mobile-windows-universal-dotnet-authenticate-app-with-token.md

1. In the MainPage.xaml.cs project file, add the following using statements:

    using System.Linq;        
    using Windows.Security.Credentials;
   

2. Replace the AuthenticateAsync method with the following code:

    private async System.Threading.Tasks.Task<bool> AuthenticateAsync()
    {
        string message;
        bool success = false;
   
        // This sample uses the Facebook provider.
        var provider = MobileServiceAuthenticationProvider.Facebook;
   
        // Use the PasswordVault to securely store and access credentials.
        PasswordVault vault = new PasswordVault();
        PasswordCredential credential = null;
   
        try
        {
            // Try to get an existing credential from the vault.
            credential = vault.FindAllByResource(provider.ToString()).FirstOrDefault();
        }
        catch (Exception)
        {
            // When there is no matching resource an error occurs, which we ignore.
        }
   
        if (credential != null)
        {
            // Create a user from the stored credentials.
            user = new MobileServiceUser(credential.UserName);
            credential.RetrievePassword();
            user.MobileServiceAuthenticationToken = credential.Password;
   
            // Set the user from the stored credentials.
            App.MobileService.CurrentUser = user;
   
            // Consider adding a check to determine if the token is 
            // expired, as shown in this post: http://aka.ms/jww5vp.
   
            success = true;
            message = string.Format("Cached credentials for user - {0}", user.UserId);
        }
        else
        {
            try
            {
                // Login with the identity provider.
                user = await App.MobileService
                    .LoginAsync(provider);
   
                // Create and store the user credentials.
                credential = new PasswordCredential(provider.ToString(),
                    user.UserId, user.MobileServiceAuthenticationToken);
                vault.Add(credential);
   
                success = true;
                message = string.Format("You are now logged in - {0}", user.UserId);
            }
            catch (MobileServiceInvalidOperationException)
            {
                message = "You must log in. Login Required";
            }
        }
   
        var dialog = new MessageDialog(message);
        dialog.Commands.Add(new UICommand("OK"));
        await dialog.ShowAsync();
   
        return success;
    }
   

   In this version of AuthenticateAsync, the app tries to use credentials stored in the PasswordVault to access the service. A regular sign-in is also performed when there is no stored credential.

   [!NOTE] A cached token may be expired, and token expiration can also occur after authentication when the app is in use. To learn how to determine if a token is expired, see Check for expired authentication tokens. For a solution to handling authorization errors related to expiring tokens, see the post Caching and handling expired tokens in Azure Mobile Services managed SDK.

3. Restart the app twice.

   Notice that on the first start-up, sign-in with the provider is again required. However, on the second restart the cached credentials are used and sign-in is bypassed.