http://ufcpp.net/study/csharp/sp5_async.html

DownloadString_Sync_Async.cs

private void btnSync_Click(object sender_, RoutedEventArgs e_)
{
    var client = new WebClient();
    client.Encoding = Encoding.UTF8;
    var json = client.DownloadString(this.txtUrl.Text);
    this.txtJson.Text = json;
}

private void btnAsync_Click(object sender_, RoutedEventArgs e_)
{
    var client = new WebClient();
    client.Encoding = Encoding.UTF8;
    client.DownloadStringCompleted += (sender, e) =>
    {
        this.txtJson.Text = e.Result;
    };
    client.DownloadStringAsync(new Uri(this.txtUrl.Text));
}