Using a filter in a C# Windows 8 application to automatically log a user back into the Mobile Service if any request returns a 401 (for example, due to a timeout of the token).

App.xaml.cs

// This is a modified App.xaml.cs file from the Mobile Services quickstart, showing how to add the AuthFilter above

private static MobileServiceClient GetClient()
{
    var client = new MobileServiceClient(
        "<your-app-url>",
        "<your-app-key>"
    );

    var filter = new AuthFilter();
    var filtered = client.WithFilter(filter);
    filter.Client = filtered;

    return filtered;
}

// This MobileServiceClient has been configured to communicate with your Mobile Service's url
// and application key. You're all set to start working with your Mobile Service!
public static MobileServiceClient MobileService = GetClient();

AuthFilter.cs

public class AuthFilter : IServiceFilter
{
    public MobileServiceClient Client { get; set; }

    private async System.Threading.Tasks.Task<IServiceFilterResponse> HandleAsync(
        IServiceFilterRequest request,
        IServiceFilterContinuation continuation)
    {
        IServiceFilterResponse response = await continuation.Handle(
            request).AsTask();

        while (response.StatusCode == 401)
        {
            // oh noes, user is not logged in - we got a 401
            // log them in, this time hardcoded Twitter but you 
            // would trigger your login presentation in your application
            
            // first, we need to log the user in
            try 
            {
                await Client.LoginAsync(MobileServiceAuthenticationProvider.Twitter);
            }
            catch (InvalidOperationException exc) 
            {
                // user cancelled auth, so lets return the original response
                return response;                
            }
            // then we want to replay the last failed request and update it's old auth header
            // to have the new auth token
            request.Headers["X-ZUMO-AUTH"] = Client.CurrentUser.MobileServiceAuthenticationToken;
            // try again!
            response = await continuation.Handle(request).AsTask();
        }

        return response;
    }

    public IAsyncOperation<IServiceFilterResponse> Handle(
      IServiceFilterRequest request, 
      IServiceFilterContinuation continuation)
    {
        return HandleAsync(request, continuation).AsAsyncOperation();
    }
}