamelCase' naming convention.
#### Example
The following code produces this diagnostic because the name of the
extension doesn't start with an uppercase letter:
```dart
extension [!stringExtension!] on String {}
```
#### Common fixes
If the extension needs to have a name (needs to be visible outside this
library), then rename the extension so that it has a valid name:
```dart
extension StringExtension on String {}
```
If the extension doesn't need to have a name, then remove the name of the
extension:
```dart
extension on String {}
```
camel_case_types:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a type (a class,
mixin, enum, or typedef) doesn't use the 'UpperCamelCase' naming
convention.
#### Example
The following code produces this diagnostic because the name of the class
doesn't start with an uppercase letter:
```dart
class [!c!] {}
```
#### Common fixes
Rename the type so that it has a valid name:
```dart
class C {}
```
cancel_subscriptions:
documentation: |-
#### Description
The analyzer produces this diagnostic when an instance of
`StreamSubscription` is created but the method `cancel` isn't invoked.
#### Example
The following code produces this diagnostic because the `subscription`
isn't canceled:
```dart
import 'dart:async';
void f(Stream stream) {
// ignore: unused_local_variable
var [!subscription = stream.listen((_) {})!];
}
```
#### Common fixes
Cancel the subscription:
```dart
import 'dart:async';
void f(Stream stream) {
var subscription = stream.listen((_) {});
subscription.cancel();
}
```
close_sinks:
documentation: |-
#### Description
The analyzer produces this diagnostic when an instance of `Sink` is
created but the method `close` isn't invoked.
#### Example
The following code produces this diagnostic because the `sink` isn't
closed:
```dart
import 'dart:io';
void g(File f) {
var [!sink = f.openWrite()!];
sink.write('x');
}
```
#### Common fixes
Close the sink:
```dart
import 'dart:io';
void g(File f) {
var sink = f.openWrite();
sink.write('x');
sink.close();
}
```
collection_methods_unrelated_type:
documentation: |-
#### Description
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
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
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');
```
constant_identifier_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a constant doesn't
follow the lowerCamelCase naming convention.
#### Example
The following code produces this diagnostic because the name of the
top-level variable isn't a lowerCamelCase identifier:
```dart
const [!EMPTY_STRING!] = '';
```
#### Common fixes
Rewrite the name to follow the lowerCamelCase naming convention:
```dart
const emptyString = '';
```
control_flow_in_finally:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `finally` clause contains a
`return`, `break`, or `continue` statement.
#### Example
The following code produces this diagnostic because there is a `return`
statement inside a `finally` block:
```dart
int f() {
try {
return 1;
} catch (e) {
print(e);
} finally {
[!return 0;!]
}
}
```
#### Common fixes
If the statement isn't needed, then remove the statement, and remove the
`finally` clause if the block is empty:
```dart
int f() {
try {
return 1;
} catch (e) {
print(e);
}
}
```
If the statement is needed, then move the statement outside the `finally`
block:
```dart
int f() {
try {
return 1;
} catch (e) {
print(e);
}
return 0;
}
```
curly_braces_in_flow_control_structures:
documentation: |-
#### Description
The analyzer produces this diagnostic when a control structure (`if`,
`for`, `while`, or `do` statement) has a statement other than a block.
#### Example
The following code produces this diagnostic because the `then` statement
is not enclosed in a block:
```dart
int f(bool b) {
if (b)
[!return 1;!]
return 0;
}
```
#### Common fixes
Add braces around the statement that should be a block:
```dart
int f(bool b) {
if (b) {
return 1;
}
return 0;
}
```
dangling_library_doc_comments:
documentation: |-
#### Description
The analyzer produces this diagnostic when a documentation comment that
appears to be library documentation isn't followed by a `library`
directive. More specifically, it is produced when a documentation comment
appears before the first directive in the library, assuming that it isn't
a `library` directive, or before the first top-level declaration and is
separated from the declaration by one or more blank lines.
#### Example
The following code produces this diagnostic because there's a
documentation comment before the first `import` directive:
```dart
[!/// This is a great library.!]
import 'dart:core';
```
The following code produces this diagnostic because there's a
documentation comment before the first class declaration, but there's a
blank line between the comment and the declaration.
```dart
[!/// This is a great library.!]
class C {}
```
#### Common fixes
If the comment is library documentation, then add a `library` directive
without a name:
```dart
/// This is a great library.
library;
import 'dart:core';
```
If the comment is documentation for the following declaration, then remove
the blank line:
```dart
/// This is a great library.
class C {}
```
depend_on_referenced_packages:
documentation: |-
#### Description
The analyzer produces this diagnostic when a package import refers to a
package that is not specified in the `pubspec.yaml` file.
Depending explicitly on packages that you reference ensures they will
always exist and allows you to put a dependency constraint on them to
guard against breaking changes.
#### Example
Given a `pubspec.yaml` file containing the following:
```yaml
dependencies:
meta: ^3.0.0
```
The following code produces this diagnostic because there is no dependency
on the package `a`:
```dart
import 'package:a/a.dart';
```
#### Common fixes
Whether the dependency should be a regular dependency or dev dependency
depends on whether the package is referenced from a public library (one
under either `lib` or `bin`), or only private libraries, (such as one
under `test`).
If the package is referenced from at least one public library, then add a
regular dependency on the package to the `pubspec.yaml` file under the
`dependencies` field:
```yaml
dependencies:
a: ^1.0.0
meta: ^3.0.0
```
If the package is referenced only from private libraries, then add a
dev dependency on the package to the `pubspec.yaml` file under the
`dev_dependencies` field:
```yaml
dependencies:
meta: ^3.0.0
dev_dependencies:
a: ^1.0.0
```
empty_catches:
documentation: |-
#### Description
The analyzer produces this diagnostic when the block in a `catch` clause
is empty.
#### Example
The following code produces this diagnostic because the catch block is
empty:
```dart
void f() {
try {
print('Hello');
} catch (exception) [!{}!]
}
```
#### Common fixes
If the exception shouldn't be ignored, then add code to handle the
exception:
```dart
void f() {
try {
print('We can print.');
} catch (exception) {
print("We can't print.");
}
}
```
If the exception is intended to be ignored, then add a comment explaining
why:
```dart
void f() {
try {
print('We can print.');
} catch (exception) {
// Nothing to do.
}
}
```
If the exception is intended to be ignored and there isn't any good
explanation for why, then rename the exception parameter:
```dart
void f() {
try {
print('We can print.');
} catch (_) {}
}
```
empty_constructor_bodies:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constructor has an empty
block body.
#### Example
The following code produces this diagnostic because the constructor for
`C` has a block body that is empty:
```dart
class C {
C() [!{}!]
}
```
#### Common fixes
Replace the block with a semicolon:
```dart
class C {
C();
}
```
empty_statements:
documentation: |-
#### Description
The analyzer produces this diagnostic when an empty statement is found.
#### Example
The following code produces this diagnostic because the statement
controlled by the `while` loop is an empty statement:
```dart
void f(bool condition) {
while (condition)[!;!]
g();
}
void g() {}
```
#### Common fixes
If there are no statements that need to be controlled, then remove both
the empty statement and the control structure it's part of (being careful
that any other code being removed doesn't have a side-effect that needs to
be preserved):
```dart
void f(bool condition) {
g();
}
void g() {}
```
If there are no statements that need to be controlled but the control
structure is still required for other reasons, then replace the empty
statement with a block to make the structure of the code more obvious:
```dart
void f(bool condition) {
while (condition) {}
g();
}
void g() {}
```
If there are statements that need to be controlled, remove the empty
statement and adjust the code so that the appropriate statements are being
controlled, possibly adding a block:
```dart
void f(bool condition) {
while (condition) {
g();
}
}
void g() {}
```
file_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a `.dart` file
doesn't use lower_case_with_underscores.
#### Example
A file named `SliderMenu.dart` produces this diagnostic because the file
name uses the UpperCamelCase convention.
#### Common fixes
Rename the file to use the lower_case_with_underscores convention, such as
`slider_menu.dart`.
hash_and_equals:
documentation: |-
#### Description
The analyzer produces this diagnostic when a class or mixin either
overrides the definition of `==` but doesn't override the definition of
`hashCode`, or conversely overrides the definition of `hashCode` but
doesn't override the definition of `==`.
