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

User · Answer

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

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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

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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

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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

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Just to clarify a bit the difference between is and runtimeType  As someone said already  and this was tested with Dart V2   the following code    class Foo      Type get runtimeType   gt  String    main       var foo   new Foo      if  foo is Foo        print  it s a foo           print  type is   foo runtimeType          will output   it s a foo   type is String   Which is wrong  Now  I can t see the reason why one should do such a thing

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object runtimeType returns the type of object  For example   print  HELLO  runtimeType     prints String var x 0 0  print x runtimeType     prints double

User · Answer

Simply call  print unknownDataType runtimeType   on the data

[dynamic] How to perform runtime type checking in Dart?

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