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?

Answer 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!");
  }
}

Answer 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);

Answer 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.");
}

Answer 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.

Answer 5

object.runtimeType returns the type of object

For example:

print("HELLO".runtimeType); //prints String
var x=0.0;
print(x.runtimeType); //prints double

Answer 6

A small package can help solve some problems.

import 'dart:async';

import 'package:type_helper/type_helper.dart';

void main() {
  if (isTypeOf<B<int>, A<num>>()) {
    print('B<int> is type of A<num>');
  }

  if (!isTypeOf<B<int>, A<double>>()) {
    print('B<int> is not a type of A<double>');
  }

  if (isTypeOf<String, Comparable<String>>()) {
    print('String is type of Comparable<String>');
  }

  var b = B<Stream<int>>();
  b.doIt();
}

class A<T> {
  //
}

class B<T> extends A<T> {
  void doIt() {
    if (isTypeOf<T, Stream>()) {
      print('($T): T is type of Stream');
    }

    if (isTypeOf<T, Stream<int>>()) {
      print('($T): T is type of Stream<int>');
    }
  }
}

Result:

B<int> is type of A<num> B<int> is not a type of A<double> String is type of Comparable<String> (Stream<int>): T is type of Stream (Stream<int>): T is type of Stream<int>