Both the `==` operator and the `hashCode` property of objects must be
consistent for a common hash map implementation to function properly. As a
result, when overriding either method, both should be overridden.
#### Example
The following code produces this diagnostic because the class `C`
overrides the `==` operator but doesn't override the getter `hashCode`:
```dart
class C {
final int value;
C(this.value);
@override
bool operator [!==!](Object other) =>
other is C &&
other.runtimeType == runtimeType &&
other.value == value;
}
```
#### Common fixes
If you need to override one of the members, then add an override of the
other:
```dart
class C {
final int value;
C(this.value);
@override
bool operator ==(Object other) =>
other is C &&
other.runtimeType == runtimeType &&
other.value == value;
@override
int get hashCode => value.hashCode;
}
```
If you don't need to override either of the members, then remove the
unnecessary override:
```dart
class C {
final int value;
C(this.value);
}
```
implementation_imports:
documentation: |-
#### Description
The analyzer produces this diagnostic when an import references a library
that's inside the `lib/src` directory of a different package, which
violates [the convention for pub
packages](https://dart.dev/tools/pub/package-layout#implementation-files).
#### Example
The following code, assuming that it isn't part of the `ffi` package,
produces this diagnostic because the library being imported is inside the
top-level `src` directory:
```dart
import [!'package:ffi/src/allocation.dart'!];
```
#### Common fixes
If the library being imported contains code that's part of the public API,
then import the public library that exports the public API:
```dart
import 'package:ffi/ffi.dart';
```
If the library being imported isn't part of the public API of the package,
then either find a different way to accomplish your goal, assuming that
it's possible, or open an issue asking the package authors to make it part
of the public API.
implicit_call_tearoffs:
documentation: |-
#### Description
The analyzer produces this diagnostic when an object with a `call` method
is assigned to a function-typed variable, implicitly tearing off the
`call` method.
#### Example
The following code produces this diagnostic because an instance of
`Callable` is passed to a function expecting a `Function`:
```dart
class Callable {
void call() {}
}
void callIt(void Function() f) {
f();
}
void f() {
callIt([!Callable()!]);
}
```
#### Common fixes
Explicitly tear off the `call` method:
```dart
class Callable {
void call() {}
}
void callIt(void Function() f) {
f();
}
void f() {
callIt(Callable().call);
}
```
library_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a library doesn't
use the lower_case_with_underscores naming convention.
#### Example
The following code produces this diagnostic because the library name
`libraryName` isn't a lower_case_with_underscores identifier:
```dart
library [!libraryName!];
```
#### Common fixes
If the library name is not required, then remove the library name:
```dart
library;
```
If the library name is required, then convert it to use the
lower_case_with_underscores naming convention:
```dart
library library_name;
```
library_prefixes:
documentation: |-
#### Description
The analyzer produces this diagnostic when an import prefix doesn't use
the lower_case_with_underscores naming convention.
#### Example
The following code produces this diagnostic because the prefix
`ffiSupport` isn't a lower_case_with_underscores identifier:
```dart
import 'package:ffi/ffi.dart' as [!ffiSupport!];
```
#### Common fixes
Convert the prefix to use the lower_case_with_underscores naming
convention:
```dart
import 'package:ffi/ffi.dart' as ffi_support;
```
library_private_types_in_public_api:
documentation: |-
#### Description
The analyzer produces this diagnostic when a type that is not part of the
public API of a library is referenced in the public API of that library.
Using a private type in a public API can make the API unusable outside the
defining library.
#### Example
The following code produces this diagnostic because the parameter `c` of
the public function `f` has a type that is library private (`_C`):
```dart
void f([!_C!] c) {}
class _C {}
```
#### Common fixes
If the API doesn't need to be used outside the defining library, then make
it private:
```dart
void _f(_C c) {}
class _C {}
```
If the API needs to be part of the public API of the library, then either
use a different type that's public, or make the referenced type public:
```dart
void f(C c) {}
class C {}
```
literal_only_boolean_expressions:
documentation: |-
#### Description
The analyzer produces this diagnostic when the value of the condition in
an `if` or loop statement is known to be either always `true` or always
`false`. An exception is made for a `while` loop whose condition is the
Boolean literal `true`.
#### Examples
The following code produces this diagnostic because the condition will
always evaluate to `true`:
```dart
void f() {
[!if (true) {
print('true');
}!]
}
```
The lint will evaluate a subset of expressions that are composed of
constants, so the following code will also produce this diagnostic because
the condition will always evaluate to `false`:
```dart
void g(int i) {
[!if (1 == 0 || 3 > 4) {
print('false');
}!]
}
```
#### Common fixes
If the condition is wrong, then correct the condition so that it's value
can't be known at compile time:
```dart
void g(int i) {
if (i == 0 || i > 4) {
print('false');
}
}
```
If the condition is correct, then simplify the code to not evaluate the
condition:
```dart
void f() {
print('true');
}
```
no_adjacent_strings_in_list:
documentation: |-
#### Description
The analyzer produces this diagnostic when two string literals are
adjacent in a list literal. Adjacent strings in Dart are concatenated
together to form a single string, but the intent might be for each string
to be a separate element in the list.
#### Example
The following code produces this diagnostic because the strings `'a'` and
`'b'` are adjacent:
```dart
List<String> list = [[!'a' 'b'!], 'c'];
```
#### Common fixes
If the two strings are intended to be separate elements of the list, then
add a comma between them:
```dart
List<String> list = ['a', 'b', 'c'];
```
If the two strings are intended to be a single concatenated string, then
either manually merge the strings:
```dart
List<String> list = ['ab', 'c'];
```
Or use the `+` operator to concatenate the strings:
```dart
List<String> list = ['a' + 'b', 'c'];
```
no_duplicate_case_values:
documentation: |-
#### Description
The analyzer produces this diagnostic when two or more `case` clauses in
the same `switch` statement have the same value.
Any `case` clauses after the first can't be executed, so having duplicate
`case` clauses is misleading.
This diagnostic is often the result of either a typo or a change to the
value of a constant.
#### Example
The following code produces this diagnostic because two case clauses have
the same value (1):
```dart
// @dart = 2.14
void f(int v) {
switch (v) {
case 1:
break;
case [!1!]:
break;
}
}
```
#### Common fixes
If one of the clauses should have a different value, then change the value
of the clause:
```dart
void f(int v) {
switch (v) {
case 1:
break;
case 2:
break;
}
}
```
If the value is correct, then merge the statements into a single clause:
```dart
void f(int v) {
switch (v) {
case 1:
break;
}
}
```
no_leading_underscores_for_library_prefixes:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a prefix declared
on an import starts with an underscore.
Library prefixes are inherently not visible outside the declaring library,
so a leading underscore indicating private adds no value.
#### Example
The following code produces this diagnostic because the prefix `_core`
starts with an underscore:
```dart
import 'dart:core' as [!_core!];
```
#### Common fixes
Remove the underscore:
```dart
import 'dart:core' as core;
```
no_leading_underscores_for_local_identifiers:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a local variable
starts with an underscore.
Local variables are inherently not visible outside the declaring library,
so a leading underscore indicating private adds no value.
#### Example
The following code produces this diagnostic because the parameter `_s`
starts with an underscore:
```dart
int f(String [!_s!]) => _s.length;
```
#### Common fixes
Remove the underscore:
```dart
int f(String s) => s.length;
```
no_logic_in_create_state:
documentation: |-
#### Description
The analyzer produces this diagnostic when an implementation of
`createState` in a subclass of `StatefulWidget` contains any logic other
than the return of the result of invoking a zero argument constructor.
#### Examples
The following code produces this diagnostic because the constructor
invocation has arguments:
```dart
import 'package:flutter/material.dart';
class MyWidget extends StatefulWidget {
@override
MyState createState() => [!MyState(0)!];
}
class MyState extends State {
int x;
MyState(this.x);
}
```
#### Common fixes
Rewrite the code so that `createState` doesn't contain any logic:
```dart
import 'package:flutter/material.dart';
class MyWidget extends StatefulWidget {
@override
MyState createState() => MyState();
}
class MyState extends State {
int x = 0;
MyState();
}
```
no_wildcard_variable_uses:
documentation: |-
#### Description
The analyzer produces this diagnostic when either a parameter or local
variable whose name consists of only underscores is referenced. Such
names will become non-binding in a future version of the Dart language,
making the reference illegal.
