在 Dart 语言中，问号 (?) 标记和点 (.) 是什么意思？

Question

say I defined ...

final DocumentSnapshot doc;

variable doc might be null so I use a question mark and dot...

print(widget.doc); // null
print(widget.doc == null); // true
print(widget.doc?.data['name']);

why widget.doc?.data['name'] throw error Tried calling: []("name") instead returning null ?

for my understanding ?. to check whether null and if it so will return null

Answer 1

In the current version of Dart (2.3) Null-aware access doesn't short-circuit call chain.

So a?.bc will throw an exception if a is null because it's the same as (a != null ? ab : null).c .

In your case widget.doc?.data['name'] is the same as ((e) { return e != null ? e.data : null; }(widget.doc))['name'] .

To make your code working you need to introduce a variable.

var a = widget.doc?.data;
print(a == null ? null : a['name']);

NB: you may be interested by #36541: Map does not have a null-aware-chainable "get" method

Answer 2

To guard access to a property or method of an object that might be null, put a question mark ( ? ) before the dot ( . ):

myObject?.someProperty

The preceding code is equivalent to the following:

(myObject != null) ? myObject.someProperty : null

You can chain multiple uses of ?. together in a single expression:

myObject?.someProperty?.someMethod()

The preceding code returns null (and never calls someMethod() ) if either myObject or myObject.someProperty is null.

Code example Try using conditional property access to finish the code snippet below.

// This method should return the uppercase version of `str`
// or null if `str` is null.
String upperCaseIt(String str) {
  return str?.toUpperCase();
}