Unauthorizedaccessexception{"Invalid cross-thread access."}...is occur - windows-phone-7

Question

i want to short my url with bitly but an exception is occur when i want to set out string to my text block
private void button1_Click(object sender, RoutedEventArgs e)
{
ShortenUrl(textBox1.Text);
}
enum Format
{
XML,
JSON,
TXT
}
enum Domain
{
BITLY,
JMP
}
void ShortenUrl(string longURL)
{
Format format = Format.XML;
Domain domain = Domain.BITLY;
string _domain;
//string output;
// Build the domain string depending on the selected domain type
if (domain == Domain.BITLY)
_domain = "bit.ly";
else
_domain = "j.mp";
HttpWebRequest request = (HttpWebRequest)WebRequest.Create(
string.Format(#"http://api.bit.ly/v3/shorten?login={0}&apiKey={1}&longUrl={2}&format={3}&domain={4}",
"username", "appkey", HttpUtility.UrlEncode(longURL), format.ToString().ToLower(), _domain));
request.BeginGetResponse(new AsyncCallback(GetResponse), request);
}
void GetResponse(IAsyncResult result)
{
XDocument doc;
HttpWebRequest request = (HttpWebRequest)result.AsyncState;
HttpWebResponse response = (HttpWebResponse)request.EndGetResponse(result);
using (StreamReader reader = new StreamReader(response.GetResponseStream()))
{
string responseString = reader.ReadToEnd();
doc = XDocument.Load(reader.BaseStream);
}
//// var x = from c in doc.Root.Element("data").Elements()
// where c.Name == "url"
// select c;
//XElement n = ((IEnumerable<XElement>)x).ElementAt(0);
// textBox2.Text = ((IEnumerable<String>)x).ElementAt(0);
lista = (from Born_rich in doc.Descendants("url")
select new a()
{
shrtenurl = Born_rich.Value
}).ToList();
output = lista.ElementAt(0).shrtenurl;
textBox2.Text = output;
//
//
// textBox2.Text = s;
}
List<a> lista = new List<a>();
String output;
}
public class a
{
public String shrtenurl { set; get; }
}

Answer 1

The calback from HttpWebRequest occurs on a non-UI thread. If you want to change soemthing in the UI you must do it on the UI thread. Fortunatley there is an easy way to do this. You simply use the dispatcher to invoke the code in question on the UI.
Dispatcher.BeginInvoke(() => textBox2.Text = output);