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. S

Answer 1

Just use the as keyword

final tweet = tweets[index] as Tweet;

Answer 2

A combination of both prior two posts, without the logging.

Fallback defaults to null when not provided.

T cast<T>(dynamic x, {T fallback}) => x is T ? x : fallback;

Answer 3

EDIT

Figured out my answer had a huge problem, type check was incorrect. For example T is num returns false even if T is passed num. I temporarily fixed this issue for now by explicitly checking equality of T with the desired type like T == int && T == double but this might not work for checking subclassed types like T == num. Overall I edited my answer so it would work as is now. Although the answer is ongoing work and will be improved better when I find a solution for this type checking problem.

---

Here's my modified version that supports parsing num (i.e. int, double) and DateTime within the tryCast:

Usage

var decimalNumber = tryCast<double>("42.42", fallback: 0);
var integerNumber = tryCast<int>("42", fallback: 0);
var currentTimestamp = tryCast<DateTime>(DateTime.now().toIso8601String(), fallback: DateTime.now());
var someType = tryCast<SomeType>(jsonString['someType'], fallback: null);

Solution

/// Casts `x` of [dynamic] type to [T] type. With fallback value `fallback` if casting failed
/// Supports tryParse from [String] to {[num],[DateTime], [Uri]}
/// Edited from [this ](https://stackoverflow.com/a/59714798/10830091) StackOverflow answer
T tryCast<T>(dynamic x, {T fallback}) {

  // tryParse from [String] `x`
  if (x is String) {
    if (T == int || T == double) {
      // tryParse to [num] (i.e. [int], [double])
      return num.tryParse(x) as T ?? fallback;
    } else if (T == DateTime) {
      // tryParse to [DateTime]
      return DateTime.tryParse(x) as T ?? fallback;
    } else if (T == Uri) {
      // tryParse to [Uri]
      return Uri.tryParse(x) as T ?? fallback;
    }
  }

  try {
    return (x as T) ?? fallback;
  } on CastError catch (e) {
    print('CastError when trying to cast $x to $T! Exception catched: $e');
    return fallback;
  }
}

Inspired from Magnus's answer.

Answer 4

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 5

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');