#### Example
The following code produces this diagnostic because the name of the
parameter consists of two underscores:
```dart
void f(int __) {
print([!__!]);
}
```
The following code produces this diagnostic because the name of the
local variable consists of a single underscore:
```dart
void f() {
int _ = 0;
print([!_!]);
}
```
#### Common fixes
If the variable or parameter is intended to be referenced, then give it a
name that has at least one non-underscore character:
```dart
void f(int p) {
print(p);
}
```
If the variable or parameter is not intended to be referenced, then
replace the reference with a different expression:
```dart
void f() {
print(0);
}
```
non_constant_identifier_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a class member,
top-level declaration, variable, parameter, named parameter, or named
constructor that isn't declared to be `const`, doesn't use the
lowerCamelCase convention.
#### Example
The following code produces this diagnostic because the top-level variable
`Count` doesn't start with a lowercase letter:
```dart
var [!Count!] = 0;
```
#### Common fixes
Change the name in the declaration to follow the lowerCamelCase
convention:
```dart
var count = 0;
```
null_check_on_nullable_type_parameter:
documentation: |-
#### Description
The analyzer produces this diagnostic when a null check operator is used
on a variable whose type is `T?`, where `T` is a type parameter that
allows the type argument to be nullable (either has no bound or has a
bound that is nullable).
Given a generic type parameter `T` which has a nullable bound, it is very
easy to introduce erroneous null checks when working with a variable of
type `T?`. Specifically, it is not uncommon to have `T? x;` and want to
assert that `x` has been set to a valid value of type `T`. A common
mistake is to do so using `x!`. This is almost always incorrect, because
if `T` is a nullable type, `x` may validly hold `null` as a value of type
`T`.
#### Example
The following code produces this diagnostic because `t` has the type `T?`
and `T` allows the type argument to be nullable (because it has no
`extends` clause):
```dart
T f<T>(T? t) => t[!!!];
```
#### Common fixes
Use the type parameter to cast the variable:
```dart
T f<T>(T? t) => t as T;
```
overridden_fields:
documentation: |-
#### Description
The analyzer produces this diagnostic when a class defines a field that
overrides a field from a superclass.
Overriding a field with another field causes the object to have two
distinct fields, but because the fields have the same name only one of the
fields can be referenced in a given scope. That can lead to confusion
where a reference to one of the fields can be mistaken for a reference to
the other.
#### Example
The following code produces this diagnostic because the field `f` in `B`
shadows the field `f` in `A`:
```dart
class A {
int f = 1;
}
class B extends A {
@override
int [!f!] = 2;
}
```
#### Common fixes
If the two fields are representing the same property, then remove the
field from the subclass:
```dart
class A {
int f = 1;
}
class B extends A {}
```
If the two fields should be distinct, then rename one of the fields:
```dart
class A {
int f = 1;
}
class B extends A {
int g = 2;
}
```
If the two fields are related in some way, but can't be the same, then
find a different way to implement the semantics you need.
package_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when the name of a package doesn't
use the lower_case_with_underscores naming convention.
#### Example
The following code produces this diagnostic because the name of the
package uses the lowerCamelCase naming convention:
```yaml
name: [!somePackage!]
```
#### Common fixes
Rewrite the name of the package using the lower_case_with_underscores
naming convention:
```yaml
name: some_package
```
package_prefixed_library_names:
documentation: |-
#### Description
The analyzer produces this diagnostic when a library has a name that
doesn't follow these guidelines:
- Prefix all library names with the package name.
- Make the entry library have the same name as the package.
- For all other libraries in a package, after the package name add the
dot-separated path to the library's Dart file.
- For libraries under `lib`, omit the top directory name.
For example, given a package named `my_package`, here are the library
names for various files in the package:
```dart
// In lib/my_package.dart
library my_package;
// In lib/other.dart
library my_package.other;
// In lib/foo/bar.dart
library my_package.foo.bar;
// In example/foo/bar.dart
library my_package.example.foo.bar;
// In lib/src/private.dart
library my_package.src.private;
```
#### Example
Assuming that the file containing the following code is not in a file
named `special.dart` in the `lib` directory of a package named `something`
(which would be an exception to the rule), the analyzer produces this
diagnostic because the name of the library doesn't conform to the
guidelines above:
```dart
library [!something.special!];
```
#### Common fixes
Change the name of the library to conform to the guidelines.
prefer_adjacent_string_concatenation:
documentation: |-
#### Description
The analyzer produces this diagnostic when the `+` operator is used to
concatenate two string literals.
#### Example
The following code produces this diagnostic because two string literals
are being concatenated by using the `+` operator:
```dart
var s = 'a' [!+!] 'b';
```
#### Common fixes
Remove the operator:
```dart
var s = 'a' 'b';
```
prefer_collection_literals:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constructor is used to create
a list, map, or set, but a literal would produce the same result.
#### Example
The following code produces this diagnostic because the constructor for
`Map` is being used to create a map that could also be created using a
literal:
```dart
var m = [!Map<String, String>()!];
```
#### Common fixes
Use the literal representation:
```dart
var m = <String, String>{};
```
prefer_conditional_assignment:
documentation: |-
#### Description
The analyzer produces this diagnostic when an assignment to a variable is
conditional based on whether the variable has the value `null` and the
`??=` operator could be used instead.
#### Example
The following code produces this diagnostic because the parameter `s` is
being compared to `null` in order to determine whether to assign a
different value:
```dart
int f(String? s) {
[!if (s == null) {
s = '';
}!]
return s.length;
}
```
#### Common fixes
Use the `??=` operator instead of an explicit `if` statement:
```dart
int f(String? s) {
s ??= '';
return s.length;
}
```
prefer_const_constructors:
documentation: |-
#### Description
The analyzer produces this diagnostic when an invocation of a const
constructor isn't either preceded by `const` or in a [constant context][].
#### Example
The following code produces this diagnostic because the invocation of the
`const` constructor is neither prefixed by `const` nor in a
[constant context][]:
```dart
class C {
const C();
}
C c = [!C()!];
```
#### Common fixes
If the context can be made a [constant context][], then do so:
```dart
class C {
const C();
}
const C c = C();
```
If the context can't be made a [constant context][], then add `const`
before the constructor invocation:
```dart
class C {
const C();
}
C c = const C();
```
prefer_const_constructors_in_immutables:
documentation: |-
#### Description
The analyzer produces this diagnostic when a non-`const` constructor is
found in a class that has the `@immutable` annotation.
#### Example
The following code produces this diagnostic because the constructor in `C`
isn't declared as `const` even though `C` has the `@immutable` annotation:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
[!C!](this.f);
}
```
#### Common fixes
If the class really is intended to be immutable, then add the `const`
modifier to the constructor:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
const C(this.f);
}
```
If the class is mutable, then remove the `@immutable` annotation:
```dart
class C {
final f;
C(this.f);
}
```
prefer_const_declarations:
documentation: |-
#### Description
The analyzer produces this diagnostic when a top-level variable, static
field, or local variable is marked as `final` and is initialized to a
constant value.
#### Examples
The following code produces this diagnostic because the top-level variable
`v` is both `final` and initialized to a constant value:
```dart
[!final v = const <int>[]!];
```
The following code produces this diagnostic because the static field `f`
is both `final` and initialized to a constant value:
```dart
class C {
static [!final f = const <int>[]!];
}
```
The following code produces this diagnostic because the local variable `v`
is both `final` and initialized to a constant value:
```dart
void f() {
[!final v = const <int>[]!];
print(v);
}
```
#### Common fixes
Replace the keyword `final` with `const` and remove `const` from the
initializer:
```dart
class C {
static const f = <int>[];
}
```
prefer_const_literals_to_create_immutables:
documentation: |-
#### Description
The analyzer produces this diagnostic when a non-const list, map, or set
literal is passed as an argument to a constructor declared in a class
annotated with `@immutable`.
