无法将“OAuthCredential”类型的值分配给“GoogleAuthCredential”类型的变量

Question

This is my code for google sign-in..

             onPressed: () async{
                final GoogleSignInAccount newuser= await GoogleSignIn().signIn();
                final  GoogleSignInAuthentication newuserauth= await 
         googleUser.authentication;
                final GoogleAuthCredential cred= GoogleAuthProvider.credential(accessToken: 
                 newuserauth.accessToken,idToken: newuserauth.idToken);
                await FirebaseAuth.instance.signInWithCredential(cred);
              },

And the errors i am getting are as follows..

        error: A value of type 'OAuthCredential' can't be assigned to a variable of type 
        'GoogleAuthCredential'. (invalid_assignment at [firebase] lib\firstpage.dart:147)

        error: Undefined name 'googleUser'. (undefined_identifier at [firebase] 
            lib\firstpage.dart:145)
         error: A value of type 'GoogleSignInAccount?' can't be assigned to a variable of type 
        'GoogleSignInAccount'. (invalid_assignment at [firebase] lib\firstpage.dart:144)

These are the dependencies in my pubspec.yaml..

      dependencies:
        flutter:
          sdk: flutter
           firebase_auth: ^3.2.0
           firebase_core : ^1.10.0
           flutter_spinkit: ^5.1.0
           cloud_firestore: ^3.1.0
           google_sign_in: ^5.2.1

Answer 1

There is multiple problems in you code as follows:

Line 2: GoogleSignIn().signIn() returns GoogleSignInAccount? which mean it could be null but you're using GoogleSignInAccount which mean it can't be null,,, so change it to (This is the last error you got):

   final GoogleSignInAccount? newuser= await GoogleSignIn().signIn();

Line 3 & 4: You used the variable name newuser not googleUser change one of them (the second error)

Line 5 & 6: GoogleAuthProvider.credential(..) returns OAuthCredential not GoogleAuthCredential and that is the first error you got.

However, with final you don't need to specify the variable type which is one of the Dart advantages.

Moreover, you'll get error on newuser.authentication because newuser, as mentioned before, could be null so you can't access authentication ... the way I love to do it, because I don't like to deal with null values is to return from the function before using it if its null.

So the whole code would be (I added the types so you could see the difference, but you don't need them ):

final GoogleSignInAccount? googleUser = await GoogleSignIn().signIn();
if (googleUser == null) return null;
  
final GoogleSignInAuthentication googleAuth = await googleUser.authentication;
final OAuthCredential credential = GoogleAuthProvider.credential(
    accessToken: googleAuth.accessToken,
    idToken: googleAuth.idToken,
  );
await FirebaseAuth.instance.signInWithCredential(credential);