Error

class Error

Error objects thrown in the case of a program failure.

An Error object represents a program failure that the programmer should have avoided.

Examples include calling a function with invalid arguments, or even with the wrong number of arguments, or calling it at a time when it is not allowed.

These are not errors that a caller should expect or catch — if they occur, the program is erroneous, and terminating the program may be the safest response.

When deciding that a function should throw an error, the conditions where it happens should be clearly described, and they should be detectable and predictable, so the programmer using the function can avoid triggering the error.

Such descriptions often uses words like "must" or "must not" to describe the condition, and if you see words like that in a function's documentation, then not satisfying the requirement is very likely to cause an error to be thrown.

Example (from String.contains):

`startIndex` must not be negative or greater than `length`.

In this case, an error will be thrown if startIndex is negative or too large.

If the conditions are not detectable before calling a function, the called function should not throw an Error. It may still throw, but the caller will have to catch the thrown value, effectively making it an alternative result rather than an error. If so, we consider the thrown object an exception rather than an error. The thrown object can choose to implement Exception to document that it represents an exceptional, but not erroneous, occurrence, but being an Exception has no other effect than documentation.

All non-null values can be thrown in Dart. Objects extending the Error class are handled specially: The first time they are thrown, the stack trace at the throw point is recorded and stored in the error object. It can be retrieved using the stackTrace getter. An error object that merely implements Error, and doesn't extend it, will not store the stack trace automatically.

Error objects are also used for system wide failures like stack overflow or an out-of-memory situation, which the user is also not expected to catch or handle.

Since errors are not created to be caught, there is no need for subclasses to distinguish the errors. Instead subclasses have been created in order to make groups of related errors easy to create with consistent error messages. For example, the String.contains method will use a RangeError if its startIndex isn't in the range 0..length, which is easily created by RangeError.range(startIndex, 0, length). Catching specific subclasses of Error is not intended, and shouldn't happen outside of testing your own code.

Implementers

• AbstractClassInstantiationError
• ArgumentError
• AssertionError
• AsyncError
• ConcurrentModificationError
• JsonUnsupportedObjectError
• NoSuchMethodError
• OutOfMemoryError
• ParallelWaitError<V, E>
• RemoteError
• StackOverflowError
• StateError
• TypeError
• UnimplementedError
• UnsupportedError

Constructors

Error()

Error()

Implementation

Error();

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;

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

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.

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() inherited

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.

Inherited from Object.

Implementation

external String toString();

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

safeToString()

String safeToString(Object? object)

Safely convert a value to a String description.

The conversion is guaranteed to not throw, so it won't use the object's toString method except for specific known and trusted types.

Implementation

static String safeToString(Object? object) {
  if (object is num || object is bool || null == object) {
    return object.toString();
  }
  if (object is String) {
    return _stringToSafeString(object);
  }
  return _objectToString(object);
}

throwWithStackTrace()

Never throwWithStackTrace(Object error, StackTrace stackTrace)

Throws error with associated stack trace stackTrace.

Behaves like throw error would if the StackTrace.current was stackTrace at the time of the throw.

Like for a throw, if error extends Error, and it has not been thrown before, its Error.stackTrace property will be set to the stackTrace.

This function does not guarantee to preserve the identity of stackTrace. The StackTrace object that is caught by a try/catch of this error, or which is set as the Error.stackTrace of an error, may not be the same stackTrace object provided as argument, but it will have the same contents according to StackTrace.toString.

Implementation

@Since("2.16")
static Never throwWithStackTrace(Object error, StackTrace stackTrace) {
  checkNotNullable(error, "error");
  checkNotNullable(stackTrace, "stackTrace");
  _throw(error, stackTrace);
}