In Dart, syntactically nice way to cast dynamic to given type or return null?

Question

I have a dynamic x and I would like to assign x to T s if x is T, and otherwise assign null to s. Specifically, I would like to avoid having to type x twice, and to avoid creating a temporary. (For example, I don't want to have to write String s = map['key'] is String ? map['key'] : null; over and over, because I will have many such expressions.) I don't want there to be any possibility of a runtime error.

The following works:

class Cast<T> {   T f(x) {     if (x is T) {       return x;     } else {       return null;     }   } }  // ...  dynamic x = something(); String s = Cast<String>().f(x); 

Is there a syntactically nicer way to do this?

Answer 1

Dart 2 has generic functions which allows

T? cast<T>(x) => x is T ? x : null; 

dynamic x = something(); String s = cast<String>(x); 

you can also use

var /* or final */ s = cast<String>(x); 

and get String inferred for s

Answer 2

I use the following utility function, which allows for an optional fallback value and error logging.

T tryCast<T>(dynamic x, {T fallback}){     try{         return (x as T);     }     on CastError catch(e){         print('CastError when trying to cast $x to $T!');         return fallback;     } } 

var x = something(); String s = tryCast(x, fallback: 'nothing');