Sdk: Directly passing a List to a method fails but wrapping it into a variable works.

[dark-chocolate]

Some magic is going on which is shaking my basics:

void main() {
  method(['1', '2']); // fails

  final list = ['1', '2'];
  method(list); // works
}

void method(List<Object> objects) {
  List<String> strings = objects;
}

Can anyone explain the reason behind this magic?

[lrhn]

It's type inference and implicit downcast.

When you do method(['1', '2']);, the signature of method asks for a List<Object>.
So, the ['1', '2'] is evaluated in a context where a List<Object> is expected, and it therefore creates a List<Object> containing two strings. Then, inside method, you try to downcast that to a List<String>. It fails because a List<Object> is-not-a List<String>.

When you write final list = ['1', '2']; you evaluate ['1', '2'] in a context with no type expectation.
The list literal contains two strings, and absent any other type hint, it creates a List<String>.
You then pass that to method as a List<Object>, which succeeds because a List<String> is-a List<Object>, and method then down-casts it to List<String> again, which works because it is one.

[dark-chocolate]

Thank you, and that makes sense!

However, I am bit disappointed by Dart team members, you have a team of great developers, why don't you allow some of them to answer questions on SO, I know there is one great person @jamesderlin who keeps on answering my questions but he is also a human being, he may sometimes forget to read all the Dart questions (that actually need answer from expert like you guys).

Or should I start posting questions here on Github issues because there exists an answer to all the questions which only you guys can answer and not the other SO users (non-Google members). So, in case my question isn't answered on SO within 2 days, is this the right place to ask? By the way here is the question link

[lrhn]

Some of us read and answer on Stack Overflow as well, but we are only human too :smile:.