// Sample call: UploadFileInBackground("http://www.contoso.com/fileUpload.aspx", "data.txt")
public static void UploadFileInBackground (string address, string fileName)
{
    System.Threading.AutoResetEvent waiter = new System.Threading.AutoResetEvent (false);
    WebClient client = new WebClient ();
    Uri uri = new Uri(address);
    string method = "POST";

    // Specify that that UploadFileCallback method gets called
    // when the file upload completes.
    client.UploadFileCompleted += new UploadFileCompletedEventHandler (UploadFileCallback);
    client.UploadFileAsync (uri, method, fileName, waiter);

    // Block the main application thread. Real applications
    // can perform other tasks while waiting for the upload to complete.
    waiter.WaitOne ();
    Console.WriteLine ("File upload is complete.");
}

private static void UploadFileCallback (Object sender, UploadFileCompletedEventArgs e)
{
    System.Threading.AutoResetEvent waiter = (System.Threading.AutoResetEvent)e.UserState;;
    try
    {
        string reply = System.Text.Encoding.UTF8.GetString (e.Result);

        Console.WriteLine (reply);
    }
    finally
    {
        // If this thread throws an exception, make sure that
        // you let the main application thread resume.
        waiter.Set ();
    }
}