#### Example
The following code produces this diagnostic because the list literal
(`[1]`) is being passed to a constructor in an immutable class but isn't
a constant list:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
const C(this.f);
}
C c = C([![1]!]);
```
#### Common fixes
If the context can be made a [constant context][], then do so:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
const C(this.f);
}
const C c = C([1]);
```
If the context can't be made a [constant context][] but the constructor
can be invoked using `const`, then add `const` before the constructor
invocation:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
const C(this.f);
}
C c = const C([1]);
```
If the context can't be made a [constant context][] and the constructor
can't be invoked using `const`, then add the keyword `const` before the
collection literal:
```dart
import 'package:meta/meta.dart';
@immutable
class C {
final f;
const C(this.f);
}
C c = C(const [1]);
```
prefer_contains:
documentation: |-
#### Description
The analyzer produces this diagnostic when the method `indexOf` is used and
the result is only compared with `-1` or `0` in a way where the semantics
are equivalent to using `contains`.
#### Example
The following code produces this diagnostic because the condition in the
`if` statement is checking to see whether the list contains the string:
```dart
void f(List<String> l, String s) {
if ([!l.indexOf(s) < 0!]) {
// ...
}
}
```
#### Common fixes
Use `contains` instead, negating the condition when necessary:
```dart
void f(List<String> l, String s) {
if (l.contains(s)) {
// ...
}
}
```
prefer_double_quotes:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string literal uses single
quotes (`'`) when it could use double quotes (`"`) without needing extra
escapes and without hurting readability.
#### Example
The following code produces this diagnostic because the string literal
uses single quotes but doesn't need to:
```dart
void f(String name) {
print([!'Hello $name'!]);
}
```
#### Common fixes
Use double quotes in place of single quotes:
```dart
void f(String name) {
print("Hello $name");
}
```
prefer_final_fields:
documentation: |-
#### Description
The analyzer produces this diagnostic when a private field is only
assigned one time. The field can be initialized in multiple constructors
and still be flagged because only one of those constructors can ever run.
#### Example
The following code produces this diagnostic because the field `_f` is only
assigned one time, in the field's initializer:
```dart
class C {
int [!_f = 1!];
int get f => _f;
}
```
#### Common fixes
Mark the field `final`:
```dart
class C {
final int _f = 1;
int get f => _f;
}
```
prefer_for_elements_to_map_fromIterable:
documentation: |-
#### Description
The analyzer produces this diagnostic when `Map.fromIterable` is used to
build a map that could be built using the `for` element.
#### Example
The following code produces this diagnostic because `fromIterable` is
being used to build a map that could be built using a `for` element:
```dart
void f(Iterable<String> data) {
[!Map<String, int>.fromIterable(
data,
key: (element) => element,
value: (element) => element.length,
)!];
}
```
#### Common fixes
Use a `for` element to build the map:
```dart
void f(Iterable<String> data) {
<String, int>{
for (var element in data)
element: element.length
};
}
```
prefer_function_declarations_over_variables:
documentation: |-
#### Description
The analyzer produces this diagnostic when a closure is assigned to a
local variable and the local variable is not re-assigned anywhere.
#### Example
The following code produces this diagnostic because the local variable `f`
is initialized to be a closure and isn't assigned any other value:
```dart
void g() {
var [!f = (int i) => i * 2!];
f(1);
}
```
#### Common fixes
Replace the local variable with a local function:
```dart
void g() {
int f(int i) => i * 2;
f(1);
}
```
prefer_generic_function_type_aliases:
documentation: |-
#### Description
The analyzer produces this diagnostic when a typedef is written using the
older syntax for function type aliases in which the name being declared is
embedded in the function type.
#### Example
The following code produces this diagnostic because it uses the older
syntax:
```dart
typedef void [!F!]<T>();
```
#### Common fixes
Rewrite the typedef to use the newer syntax:
```dart
typedef F<T> = void Function();
```
prefer_if_null_operators:
documentation: |-
#### Description
The analyzer produces this diagnostic when a conditional expression (using
the `?:` operator) is used to select a different value when a local
variable is `null`.
#### Example
The following code produces this diagnostic because the variable `s` is
being compared to `null` so that a different value can be returned when
`s` is `null`:
```dart
String f(String? s) => [!s == null ? '' : s!];
```
#### Common fixes
Use the if-null operator instead:
```dart
String f(String? s) => s ?? '';
```
prefer_initializing_formals:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constructor parameter is used
to initialize a field without modification.
#### Example
The following code produces this diagnostic because the parameter `c` is
only used to set the field `c`:
```dart
class C {
int c;
C(int c) : [!this.c = c!];
}
```
#### Common fixes
Use an initializing formal parameter to initialize the field:
```dart
class C {
int c;
C(this.c);
}
```
prefer_inlined_adds:
documentation: |-
#### Description
The analyzer produces this diagnostic when the methods `add` and `addAll`
are invoked on a list literal where the elements being added could be
included in the list literal.
#### Example
The following code produces this diagnostic because the `add` method is
being used to add `b`, when it could have been included directly in the
list literal:
```dart
List<String> f(String a, String b) {
return [a]..[!add!](b);
}
```
The following code produces this diagnostic because the `addAll` method is
being used to add the elements of `b`, when it could have been included
directly in the list literal:
```dart
List<String> f(String a, List<String> b) {
return [a]..[!addAll!](b);
}
```
#### Common fixes
If the `add` method is being used, then make the argument an element of
the list and remove the invocation:
```dart
List<String> f(String a, String b) {
return [a, b];
}
```
If the `addAll` method is being used, then use the spread operator on the
argument to add its elements to the list and remove the invocation:
```dart
List<String> f(String a, List<String> b) {
return [a, ...b];
}
```
prefer_interpolation_to_compose_strings:
documentation: |-
#### Description
The analyzer produces this diagnostic when string literals and computed
strings are being concatenated using the `+` operator, but string
interpolation would achieve the same result.
#### Example
The following code produces this diagnostic because the elements of the
list `l` are being concatenated with other strings using the `+` operator:
```dart
String f(List<String> l) {
return [!'(' + l[0] + ', ' + l[1] + ')'!];
}
```
#### Common fixes
Use string interpolation:
```dart
String f(List<String> l) {
return '(${l[0]}, ${l[1]})';
}
```
prefer_is_empty:
documentation: |-
#### Description
The analyzer produces this diagnostic when the result of invoking either
`Iterable.length` or `Map.length` is compared for equality with zero
(`0`).
#### Example
The following code produces this diagnostic because the result of invoking
`length` is checked for equality with zero:
```dart
int f(Iterable<int> p) => [!p.length == 0!] ? 0 : p.first;
```
#### Common fixes
Replace the use of `length` with a use of either `isEmpty` or
`isNotEmpty`:
```dart
void f(Iterable<int> p) => p.isEmpty ? 0 : p.first;
```
prefer_is_not_empty:
documentation: |-
#### Description
The analyzer produces this diagnostic when the result of invoking
`Iterable.isEmpty` or `Map.isEmpty` is negated.
#### Example
The following code produces this diagnostic because the result of invoking
`Iterable.isEmpty` is negated:
```dart
void f(Iterable<int> p) => [!!p.isEmpty!] ? p.first : 0;
```
#### Common fixes
Rewrite the code to use `isNotEmpty`:
```dart
void f(Iterable<int> p) => p.isNotEmpty ? p.first : 0;
```
prefer_is_not_operator:
documentation: |-
#### Description
The analyzer produces this diagnostic when the prefix `!` operator is used
to negate the result of an `is` test.
#### Example
The following code produces this diagnostic because the result of testing
to see whether `o` is a `String` is negated using the prefix `!` operator:
```dart
String f(Object o) {
if ([!!(o is String)!]) {
return o.toString();
}
return o;
}
```
#### Common fixes
Use the `is!` operator instead:
```dart
String f(Object o) {
if (o is! String) {
return o.toString();
}
return o;
}
```
prefer_iterable_whereType:
documentation: |-
#### Description
The analyzer produces this diagnostic when the method `Iterable.where` is
being used to filter elements based on their type.
#### Example
The following code produces this diagnostic because the method `where` is
being used to access only the strings within the iterable:
```dart
Iterable<Object> f(Iterable<Object> p) => p.[!where!]((e) => e is String);
```
#### Common fixes
Rewrite the code to use `whereType`:
```dart
Iterable<String> f(Iterable<Object> p) => p.whereType<String>();
```
This might also allow you to tighten the types in your code or remove
other type checks.
prefer_null_aware_operators:
documentation: |-
#### Description
The analyzer produces this diagnostic when a comparison with `null` is
used to guard a member reference, and `null` is used as a result when the
guarded target is `null`.
