ArgumentError

class ArgumentError extends Error

Error thrown when a function is passed an unacceptable argument.

The method should document restrictions on the arguments it accepts, for example if an integer argument must be non-nullable, a string argument must be non-empty, or a dynamic-typed argument must actually have one of a few accepted types.

The user should be able to predict which arguments will cause an error to be throw, and avoid calling with those.

It's almost always a good idea to provide the unacceptable value as part of the error, to help the user figure out what went wrong, so the ArgumentError.value constructor is the preferred constructor. Use ArgumentError.new only when the value cannot be provided for some reason.

Inheritance

Object → Error → ArgumentError

Implementers

• IndexError
• RangeError

Constructors

ArgumentError()

ArgumentError([dynamic message, String? name])

Creates an error with message describing the problem with an argument.

Existing code may be using message to hold the invalid value. If the message is not a String, it is assumed to be a value instead of a message.

If name is provided, it should be the name of the parameter which received an invalid argument.

Prefer using ArgumentError.value instead to retain and document the invalid value as well.

Implementation

@pragma("vm:entry-point")
ArgumentError([this.message, @Since("2.14") this.name])
  : invalidValue = null,
    _hasValue = false;

ArgumentError.notNull()

ArgumentError.notNull([String? name])

Creates an argument error for a null argument that must not be null.

Implementation

ArgumentError.notNull([this.name])
  : _hasValue = false,
    message = "Must not be null",
    invalidValue = null;

ArgumentError.value()

ArgumentError.value(dynamic value, [String? name, dynamic message])

Creates error containing the invalid value.

A message is built by suffixing the message argument with the name argument (if provided) and the value. Example:

Invalid argument (foo): null

The name should match the argument name of the function, but if the function is a method implementing an interface, and its argument names differ from the interface, it might be more useful to use the interface method's argument name (or just rename arguments to match).

Implementation

@pragma("vm:entry-point")
ArgumentError.value(value, [this.name, this.message])
  : invalidValue = value,
    _hasValue = true;

Properties

hashCode no setter inherited

int get hashCode

The hash code for this object.

A hash code is a single integer which represents the state of the object that affects operator == comparisons.

All objects have hash codes. The default hash code implemented by Object represents only the identity of the object, the same way as the default operator == implementation only considers objects equal if they are identical (see identityHashCode).

If operator == is overridden to use the object state instead, the hash code must also be changed to represent that state, otherwise the object cannot be used in hash based data structures like the default Set and Map implementations.

Hash codes must be the same for objects that are equal to each other according to operator ==. The hash code of an object should only change if the object changes in a way that affects equality. There are no further requirements for the hash codes. They need not be consistent between executions of the same program and there are no distribution guarantees.

Objects that are not equal are allowed to have the same hash code. It is even technically allowed that all instances have the same hash code, but if clashes happen too often, it may reduce the efficiency of hash-based data structures like HashSet or HashMap.

If a subclass overrides hashCode, it should override the operator == operator as well to maintain consistency.

Inherited from Object.

Implementation

external int get hashCode;

invalidValue final

final dynamic invalidValue

The invalid value.

Implementation

final dynamic invalidValue;

message final

final dynamic message

Message describing the problem.

Implementation

final dynamic message;

name final

final String? name

Name of the invalid argument, if available.

Implementation

final String? name;

runtimeType no setter inherited

Type get runtimeType

A representation of the runtime type of the object.

Inherited from Object.

Implementation

external Type get runtimeType;

stackTrace no setter inherited

StackTrace? get stackTrace

The stack trace at the point where this error was first thrown.

Classes which extend Error will automatically have a stack trace filled in the first time they are thrown by a throw expression.

Inherited from Error.

Implementation

external StackTrace? get stackTrace;

Methods

noSuchMethod() inherited

dynamic noSuchMethod(Invocation invocation)

Invoked when a nonexistent method or property is accessed.

A dynamic member invocation can attempt to call a member which doesn't exist on the receiving object. Example:

dynamic object = 1;
object.add(42); // Statically allowed, run-time error

This invalid code will invoke the noSuchMethod method of the integer 1 with an Invocation representing the .add(42) call and arguments (which then throws).

Classes can override noSuchMethod to provide custom behavior for such invalid dynamic invocations.

A class with a non-default noSuchMethod invocation can also omit implementations for members of its interface. Example:

class MockList<T> implements List<T> {
  noSuchMethod(Invocation invocation) {
    log(invocation);
    super.noSuchMethod(invocation); // Will throw.
  }
}
void main() {
  MockList().add(42);
}

This code has no compile-time warnings or errors even though the MockList class has no concrete implementation of any of the List interface methods. Calls to List methods are forwarded to noSuchMethod, so this code will log an invocation similar to Invocation.method(#add, [42]) and then throw.

If a value is returned from noSuchMethod, it becomes the result of the original invocation. If the value is not of a type that can be returned by the original invocation, a type error occurs at the invocation.

The default behavior is to throw a NoSuchMethodError.

Inherited from Object.

Implementation

@pragma("vm:entry-point")
@pragma("wasm:entry-point")
external dynamic noSuchMethod(Invocation invocation);

toString() override

String toString()

A string representation of this object.

Some classes have a default textual representation, often paired with a static parse function (like int.parse). These classes will provide the textual representation as their string representation.

Other classes have no meaningful textual representation that a program will care about. Such classes will typically override toString to provide useful information when inspecting the object, mainly for debugging or logging.

Implementation

String toString() {
  String? name = this.name;
  String nameString = (name == null) ? "" : " ($name)";
  Object? message = this.message;
  var messageString = (message == null) ? "" : ": ${message}";
  String prefix = "$_errorName$nameString$messageString";
  if (!_hasValue) return prefix;
  // If we know the invalid value, we can try to describe the problem.
  String explanation = _errorExplanation;
  String errorValue = Error.safeToString(invalidValue);
  return "$prefix$explanation: $errorValue";
}

Operators

operator ==() inherited

bool operator ==(Object other)

The equality operator.

The default behavior for all Objects is to return true if and only if this object and other are the same object.

Override this method to specify a different equality relation on a class. The overriding method must still be an equivalence relation. That is, it must be:

• Total: It must return a boolean for all arguments. It should never throw.

• Reflexive: For all objects o, o == o must be true.

• Symmetric: For all objects o1 and o2, o1 == o2 and o2 == o1 must either both be true, or both be false.

• Transitive: For all objects o1, o2, and o3, if o1 == o2 and o2 == o3 are true, then o1 == o3 must be true.

The method should also be consistent over time, so whether two objects are equal should only change if at least one of the objects was modified.

If a subclass overrides the equality operator, it should override the hashCode method as well to maintain consistency.

Inherited from Object.

Implementation

external bool operator ==(Object other);

Static Methods

checkNotNull()

T checkNotNull<T>(T? argument, [String? name])

Throws if argument is null.

If name is supplied, it is used as the parameter name in the error message.

Returns the argument if it is not null.

Implementation

static T checkNotNull<T>(T? argument, [String? name]) =>
    argument ?? (throw ArgumentError.notNull(name));