How to use String in WebClient.DownloadStringAsync URL

I currently have this for my WebClient URL:

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

What I want to do is use this string:

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

As part of that WebClient URL.

Example:

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

Is there anyway to use this string in URL as show above?

Complete code:

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)
            s开发者_如何学编程elect 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;
        }

        //Display specific 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.
    }
}

---

another way it to use Task:

using (WebClient client = new WebClient())
{                
    result = await client.DownloadStringTaskAsync(urlString);
}

---

This is what you need to do:

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

void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
    string text = e.Result;
    // … do something with result.
}

There you will have your text.

---

Are they on the same page? If so, then you can make the id variable a class-level variable so that it can be accessed from anywhere in the class. Alternatively, you could put the WebClient section into a method that accepts a string. For example:

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

Then in that ListboxSelectionChanged handler, simply call the above method.

if(lbi != null)
{
    string id = lbi.ListingId.ToString();
    DownloadInformation(id);
}

As a side note, it's probably best to have the WebClient set up outside the method, such as in the Page's Loaded event. This way, a new instance won't have to be created each time.