#### Example
The following code produces this diagnostic because the invocation of
`length` is guarded by a `null` comparison even though the default value
is `null`:
```dart
int? f(List<int>? p) {
return [!p == null ? null : p.length!];
}
```
#### Common fixes
Use a null-aware access operator instead:
```dart
int? f(List<int>? p) {
return p?.length;
}
```
prefer_relative_imports:
documentation: |-
#### Description
The analyzer produces this diagnostic when an `import` in a library inside
the `lib` directory uses a `package:` URI to refer to another library in
the same package.
#### Example
The following code produces this diagnostic because it uses a `package:`
URI when a relative URI could have been used:
```dart
import 'package:my_package/bar.dart';
```
#### Common fixes
Use a relative URI to import the library:
```dart
import 'bar.dart';
```
prefer_single_quotes:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string literal uses double
quotes (`"`) when it could use single quotes (`'`) without needing extra
escapes and without hurting readability.
#### Example
The following code produces this diagnostic because the string literal
uses double quotes but doesn't need to:
```dart
void f(String name) {
print([!"Hello $name"!]);
}
```
#### Common fixes
Use single quotes in place of double quotes:
```dart
void f(String name) {
print('Hello $name');
}
```
prefer_typing_uninitialized_variables:
documentation: |-
#### Description
The analyzer produces this diagnostic when a variable without an
initializer doesn't have an explicit type annotation.
Without either a type annotation or an initializer, a variable has the
type `dynamic`, which allows any value to be assigned to the variable,
often causing hard to identify bugs.
#### Example
The following code produces this diagnostic because the variable `r`
doesn't have either a type annotation or an initializer:
```dart
Object f() {
var [!r!];
r = '';
return r;
}
```
#### Common fixes
If the variable can be initialized, then add an initializer:
```dart
Object f() {
var r = '';
return r;
}
```
If the variable can't be initialized, then add an explicit type
annotation:
```dart
Object f() {
String r;
r = '';
return r;
}
```
prefer_void_to_null:
documentation: |-
#### Description
The analyzer produces this diagnostic when `Null` is used in a location
where `void` would be a valid choice.
#### Example
The following code produces this diagnostic because the function `f` is
declared to return `null` (at some future time):
```dart
Future<[!Null!]> f() async {}
```
#### Common fixes
Replace the use of `Null` with a use of `void`:
```dart
Future<void> f() async {}
```
provide_deprecation_message:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `deprecated` annotation is
used instead of the `Deprecated` annotation.
#### Example
The following code produces this diagnostic because the function `f` is
annotated with `deprecated`:
```dart
[!@deprecated!]
void f() {}
```
#### Common fixes
Convert the code to use the longer form:
```dart
@Deprecated('Use g instead. Will be removed in 4.0.0.')
void f() {}
```
recursive_getters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a getter invokes itself,
resulting in an infinite loop.
#### Example
The following code produces this diagnostic because the getter `count`
invokes itself:
```dart
class C {
int _count = 0;
int get [!count!] => count;
}
```
#### Common fixes
Change the getter to not invoke itself:
```dart
class C {
int _count = 0;
int get count => _count;
}
```
secure_pubspec_urls:
documentation: |-
#### Description
The analyzer produces this diagnostic when a URL in a `pubspec.yaml` file is
using a non-secure scheme, such as `http`.
#### Example
The following code produces this diagnostic because the `pubspec.yaml` file
contains an `http` URL:
```yaml
dependencies:
example: any
repository: [!http://github.com/dart-lang/example!]
```
#### Common fixes
Change the scheme of the URL to use a secure scheme, such as `https`:
```yaml
dependencies:
example: any
repository: https://github.com/dart-lang/example
```
sized_box_for_whitespace:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `Container` is created using
only the `height` and/or `width` arguments.
#### Example
The following code produces this diagnostic because the `Container` has
only the `width` argument:
```dart
import 'package:flutter/material.dart';
Widget buildRow() {
return Row(
children: <Widget>[
const Text('...'),
[!Container!](
width: 4,
child: Text('...'),
),
const Expanded(
child: Text('...'),
),
],
);
}
```
#### Common fixes
Replace the `Container` with a `SizedBox` of the same dimensions:
```dart
import 'package:flutter/material.dart';
Widget buildRow() {
return Row(
children: <Widget>[
Text('...'),
SizedBox(
width: 4,
child: Text('...'),
),
Expanded(
child: Text('...'),
),
],
);
}
```
sized_box_shrink_expand:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `SizedBox` constructor
invocation specifies the values of both `height` and `width` as either
`0.0` or `double.infinity`.
#### Examples
The following code produces this diagnostic because both the `height` and
`width` are `0.0`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return [!SizedBox!](
height: 0.0,
width: 0.0,
child: const Text(''),
);
}
```
The following code produces this diagnostic because both the `height` and
`width` are `double.infinity`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return [!SizedBox!](
height: double.infinity,
width: double.infinity,
child: const Text(''),
);
}
```
#### Common fixes
If both are `0.0`, then use `SizedBox.shrink`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return SizedBox.shrink(
child: const Text(''),
);
}
```
If both are `double.infinity`, then use `SizedBox.expand`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return SizedBox.expand(
child: const Text(''),
);
}
```
slash_for_doc_comments:
documentation: |-
#### Description
The analyzer produces this diagnostic when a documentation comment uses
the block comment style (delimited by `/**` and `*/`).
#### Example
The following code produces this diagnostic because the documentation
comment for `f` uses a block comment style:
```dart
[!/**
* Example.
*/!]
void f() {}
```
#### Common fixes
Use an end-of-line comment style:
```dart
/// Example.
void f() {}
```
sort_child_properties_last:
documentation: |-
#### Description
The analyzer produces this diagnostic when the `child` or `children`
argument isn't the last argument in an invocation of a widget class'
constructor. An exception is made if all of the arguments after the
`child` or `children` argument are function expressions.
#### Example
The following code produces this diagnostic because the `child` argument
isn't the last argument in the invocation of the `Center` constructor:
```dart
import 'package:flutter/material.dart';
Widget createWidget() {
return Center(
[!child: Text('...')!],
widthFactor: 0.5,
);
}
```
#### Common fixes
Move the `child` or `children` argument to be last:
```dart
import 'package:flutter/material.dart';
Widget createWidget() {
return Center(
widthFactor: 0.5,
child: Text('...'),
);
}
```
sort_constructors_first:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constructor declaration is
preceded by one or more non-constructor declarations.
#### Example
The following code produces this diagnostic because the constructor for
`C` appears after the method `m`:
```dart
class C {
void m() {}
[!C!]();
}
```
#### Common fixes
Move all of the constructor declarations before any other declarations:
```dart
class C {
C();
void m() {}
}
```
sort_pub_dependencies:
documentation: |-
#### Description
The analyzer produces this diagnostic when the keys in a dependency map in
the `pubspec.yaml` file aren't sorted alphabetically. The dependency maps
that are checked are the `dependencies`, `dev_dependencies`, and
`dependency_overrides` maps.
#### Example
The following code produces this diagnostic because the entries in the
`dependencies` map are not sorted:
```yaml
dependencies:
path: any
collection: any
```
#### Common fixes
Sort the entries:
```yaml
dependencies:
collection: any
path: any
```
sort_unnamed_constructors_first:
documentation: |-
#### Description
The analyzer produces this diagnostic when an unnamed constructor appears
after a named constructor.
#### Example
The following code produces this diagnostic because the unnamed
constructor is after the named constructor:
```dart
class C {
C.named();
[!C!]();
}
```
#### Common fixes
Move the unnamed constructor before any other constructors:
```dart
class C {
C();
C.named();
}
```
test_types_in_equals:
documentation: |-
#### Description
The analyzer produces this diagnostic when an override of the `==`
operator doesn't include a type test on the value of the parameter.
#### Example
The following code produces this diagnostic because `other` is not type
tested:
```dart
class C {
final int f;
C(this.f);
@override
bool operator ==(Object other) {
return ([!other as C!]).f == f;
}
}
```
#### Common fixes
Perform an `is` test as part of computing the return value:
```dart
class C {
final int f;
C(this.f);
@override
bool operator ==(Object other) {
return other is C && other.f == f;
}
}
```
throw_in_finally:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `throw` statement is found
inside a `finally` block.
