Building a Real-time Push App with Silverlight: Part 5

I planned on this post to be about UI, but I’m going to defer that until the next post. I said from the start of this series that I would document about everything about building the application from scratch, including my struggles.

And with that I want to mention something that got me scratching my head one too many times. It was with how I used LinqToTwitter. Here is the source code which you can immediately copy/paste into a blank project to reproduce:

public partial class MainPage : UserControl
    private readonly TwitterContext _context = new TwitterContext();
    private readonly ViewModel _vm1, _vm2, _vm3;

    public MainPage()
        _vm1 = new ViewModel(_context);
        _vm2 = new ViewModel(_context);
        _vm3 = new ViewModel(_context);
        _vm1.Callback += () => Debug.WriteLine("Callback of VM1: " + _vm1.LocalState);
        _vm2.Callback += () => Debug.WriteLine("Callback of VM2: " + _vm2.LocalState);
        _vm3.Callback += () => Debug.WriteLine("Callback of VM3: " + _vm3.LocalState);


public class ViewModel
    private readonly TwitterContext _context;
    public event Action Callback;

    public int LocalState;

    public ViewModel(TwitterContext context)
        _context = context;

    public void Start()
        var query = (from s in _context.Status
                where s.Type == StatusType.Public && s.Count == 10
                select s);
        Debug.WriteLine("Hash code of ViewModel: " + query.GetHashCode());
        query.AsyncCallback(statuses =>
                Debug.WriteLine("Hash code inside callback: " + GetHashCode());

Now, if you run this, you will see that only one of the view models will get its state updated. Huh?!

How is that possible? I started getting paranoid so I even added the local state variable “just in case.”

Well, I had to look into the source code of LinqToTwitter to figure out exactly what happened. Here is the code for AsyncCallback:

public static IQueryable<T> AsyncCallback<T>(this IQueryable<T> queryType, Action<IEnumerable<T>> callback)
     (queryType.Provider as TwitterQueryProvider)
         .AsyncCallback = callback;

     return queryType;

See what happened? The callback gets overwritten every time you call this method. Even though the call to FirstOrDefault() causes all 3 expressions to evaluate, only the last view model will get values because that’s the with the callback attached.

Lesson of the day: The AsyncCallback extension method for LinqToTwitter is not thread-safe.

So…the question is, how do we make it thread safe? I just replaced wrapped the AsyncCallback with another extension method:

private static readonly AutoResetEvent _twitterEvt = new AutoResetEvent(true);
public static void AsyncTwitterCallback<T>(this IQueryable<T> twitter, Action<IEnumerable<T>> callback)
    Observable.Start(() =>
        twitter.AsyncCallback(results =>

Nothing complicated – just a simple wait handle to ensure only 1 thread can go through at a time.

Hopefully upstream fixes this, or at least documents it.

comments powered by Disqus