Как использовать String в URL-адресе WebClient.DownloadStringAsync

В настоящее время у меня есть это для моего URL-адреса WebClient

WebClient Detail = new WebClient();
Detail.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Detail_DownloadStringCompleted);
Detail.DownloadStringAsync(new Uri("http://api.trademe.co.nz/v1/Listings/" + ListingID.Text + ".xml"));

Я хочу использовать эту строку:

void ListBoxSelectionChanged(object sender, SelectionChangedEventArgs args)
{
    var lbi = ((sender as ListBox).SelectedItem as TradeItem);
    if(lbi != null)
    {
        string id = lbi.ListingId.ToString();
    }
}

Как часть этого URL-адреса WbeClient.

Пример:

WebClient Detail = new WebClient();
Detail.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Detail_DownloadStringCompleted);
Detail.DownloadStringAsync(new Uri("http://api.trademe.co.nz/v1/Listings/" + id + ".xml"));

А есть ли вообще использовать эту строку в URL-адресе, как показано выше?

Полный код ################################## ##

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
using System.Xml.Linq;

namespace TradeMe_Panorama
{
    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();

            // Set the data context of the listbox control to the sample data
            DataContext = App.ViewModel;
            this.Loaded += new RoutedEventHandler(MainPage_Loaded);
        }

        // Load data for the ViewModel Items
        private void MainPage_Loaded(object sender, RoutedEventArgs e)
        {
            if (!App.ViewModel.IsDataLoaded)
            {
                App.ViewModel.LoadData();
            }
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            WebClient Trademe = new WebClient();
            Trademe.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Trademe_DownloadStringCompleted);
            Trademe.DownloadStringAsync(new Uri("http://api.trademe.co.nz/v1/Search/General.xml?search_string=" + TradeSearch.Text));

            progressBar1.IsIndeterminate = true;
            progressBar1.Visibility = Visibility.Visible;
        }
        // Display listing for used general products  ##################################################################

        void Trademe_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error != null)
                return;

            var r = XDocument.Parse(e.Result);

            // Declare the namespace
            XNamespace ns = "http://api.trademe.co.nz/v1";
            TradeSearch1.ItemsSource = from TM in r.Root.Descendants(ns + "Listing").Take(20)
                                   select new TradeItem
                                   {
                                       ImageSource = TM.Element(ns + "PictureHref").Value,
                                       Title = TM.Element(ns + "Title").Value,
                                       Region = TM.Element(ns + "Region").Value,
                                       PriceDisplay = TM.Element(ns + "PriceDisplay").Value,
                                       ListingId = TM.Element(ns + "ListingId").Value,
                                   };

            progressBar1.IsIndeterminate = false;
            progressBar1.Visibility = Visibility.Collapsed;
        }

        // Display listing for used Cars ##################################################################

        private void button2_Click(object sender, RoutedEventArgs e)
        {

            WebClient Motor = new WebClient();
            Motor.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Motor_DownloadStringCompleted);
            Motor.DownloadStringAsync(new Uri("http://api.trademe.co.nz/v1/Search/Motors/Used.xml?search_string=" + MotorSearch.Text));

            progressBar1.IsIndeterminate = true;
            progressBar1.Visibility = Visibility.Visible;
        }


        void Motor_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error != null)
                return;

            var r = XDocument.Parse(e.Result);

            // Declare the namespace
            XNamespace ns = "http://api.trademe.co.nz/v1";
            MotorsListings.ItemsSource = from M in r.Root.Descendants(ns + "Car").Take(20)
                                       select new TradeItem
                                       {
                                           ImageSource = M.Element(ns + "PictureHref").Value,
                                           Title = M.Element(ns + "Title").Value,
                                           Region = M.Element(ns + "Region").Value,
                                           PriceDisplay = M.Element(ns + "PriceDisplay").Value,
                                           ListingId = M.Element(ns + "ListingId").Value,
                                       };

            progressBar1.IsIndeterminate = false;
            progressBar1.Visibility = Visibility.Collapsed;
        }
        //Disaply Specfic Details of listings ########################################################################

         private void button4_Click(object sender, RoutedEventArgs e)
        {
            WebClient Detail = new WebClient();
            Detail.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Detail_DownloadStringCompleted);
            Detail.DownloadStringAsync(new Uri("http://api.trademe.co.nz/v1/Listings/" + ListingID.Text + ".xml"));

            progressBar1.IsIndeterminate = true;
            progressBar1.Visibility = Visibility.Visible;
        }




        void Detail_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error != null)
                return;

            var r = XDocument.Parse(e.Result);

            // Declare the namespace
            XNamespace ns = "http://api.trademe.co.nz/v1";
            ListingDetails.ItemsSource = from D in r.Descendants(ns + "ListedItemDetail").Take(20)
                                       select new TradeItem
                                       {
                                           ImageSource = D.Element(ns + "Photos").Element(ns + "Photo").Element(ns + "Value").Element(ns + "Medium").Value,
                                           Title = D.Element(ns + "Title").Value,
                                           Region = D.Element(ns + "Region").Value,
                                           PriceDisplay = D.Element(ns + "Body").Value,
                                           ListingId = D.Element(ns + "ListingId").Value,
                                           CloseDate = D.Element(ns + "EndDate").Value,
                                           BuyNow = D.Element(ns + "BuyNowPrice").Value,
                                           StartPrice = D.Element(ns + "StartPrice").Value,
                                       };

            progressBar1.IsIndeterminate = false;
            progressBar1.Visibility = Visibility.Collapsed;
        }



        public class TradeItem
        {
            public string Region { get; set; }
            public string ListingId { get; set; }
            public string PriceDisplay { get; set; }
            public string Title { get; set; }
            public string ImageSource { get; set; }
            public string CloseDate { get; set;  }
            public string StartPrice { get; set; }
            public string BuyNow { get; set; }
        }



        // Run query string to pass LIstingID to next page



    }
}