#### Example
The following code produces this diagnostic because there is a `throw`
statement inside a `finally` block:
```dart
void f() {
try {
// ...
} catch (e) {
// ...
} finally {
[!throw 'error'!];
}
}
```
#### Common fixes
Rewrite the code so that the `throw` statement isn't inside a `finally`
block:
```dart
void f() {
try {
// ...
} catch (e) {
// ...
}
throw 'error';
}
```
type_init_formals:
documentation: |-
#### Description
The analyzer produces this diagnostic when an initializing formal
parameter (`this.x`) or a super parameter (`super.x`) has an explicit type
annotation that is the same as the field or overridden parameter.
If a constructor parameter is using `this.x` to initialize a field, then
the type of the parameter is implicitly the same type as the field. If a
constructor parameter is using `super.x` to forward to a super
constructor, then the type of the parameter is implicitly the same as the
super constructor parameter.
#### Example
The following code produces this diagnostic because the parameter `this.c`
has an explicit type that is the same as the field `c`:
```dart
class C {
int c;
C([!int!] this.c);
}
```
The following code produces this diagnostic because the parameter
`super.a` has an explicit type that is the same as the parameter `a` from
the superclass:
```dart
class A {
A(int a);
}
class B extends A {
B([!int!] super.a);
}
```
#### Common fixes
Remove the type annotation from the parameter:
```dart
class C {
int c;
C(this.c);
}
```
type_literal_in_constant_pattern:
documentation: |-
#### Description
The analyzer produces this diagnostic when a type literal appears as a
pattern.
#### Example
The following code produces this diagnostic because a type literal is used
as a constant pattern:
```dart
void f(Object? x) {
if (x case [!num!]) {
// ...
}
}
```
#### Common fixes
If the type literal is intended to match an object of the given type, then
use either a variable pattern:
```dart
void f(Object? x) {
if (x case num _) {
// ...
}
}
```
Or an object pattern:
```dart
void f(Object? x) {
if (x case num()) {
// ...
}
}
```
If the type literal is intended to match the type literal, then write it
as a constant pattern:
```dart
void f(Object? x) {
if (x case const (num)) {
// ...
}
}
```
unawaited_futures:
documentation: |-
#### Description
The analyzer produces this diagnostic when an instance of `Future` is
returned from an invocation within an `async` (or `async*`) method or
function and the future is neither awaited nor passed to the `unawaited`
function.
#### Example
The following code produces this diagnostic because the function `g`
returns a future, but the future isn't awaited:
```dart
Future<void> f() async {
[!g();!]
}
Future<int> g() => Future.value(0);
```
#### Common fixes
If the future needs to complete before the following code is executed,
then add an `await` before the invocation:
```dart
Future<void> f() async {
await g();
}
Future<int> g() => Future.value(0);
```
If the future doesn't need to complete before the following code is
executed, then wrap the `Future`-returning invocation in an invocation of
the `unawaited` function:
```dart
import 'dart:async';
Future<void> f() async {
unawaited(g());
}
Future<int> g() => Future.value(0);
```
unnecessary_brace_in_string_interps:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string interpolation with
braces is used to interpolate a simple identifier and isn't followed by
alphanumeric text.
#### Example
The following code produces this diagnostic because the interpolation
element `${s}` uses braces when they are not necessary:
```dart
String f(String s) {
return '"[!${s}!]"';
}
```
#### Common fixes
Remove the unnecessary braces:
```dart
String f(String s) {
return '"$s"';
}
```
unnecessary_const:
documentation: |-
#### Description
The analyzer produces this diagnostic when the keyword `const` is used in
a [constant context][]. The keyword isn't required because it's implied.
#### Example
The following code produces this diagnostic because the keyword `const` in
the list literal isn't needed:
```dart
const l = [!const!] <int>[];
```
The list is implicitly `const` because of the keyword `const` on the
variable declaration.
#### Common fixes
Remove the unnecessary keyword:
```dart
const l = <int>[];
```
unnecessary_constructor_name:
documentation: |-
#### Description
The analyzer produces this diagnostic when a reference to an unnamed
constructor uses `.new`. The only place where `.new` is required is in a
constructor tear-off.
#### Example
The following code produces this diagnostic because `.new` is being used
to refer to the unnamed constructor where it isn't required:
```dart
var o = Object.[!new!]();
```
#### Common fixes
Remove the unnecessary `.new`:
```dart
var o = Object();
```
unnecessary_final:
documentation: |-
#### Description
The analyzer produces this diagnostic when a local variable is marked as
being `final`.
#### Example
The following code produces this diagnostic because the local variable `c`
is marked as being `final`:
```dart
void f(int a, int b) {
[!final!] c = a + b;
print(c);
}
```
#### Common fixes
If the variable doesn't have a type annotation, then replace the `final`
with `var`:
```dart
void f(int a, int b) {
var c = a + b;
print(c);
}
```
If the variable has a type annotation, then remove the `final`
modifier:
```dart
void f(int a, int b) {
int c = a + b;
print(c);
}
```
unnecessary_getters_setters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a getter and setter pair
returns and sets the value of a field without any additional processing.
#### Example
The following code produces this diagnostic because the getter/setter pair
named `c` only expose the field named `_c`:
```dart
class C {
int? _c;
int? get [!c!] => _c;
set c(int? v) => _c = v;
}
```
#### Common fixes
Make the field public and remove the getter and setter:
```dart
class C {
int? c;
}
```
unnecessary_lambdas:
documentation: |-
#### Description
The analyzer produces this diagnostic when a closure (lambda) could be
replaced by a tear-off.
#### Example
The following code produces this diagnostic because the closure passed to
`forEach` contains only an invocation of the function `print` with the
parameter of the closure:
```dart
void f(List<String> strings) {
strings.forEach([!(string) {
print(string);
}!]);
}
```
#### Common fixes
Replace the closure with a tear-off of the function or method being
invoked with the closure:
```dart
void f(List<String> strings) {
strings.forEach(print);
}
```
unnecessary_late:
documentation: |-
#### Description
The analyzer produces this diagnostic when a top-level variable or static
field with an initializer is marked as `late`. Top-level variables and
static fields are implicitly late, so they don't need to be explicitly
marked.
#### Example
The following code produces this diagnostic because the static field `c`
has the modifier `late` even though it has an initializer:
```dart
class C {
static [!late!] String c = '';
}
```
#### Common fixes
Remove the keyword `late`:
```dart
class C {
static String c = '';
}
```
unnecessary_new:
documentation: |-
#### Description
The analyzer produces this diagnostic when the keyword `new` is used to
invoke a constructor.
#### Example
The following code produces this diagnostic because the keyword `new` is
used to invoke the unnamed constructor from `Object`:
```dart
var o = [!new!] Object();
```
#### Common fixes
Remove the keyword `new`:
```dart
var o = Object();
```
unnecessary_null_aware_assignments:
documentation: |-
#### Description
The analyzer produces this diagnostic when the right-hand side of a
null-aware assignment is the `null` literal.
#### Example
The following code produces this diagnostic because the null aware
operator is being used to assign `null` to `s` when `s` is already `null`:
```dart
void f(String? s) {
[!s ??= null!];
}
```
#### Common fixes
If a non-null value should be assigned to the left-hand operand, then
change the right-hand side:
```dart
void f(String? s) {
s ??= '';
}
```
If there is no non-null value to assign to the left-hand operand, then
remove the assignment:
```dart
void f(String? s) {
}
```
unnecessary_null_in_if_null_operators:
documentation: |-
#### Description
The analyzer produces this diagnostic when the right operand of the `??`
operator is the literal `null`.
#### Example
The following code produces this diagnostic because the right-hand operand
of the `??` operator is `null`:
```dart
String? f(String? s) => s ?? [!null!];
```
#### Common fixes
If a non-null value should be used for the right-hand operand, then
change the right-hand side:
```dart
String f(String? s) => s ?? '';
```
If there is no non-null value to use for the right-hand operand, then
remove the operator and the right-hand operand:
```dart
String? f(String? s) => s;
```
unnecessary_nullable_for_final_variable_declarations:
documentation: |-
#### Description
The analyzer produces this diagnostic when a final field or variable has a
nullable type but is initialized to a non-nullable value.
