How to perform runtime type checking in Dart?

Question

Dart specification states:

Reified type information reflects the types of objects at runtime and may always be queried by dynamic typechecking constructs (the analogs of instanceOf, casts, typecase etc. in other languages).

Sounds great, but there is no instanceof-like operator. So how do we perform runtime type-checking in Dart? Is it possible at all?

Solution 1

The instanceof-operator is called is in Dart. The spec isn't exactly friendly to a casual reader, so the best description right now seems to be http://www.dartlang.org/articles/optional-types/.

Here's an example:

class Foo { }

main() {
  var foo = new Foo();
  if (foo is Foo) {
    print("it's a foo!");
  }
}

Solution 2

Dart Object type has a runtimeType instance member (source is from dart-sdk v1.14, don't know if it was available earlier)

class Object {
  //...
  external Type get runtimeType;
}

Usage:

Object o = 'foo';
assert(o.runtimeType == String);

Solution 3

As others have mentioned, Dart's is operator is the equivalent of Javascript's instanceof operator. However, I haven't found a direct analogue of the typeof operator in Dart.

Thankfully the dart:mirrors reflection API has recently been added to the SDK, and is now available for download in the latest Editor+SDK package. Here's a short demo:

import 'dart:mirrors'; 

getTypeName(dynamic obj) {
  return reflect(obj).type.reflectedType.toString();
}

void main() {
  var val = "\"Dart is dynamically typed (with optional type annotations.)\"";
  if (val is String) {
    print("The value is a String, but I needed "
        "to check with an explicit condition.");
  }
  var typeName = getTypeName(val);
  print("\nThe mirrored type of the value is $typeName.");
}

Solution 4

There are two operators for type testing: E is T tests for E an instance of type T while E is! T tests for E not an instance of type T.

Note that E is Object is always true, and null is T is always false unless T===Object.

Solution 5

Exact type matching is done via runtimeType property. Checking if an instance or any of its parent types (in the inheritance chain) is of the given type is done via is operator:

class xxx {}

class yyy extends xxx {}

void main() {
  var y = yyy();
  
  print(y is xxx);
  print(y.runtimeType == xxx);
}

Returns:

true
false

View more solutions

Comments

[Idolon]

Looks like there is no mention of is operator at all in the specification. It's better to refere to the grammar file in Dart sources: code.google.com/p/dart/source/browse/trunk/dart/language/…

[Duncan]

@Idolon, the is operator is defined on page 59 of the spec, section 10.30 'Type test'

[Günter Zöchbauer]

RuntimeType is only for debugging purposes and the application code shouldn't depend on it. It can be overridden by classes to return fake values and probably returns unusable values when transpiled to JS

[sbedulin]

Thanks for your remark, I'm pretty new to Dart, and I agree that runtimeType may be overriden by classes, although I can't think of a reason why they would. (external code can't set the value sinse it's a getter) Personally, I would stick to is and reflection.

[Günter Zöchbauer]

It's fine this is mentioned here. It's not very obvious that runtimeType has these limitations.

[Matt C]

Could you explain what is meant by by T===Object? Dart doesn't have the triple equals operator, but you chose to use it rather than double equals, so I assume the difference has significance.

[Matt C]

Gunter, is it still the case that runtimeType should only be used for debugging purposes? I ask because there isn't any mention of this in the docs for Object, or elsewhere (that I could find).

[Duncan]

@MattC That was written more than 7 years ago! I think what I meant was null is Object would be true but null is T false for any other type T. tbh though I haven't been near Dart for many years now so can't be certain.

[Spyryto]

@MattC yes, I was using runtimeType to generate HTML classes on the fly, it works with development build, but fails with release build, e.g. in my case result is minified:ed instead of column

[Lii]

Is Dart a statically typed language?

[SoftWyer]

is and is! can be found in the Operators section of the Dart language tour.

[Mahdi Imani]

it is good solution but, we have error: Unsupported operation: dart:mirrors is no longer supported for web apps

[Mahdi Imani]

new syntax is getTypeName(dynamic obj) => obj.runtimeType;

[atreeon]

!= but is!...confuses me not it does

[Rob]

@Lii This answer was written for Ecma TC52. See dart.dev/faq

[vovahost]

@GünterZöchbauer comment is no longer true in Dart 2. It should be fine to use it now.

[Günter Zöchbauer]

@vovahost I'm mot aware of any related changes. Why do you think my comment is obdolete?

[vovahost]

@GünterZöchbauer I'm not expert on Dart 2 differences. I commented based on this discussion ibb.co/WcVJzhz ibb.co/zmgq168 Let me know what you think.

[Günter Zöchbauer]

I'm not sure why he thinks it's ok in Dart 2. Perhaps I missed something.

[TryHard]

Is there a way to use the runtimeType to check whether a particular named constructor was used?

[Yudhishthir Singh]

The Dart Language Tour has sections on equality operators: dart.dev/guides/language/… and type operators: dart.dev/guides/language/language-tour#type-test-operators

[Ian]

Be aware that Flutter, if you're using that, disables reflection (because it breaks tree shaking).

[Eric Aya]

As with most programming language, you don't need to add == true, the expression itself resolves to a Boolean. You just need if (thing is int) - and then you see that your solution is actually the same as the accepted answer.

[Muhammad Qasim]

How to use this operator to check Maps because maps can have several types like <Type, type> etc?

[derekantrican]

Make sure you don't combine the two different methods here and end up with foo.runtimeType is String that screwed me up for a while.