WP7 Propogate exception from BeginGetResponse callback

Question

I'm using HttpWebRequest to call a web service. If the AsyncCallback from BeginGetResponse throws an error, I want to propagate it up to my main program flow. I'm having trouble doing this because the error doesn't get propagated beyond the AsyncCallback. I've tried putting try/catch blocks at each step of the HttpWebRequest chain, but it never propagates beyond the "ResponseCallBack" method. Is it possible to get it back to the main thread?

private void StartRequest()
{
    // Code to create request object is here
    // ...

    httpRequest.BeginGetRequestStream(new AsyncCallback(GetRequestStreamCallback), httpRequest);
}

---

private void GetRequestStreamCallback(IAsyncResult result)
{
    HttpWebRequest request = (HttpWebRequest)result.AsyncState;

    // End the operation
    var postStream = request.EndGetRequestStream(result);
    string body = GenerateRequestBody();

    // Convert the string into a byte array
    byte[] postBytes = Encoding.UTF8.GetBytes(body);

    // Write to request stream
    postStream.Write(postBytes, 0, postBytes.Length);
    postStream.Close();

    // Start the asynchronous operation to get the resonse
    try
    {
        request.BeginGetResponse(new AsyncCallback(ResponseCallback), request);
    }
    catch (Exception)
    {
        throw;
    }
}

---

private void ResponseCallback(IAsyncResult result)
{
    string contents = String.Empty;
    HttpWebRequest request = (HttpWebRequest)result.AsyncState;
    HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(result);

    using (Stream stream = response.GetResponseStream())
    using (StreamReader reader = new StreamReader(stream))
    {
        contents = reader.ReadToEnd();
    }

    // Check the status
    if (response.StatusCode == HttpStatusCode.OK)
    {
        //EXCEPTION NEVER MAKES IT PASSED HERE, EVEN IF I HAVE IT IN A TRY/CATCH BLOCK AND RE-THROW IT.
        _object = ProcessResponseEntity(contents);
    }
}

Answer 1

I think you're getting confused about the way asynchronous code exectution works and how the callback execution fits in with the calling code.

Within GetRequestStreamCallback , after the call to request.BeginGetResponse the method will continue to execute and in your example just end.

It is not known when (or even if) ResponseCallback will execute or what will be happening on the UI thread when it does. Because of this, ResponseCallback will execute on a different thread.

It's possible to have code within the callback run on the UI thread (which you'll need to do you want to interact with the UI) by using Dispatcher.BeginInvoke . However you can't have this execute within the context of another method.

While I wouldn't recommend it, you may want to have a look at this discussion on making the callback appear to execute synchronously. This will block your UI thread though and so is NOT recommended.