how to check if user token already exists before signin anonymously ? about firebase-authentication-dotnet

[cabauman]

The author may have a more general solution, but I'll go ahead and put in my two cents.

Since you're using Xamarin, you can use the AccountStore in the Xamarin.Auth package:
Github
Docs

When saving, you can add your own properties to the account. For example, I have a CredentialsService class with a property to save the FirebaseAuth json:

public string FirebaseAuthJson
{
    get
    {
        return _account?.Properties["FirebaseAuthJson"];
    }

    set
    {
        if(_account != null)
        {
            _account.Properties["FirebaseAuthJson"] = value;
            AccountStore.Create().Save(_account, SERVICE_ID);
        }
    }
}

So you can set it like this:

string json = JsonConvert.SerializeObject(firebaseAuth);
credentialsService.FirebaseAuthJson = json;

And retrieve like this:

var account = AccountStore.Create().FindAccountsForService(App.AppName).FirstOrDefault();
if(account == null)
{
    SignInAnonymously();
}
else
{
    string json = account.Properties ["FirebaseAuthJson"];
    FirebaseAuth  auth = JsonConvert.DeserializeObject<FirebaseAuth>(json);
    var authProvider = new FirebaseAuthProvider(new FirebaseConfig(Config.ApiKeys.FIREBASE));
    FirebaseAuthLink  authLink = new FirebaseAuthLink(authProvider, auth);
    ...
    // Now you can do some more verification
}

[bezysoftware]

That's pretty much what I do - I save the user information into storage (ApplicationDataContainer in UWP) and during startup I check if there is any info there.

[EmilAlipiev]

thats not a correct approach, i believe. FirebaseToken is a JWT and it contains expiry date information or even though sigin functions have expiry date information. you should verify that it is not expired. if expired refresh token. even though, token can be invalidated for different reason before expiry dates. for example, if user changes, his password or deleting account on other device etc. better to always user verify token function. it doesnt have to be because if token is invalid, other actions like database access will return an exception. but at least do offline expiry date check

[cabauman]

@EmilAlipiev You're correct. My answer was outdated. You have to save the whole FirebaseAuth object because it contains the needed info for refreshing. And yes, it's good to do extra verification after the auth object is retrieved.

I edited my previous reply to reflect these points.