错误：无法在“用户？”上访问属性“uid”因为它可能为空

Question

嗨，flutter 和 firebase 新手，我想做的是简单地注册新用户，然后使用自定义 id 将其信息添加到 cloud firestore 并添加子集合名称“通知” 这里是代码示例..

final _auth = FirebaseAuth.instance;
  final _ref = FirebaseFirestore.instance;
  final userCollections = FirebaseFirestore.instance.collection("users");
  Future<void> SignupToColud(
      {@required String name = "",
      @required String email = "",
      @required String pass = ""}) async {
    try {
      await _auth
          .createUserWithEmailAndPassword(email: email, password: pass)
          .then((value) {
        String userId = value.user.uid;
        if (userId != null) {
          userCollections.doc(userId).set({'name': name, 'email': email});
          userCollections
              .doc(userId)
              .collection("notifications")
              .add({'txt': 'welcome you registered as new user succefully'});
        } else {
          throw Error();
        }
      });
      // first sign up to cloud if success

      //then add to users collections
    } catch (e) {
      print(e.toString());
    }
  }

i got this error !!
    lib/Sign_up.dart:24:36: Error: Property 'uid' cannot be accessed on 'User?' because it is potentially null.
 - 'User' is from 'package:firebase_auth/firebase_auth.dart' ('/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_auth-1.2.0/lib/firebase_auth.dart').
Try accessing using ?. instead.
        String userId = value.user.uid;
                               ^^^

this is pubspec.yaml file information
environment:
sdk: ">=2.12.0 <3.0.0"
dependencies:

    lib/Sign_up.dart:24:36: Error: Property 'uid' cannot be accessed on 'User?' because it is potentially null.
 - 'User' is from 'package:firebase_auth/firebase_auth.dart' ('/C:/src/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_auth-1.2.0/lib/firebase_auth.dart').
Try accessing using ?. instead.
        String userId = value.user.uid;
                               ^^^

我真的不知道这个问题以及如何解决它:(

Answer 1

错误来自这一行：

  flutter:
    sdk: flutter
  cupertino_icons: ^1.0.0
  firebase_core: ^1.2.0
  cloud_functions: ^1.1.0
  cloud_firestore: ^2.2.0
  firebase_storage: ^8.1.0
  firebase_auth: ^1.2.0

正如错误所说，

String userId = value.user.uid;

是一个

value.user

，这意味着它可以是一个

User?

对象或它可以是

User

。您的代码需要处理可能为空的事实，因为只有您可以决定在这种情况下做什么。

我想你想要什么：

null

因此，这会检查

  _auth
      .createUserWithEmailAndPassword(email: email, password: pass)
      .then((value) {
    if (value.user != null) {
      String userId = value.user!.uid;
      userCollections.doc(userId).set({'name': name, 'email': email});

是否为空（而不是检查

value.user

）。如果

userId

不为空，则保证

value.user

有一个值，因此我们可以在写入操作中使用它。

Answer 2

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