#### Example
The following code produces this diagnostic because the final variable `i`
has a nullable type (`int?`), but can never be `null`:
```dart
final int? [!i!] = 1;
```
#### Common fixes
Make the type non-nullable:
```dart
final int i = 1;
```
unnecessary_overrides:
documentation: |-
#### Description
The analyzer produces this diagnostic when an instance member overrides an
inherited member but only invokes the overridden member with exactly the
same arguments.
#### Example
The following code produces this diagnostic because the method `D.m`
doesn't do anything other than invoke the overridden method:
```dart
class C {
int m(int x) => x;
}
class D extends C {
@override
int [!m!](int x) => super.m(x);
}
```
#### Common fixes
If the method should do something more than what the overridden method
does, then implement the missing functionality:
```dart
class C {
int m(int x) => x;
}
class D extends C {
@override
int m(int x) => super.m(x) + 1;
}
```
If the overridden method should be modified by changing the return type or
one or more of the parameter types, making one of the parameters
`covariant`, having a documentation comment, or by having additional
annotations, then update the code:
```dart
import 'package:meta/meta.dart';
class C {
int m(int x) => x;
}
class D extends C {
@mustCallSuper
@override
int m(int x) => super.m(x);
}
```
If the overriding method doesn't change or enhance the semantics of the
code, then remove it:
```dart
class C {
int m(int x) => x;
}
class D extends C {}
```
unnecessary_parenthesis:
documentation: |-
#### Description
The analyzer produces this diagnostic when parentheses are used where they
do not affect the semantics of the code.
#### Example
The following code produces this diagnostic because the parentheses around
the binary expression are not necessary:
```dart
int f(int a, int b) => [!(a + b)!];
```
#### Common fixes
Remove the unnecessary parentheses:
```dart
int f(int a, int b) => a + b;
```
unnecessary_raw_strings:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string literal is marked as
being raw (is prefixed with an `r`), but making the string raw doesn't
change the value of the string.
#### Example
The following code produces this diagnostic because the string literal
will have the same value without the `r` as it does with the `r`:
```dart
var s = [!r'abc'!];
```
#### Common fixes
Remove the `r` in front of the string literal:
```dart
var s = 'abc';
```
unnecessary_statements:
documentation: |-
#### Description
The analyzer produces this diagnostic when an expression statement has no
clear effect.
#### Example
The following code produces this diagnostic because the addition of the
returned values from the two invocations has no clear effect:
```dart
void f(int Function() first, int Function() second) {
[!first() + second()!];
}
```
#### Common fixes
If the expression doesn't need to be computed, then remove it:
```dart
void f(int Function() first, int Function() second) {
}
```
If the value of the expression is needed, then make use of it, possibly
assigning it to a local variable first:
```dart
void f(int Function() first, int Function() second) {
print(first() + second());
}
```
If portions of the expression need to be executed, then remove the
unnecessary portions:
```dart
void f(int Function() first, int Function() second) {
first();
second();
}
```
unnecessary_string_escapes:
documentation: |-
#### Description
The analyzer produces this diagnostic when characters in a string are
escaped when escaping them is unnecessary.
#### Example
The following code produces this diagnostic because single quotes don't
need to be escaped inside strings delimited by double quotes:
```dart
var s = "Don[!\!]'t use a backslash here.";
```
#### Common fixes
Remove the unnecessary backslashes:
```dart
var s = "Don't use a backslash here.";
```
unnecessary_string_interpolations:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string literal contains a
single interpolation of a `String`-valued variable and no other
characters.
#### Example
The following code produces this diagnostic because the string literal
contains a single interpolation and doesn't contain any character outside
the interpolation:
```dart
String f(String s) => [!'$s'!];
```
#### Common fixes
Replace the string literal with the content of the interpolation:
```dart
String f(String s) => s;
```
unnecessary_this:
documentation: |-
#### Description
The analyzer produces this diagnostic when the keyword `this` is used to
access a member that isn't shadowed.
#### Example
The following code produces this diagnostic because the use of `this` to
access the field `_f` isn't necessary:
```dart
class C {
int _f = 2;
int get f => [!this!]._f;
}
```
#### Common fixes
Remove the `this.`:
```dart
class C {
int _f = 2;
int get f => _f;
}
```
unnecessary_to_list_in_spreads:
documentation: |-
#### Description
The analyzer produces this diagnostic when `toList` is used to convert an
`Iterable` to a `List` just before a spread operator is applied to the
list. The spread operator can be applied to any `Iterable`, so the
conversion isn't necessary.
#### Example
The following code produces this diagnostic because `toList` is invoked on
the result of `map`, which is an `Iterable` that the spread operator could
be applied to directly:
```dart
List<String> toLowercase(List<String> strings) {
return [
...strings.map((String s) => s.toLowerCase()).[!toList!](),
];
}
```
#### Common fixes
Remove the invocation of `toList`:
```dart
List<String> toLowercase(List<String> strings) {
return [
...strings.map((String s) => s.toLowerCase()),
];
}
```
unrelated_type_equality_checks:
documentation: |-
#### Description
The analyzer produces this diagnostic when two objects are being compared
and neither of the static types of the two objects is a subtype of the
other.
Such a comparison will usually return `false` and might not reflect the
programmer's intent.
There can be false positives. For example, a class named `Point` might
have subclasses named `CartesianPoint` and `PolarPoint`, neither of which
is a subtype of the other, but it might still be appropriate to test the
equality of instances.
As a concrete case, the classes `Int64` and `Int32` from `package:fixnum`
allow comparing instances to an `int` provided the `int` is on the
right-hand side. This case is specifically allowed by the diagnostic, but
other such cases are not.
#### Example
The following code produces this diagnostic because the string `s` is
being compared to the integer `1`:
```dart
bool f(String s) {
return s [!==!] 1;
}
```
#### Common fixes
Replace one of the operands with something compatible with the other
operand:
```dart
bool f(String s) {
return s.length == 1;
}
```
use_build_context_synchronously:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `BuildContext` is referenced
by a `StatefulWidget` after an asynchronous gap without first checking the
`mounted` property.
Storing a `BuildContext` for later use can lead to difficult to diagnose
crashes. Asynchronous gaps implicitly store a `BuildContext`, making them
easy to overlook for diagnosis.
#### Example
The following code produces this diagnostic because the `context` is
passed to a constructor after the `await`:
```dart
import 'package:flutter/material.dart';
class MyWidget extends Widget {
void onButtonTapped(BuildContext context) async {
await Future.delayed(const Duration(seconds: 1));
Navigator.of([!context!]).pop();
}
}
```
#### Common fixes
If you can remove the asynchronous gap, do so:
```dart
import 'package:flutter/material.dart';
class MyWidget extends Widget {
void onButtonTapped(BuildContext context) {
Navigator.of(context).pop();
}
}
```
If you can't remove the asynchronous gap, then use `mounted` to guard the
use of the `context`:
```dart
import 'package:flutter/material.dart';
class MyWidget extends Widget {
void onButtonTapped(BuildContext context) async {
await Future.delayed(const Duration(seconds: 1));
if (context.mounted) {
Navigator.of(context).pop();
}
}
}
```
use_colored_box:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `Container` is created that
only sets the color.
#### Example
The following code produces this diagnostic because the only attribute of
the container that is set is the `color`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return [!Container!](
color: Colors.red,
child: const Text('hello'),
);
}
```
#### Common fixes
Replace the `Container` with a `ColoredBox`:
```dart
import 'package:flutter/material.dart';
Widget build() {
return ColoredBox(
color: Colors.red,
child: const Text('hello'),
);
}
```
use_decorated_box:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `Container` is created that
only sets the decoration.
#### Example
The following code produces this diagnostic because the only attribute of
the container that is set is the `decoration`:
```dart
import 'package:flutter/material.dart';
Widget buildArea() {
return [!Container!](
decoration: const BoxDecoration(
color: Colors.red,
borderRadius: BorderRadius.all(
Radius.circular(5),
),
),
child: const Text('...'),
);
}
```
#### Common fixes
Replace the `Container` with a `DecoratedBox`:
```dart
import 'package:flutter/material.dart';
Widget buildArea() {
return DecoratedBox(
decoration: const BoxDecoration(
color: Colors.red,
borderRadius: BorderRadius.all(
Radius.circular(5),
),
),
child: const Text('...'),
);
}
```
use_full_hex_values_for_flutter_colors:
documentation: |-
#### Description
The analyzer produces this diagnostic when the argument to the constructor
of the `Color` class is a literal integer that isn't represented as an
8-digit hexadecimal integer.
