Windows Phone Background Task - Using HttpWebRequest or WebClient

Question

I need to send information to server using WCF. I am currently using WebClient to call WCF with json data. Now, with background task, I am calling the same WCF with json, but the callback function of UploadStringAsync never gets called. I also tried HttpWebRequest, but it is not working too.

I can see in the documentation that HttpWebRequest is supported in Background Tasks.

Below is the code that is handling WCF request/response:

public class Communication
{
    #region Private Variables

    /// <summary>
    /// Callback method passed to MakeHttpPostRequest will be set to below variable. 
    /// This variable holds the reference to callback function and used to invoke the method passed by MakeHttpPostRequest calling method.
    /// </summary>
    private Action<string> action;

    private Action<string, object> genericAction;

    private object returnValue;

    #endregion

    #region Methods

    /// <summary>
    /// Calls WCF service using POST method.
    /// </summary>
    /// <param name="webserviceURL">URL of WCF service.</param>
    /// <param name="json">JSON data to be posted to WCF service.</param>
    /// <param name="response">Callback function that is invoked when response is received from WCF service.</param>
    public void MakeHttpPostRequest(string webserviceURL, string json, Action<string> response)
    {
        try
        {
            this.action = response;

            if (DeviceNetworkInformation.IsNetworkAvailable)
            {
                Uri uri = new Uri(webserviceURL);
                byte[] byteArray = System.Text.Encoding.UTF8.GetBytes(json);

                string data = Encoding.UTF8.GetString(byteArray.ToArray(), 0, (int)byteArray.Length);
                WebClient webClient = new WebClient();
                webClient.UploadStringCompleted += new UploadStringCompletedEventHandler(this.WebClient_UploadStringCompleted);
                webClient.Headers["Content-type"] = "application/json";
                webClient.Encoding = Encoding.UTF8;
                webClient.UploadStringAsync(uri, "POST", data);
            }
            else
            {
                if (this.action != null)
                {
                    this.action(string.Empty);
                }
            }
        }
        catch (Exception ex)
        {
            if (this.action != null)
            {
                this.action(string.Empty);
            }

            new ErrorException.ErrorException().HandleError(ex, string.Empty, Global.Modules.General);
        }
    }

    #endregion

    #region Events

    /// <summary>
    /// Callback function that gets called when response is received from web service.
    /// </summary>
    /// <param name="sender">The object that raises the event.</param>
    /// <param name="e">Object containing Http response details.</param>
    private void WebClient_UploadStringCompleted(object sender, UploadStringCompletedEventArgs e)
    {
        try
        {
            // Check whether to invoke any method
            if (this.action != null)
            {
                // Invoke the method passed to MakeHttpPostRequest by it's calling method
                this.action(e.Result);
            }
            else if (this.genericAction != null)
            {
                // Invoke the method passed to MakeHttpPostRequest by it's calling method
                this.genericAction(e.Result, this.returnValue);
            }
        }
        catch (Exception ex)
        {
            if (this.action != null)
            {
                this.action(string.Empty);
            }

            new ErrorException.ErrorException().HandleError(ex, string.Empty, Global.Modules.General);
        }
    }

    #endregion
}

And using below code to send json to server:

// Send location data to server
new Common.Communication().MakeHttpPostRequest(Common.ServiceURL.TrackingTracingURL, postData, result);

Above code is working fine from application. But, does not work when called from Background Task.

---

There wasn't any problem with HttpWebRequest or WebClient. It was problem with calling:

NotifyComplete();

As calls on HttpWebRequest or WebClient are async, calling NotifyComplete(); was aborting execution of background task bafore response is received and was not waiting for HttpWebRequest or WebClient response.

Does anybody have workaround for this?

Answer 1

As Paul mentioned in his comment, it's likely that your background task is being terminated after 25 seconds of inactivity. If your task is forcefully terminated 3 (?) times it will be unscheduled until your application schedules it again (I believe they can also get perma-banned if it keeps up, but not I'm 100% sure on this).

Edit

NotifyComplete(); can occur in an asynchronous callback. Just move it to the end of your callback, after you've finished processing the response.