#### Example
The following code produces this diagnostic because the argument (`1`)
isn't represented as an 8-digit hexadecimal integer:
```dart
import 'package:flutter/material.dart';
Color c = Color([!1!]);
```
#### Common fixes
Convert the representation to be an 8-digit hexadecimal integer:
```dart
import 'package:flutter/material.dart';
Color c = Color(0x00000001);
```
use_function_type_syntax_for_parameters:
documentation: |-
#### Description
The analyzer produces this diagnostic when the older style function-valued
parameter syntax is used.
#### Example
The following code produces this diagnostic because the function-valued
parameter `f` is declared using an older style syntax:
```dart
void g([!bool f(String s)!]) {}
```
#### Common fixes
Use the generic function type syntax to declare the parameter:
```dart
void g(bool Function(String) f) {}
```
use_if_null_to_convert_nulls_to_bools:
documentation: |-
#### Description
The analyzer produces this diagnostic when a nullable `bool`-valued
expression is compared (using `==` or `!=`) to a boolean literal.
#### Example
The following code produces this diagnostic because the nullable boolean
variable `b` is compared to `true`:
```dart
void f(bool? b) {
if ([!b == true!]) {
// Treats `null` as `false`.
}
}
```
#### Common fixes
Rewrite the condition to use `??` instead:
```dart
void f(bool? b) {
if (b ?? false) {
// Treats `null` as `false`.
}
}
```
use_key_in_widget_constructors:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constructor in a subclass of
`Widget` that isn't private to its library doesn't have a parameter named
`key`.
#### Example
The following code produces this diagnostic because the constructor for
the class `MyWidget` doesn't have a parameter named `key`:
```dart
import 'package:flutter/material.dart';
class MyWidget extends StatelessWidget {
[!MyWidget!]({required int height});
}
```
The following code produces this diagnostic because the default
constructor for the class `MyWidget` doesn't have a parameter named `key`:
```dart
import 'package:flutter/material.dart';
class [!MyWidget!] extends StatelessWidget {}
```
#### Common fixes
Add a parameter named `key` to the constructor, explicitly declaring the
constructor if necessary:
```dart
import 'package:flutter/material.dart';
class MyWidget extends StatelessWidget {
MyWidget({super.key, required int height});
}
```
use_late_for_private_fields_and_variables:
documentation: |-
#### Description
The analyzer produces this diagnostic when a private field or variable is
marked as being nullable, but every reference assumes that the variable is
never `null`.
#### Example
The following code produces this diagnostic because the private top-level
variable `_i` is nullable, but every reference assumes that it will not be
`null`:
```dart
void f() {
_i!.abs();
}
int? [!_i!];
```
#### Common fixes
Mark the variable or field as being both non-nullable and `late` to
indicate that it will always be assigned a non-null:
```dart
void f() {
_i.abs();
}
late int _i;
```
use_named_constants:
documentation: |-
#### Description
The analyzer produces this diagnostic when a constant is created with the
same value as a known `const` variable.
#### Example
The following code produces this diagnostic because there is a known
`const` field (`Duration.zero`) whose value is the same as what the
constructor invocation will evaluate to:
```dart
Duration d = [!const Duration(seconds: 0)!];
```
#### Common fixes
Replace the constructor invocation with a reference to the known `const`
variable:
```dart
Duration d = Duration.zero;
```
use_raw_strings:
documentation: |-
#### Description
The analyzer produces this diagnostic when a string literal containing
escapes, and no interpolations, could be marked as being raw in order to
avoid the need for the escapes.
#### Example
The following code produces this diagnostic because the string contains
escaped characters that wouldn't need to be escaped if the string is
made a raw string:
```dart
var s = [!'A string with only \\ and \$'!];
```
#### Common fixes
Mark the string as being raw and remove the unnecessary backslashes:
```dart
var s = r'A string with only \ and $';
```
use_rethrow_when_possible:
documentation: |-
#### Description
The analyzer produces this diagnostic when a caught exception is thrown
using a `throw` expression rather than a `rethrow` statement.
#### Example
The following code produces this diagnostic because the caught exception
`e` is thrown using a `throw` expression:
```dart
void f() {
try {
// ...
} catch (e) {
[!throw e!];
}
}
```
#### Common fixes
Use `rethrow` instead of `throw`:
```dart
void f() {
try {
// ...
} catch (e) {
rethrow;
}
}
```
use_setters_to_change_properties:
documentation: |-
#### Description
The analyzer produces this diagnostic when a method is used to set the
value of a field, or a function is used to set the value of a top-level
variable, and nothing else.
#### Example
The following code produces this diagnostic because the method `setF` is
used to set the value of the field `_f` and does no other work:
```dart
class C {
int _f = 0;
void [!setF!](int value) => _f = value;
}
```
#### Common fixes
Convert the method to a setter:
```dart
class C {
int _f = 0;
set f(int value) => _f = value;
}
```
use_string_buffers:
documentation: |-
#### Description
The analyzer produces this diagnostic when values are concatenated to a
string inside a loop without using a `StringBuffer` to do the
concatenation.
#### Example
The following code produces this diagnostic because the string `result` is
computed by repeated concatenation within the `for` loop:
```dart
String f() {
var result = '';
for (int i = 0; i < 10; i++) {
[!result += 'a'!];
}
return result;
}
```
#### Common fixes
Use a `StringBuffer` to compute the result:
```dart
String f() {
var buffer = StringBuffer();
for (int i = 0; i < 10; i++) {
buffer.write('a');
}
return buffer.toString();
}
```
use_string_in_part_of_directives:
documentation: |-
#### Description
The analyzer produces this diagnostic when a `part of` directive uses a
library name to refer to the library that the part is a part of.
#### Example
Given a file named `lib.dart` that contains the following:
```dart
%uri="lib/lib.dart"
library lib;
part 'test.dart';
```
The following code produces this diagnostic because the `part of`
directive uses the name of the library rather than the URI of the library
it's part of:
```dart
[!part of lib;!]
```
#### Common fixes
Use a URI to reference the library:
```dart
part of 'lib.dart';
```
use_super_parameters:
documentation: |-
#### Description
The analyzer produces this diagnostic when a parameter to a constructor is
passed to a super constructor without being referenced or modified and a
`super` parameter isn't used.
#### Example
The following code produces this diagnostic because the parameters of the
constructor for `B` are only used as arguments to the super constructor:
```dart
class A {
A({int? x, int? y});
}
class B extends A {
[!B!]({int? x, int? y}) : super(x: x, y: y);
}
```
#### Common fixes
Use a `super` parameter to pass the arguments:
```dart
class A {
A({int? x, int? y});
}
class B extends A {
B({super.x, super.y});
}
```
valid_regexps:
documentation: |-
#### Description
The analyzer produces this diagnostic when the string passed to the
default constructor of the class `RegExp` doesn't contain a valid regular
expression.
A regular expression created with invalid syntax will throw a
`FormatException` at runtime.
#### Example
The following code produces this diagnostic because the regular expression
isn't valid:
```dart
var r = RegExp([!r'('!]);
```
#### Common fixes
Fix the regular expression:
```dart
var r = RegExp(r'\(');
```
void_checks:
documentation: |-
#### Description
The analyzer produces this diagnostic when a value is assigned to a
variable of type `void`.
It isn't possible to access the value of such a variable, so the
assignment has no value.
#### Example
The following code produces this diagnostic because the field `value` has
the type `void`, but a value is being assigned to it:
```dart
class A<T> {
T? value;
}
void f(A<void> a) {
[!a.value = 1!];
}
```
The following code produces this diagnostic because the type of the
parameter `p` in the method `m` is `void`, but a value is being assigned
to it in the invocation:
```dart
class A<T> {
void m(T p) { }
}
void f(A<void> a) {
a.m([!1!]);
}
```
#### Common fixes
If the type of the variable is incorrect, then change the type of the
variable:
```dart
class A<T> {
T? value;
}
void f(A<int> a) {
a.value = 1;
}
```
If the type of the variable is correct, then remove the assignment:
```dart
class A<T> {
T? value;
}
void f(A<void> a) {}
```