get html content of a page with Silverlight

Question

Hi,

I'm trying to get the html content of a page using silverlight. Webresponse and request classes don't work in silverlight.

I did some googling and I found something. This is what i tried:

public partial class MainPage : UserControl
 {
  string result;
  WebClient client;

  public MainPage()
  {
   InitializeComponent();
   this.result = string.Empty;
   this.client = new WebClient();
   this.client.DownloadStringCompleted += ClientDownloadStringCompleted;
  }

  private void btn1_Click(object sender, RoutedEventArgs e)
  {
   string url = "http://www.nu.nl/feeds/rss/algemeen.rss";

   this.client.DownloadStringAsync(new Uri(url, UriKind.Absolute));

   if (this.result != string.Empty && this.result != null)
   {
    this.txbSummery.Text = this.result;
   }
  }

  private void ClientDownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
  {
   this.result = e.Result;
   //handle the response.
  }
 }

It gives me a runtime error after pressing the button:

Microsoft JScript runtime error: Unhandled Error in Silverlight Application An exception occurred during the operation, making the result invalid. Check InnerException for exception details. at System.ComponentModel.AsyncCompletedEventArgs.RaiseExceptionIfNecessary() at System.Net.DownloadStringCompletedEventArgs.get_Result() at JWTG.MainPage.ClientDownloadStringCompleted(Object sender, DownloadStringCompletedEventArgs e) at System.Net.WebClient.OnDownloadStringCompleted(DownloadStringCompletedEventArgs e) at System.Net.WebClient.DownloadStringOperationCompleted(Object arg)

I've tried numerous things but all failed.

What am i missing? Or does anyone know how i could achieve this in a different way?

Thanks in advance!

Answer 1

In this line

this.client.DownloadStringAsync(new Uri(url, UriKind.Absolute)); 

you are stating an asynchroneous download in a background thread. And in the next line you somehow expect that it is aready completed?

If you have no knowledge about Threading just try with DownloadString first. Then your code will work.

Answer 2

Hi,

I tried that:

private void btn1_Click(object sender, RoutedEventArgs e)
        {
            string url = "http://www.nu.nl/feeds/rss/algemeen.rss";

            this.client.DownloadStringAsync(new Uri(url, UriKind.Absolute));

        }

        private void ClientDownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            this.result = e.Result;
            //handle the response.
            if (this.result != string.Empty && this.result != null)
            {
                this.txbSummery.Text = this.result;
            }
        }

Still produces that runtime error..

At this line:

this.result = e.Result;

Answer 3

Try this one, instead of your btn1_Click and ClientDownloadStringCompleted methods. It invokes the GUI thread after the feed is downloaded to update the textbox. If it fails because of an error on the network, it will unpack the exception (contained as an inner exception in a TargetInvocationException) and rethrow the inner exception.

private void btn1_Click(object sender, RoutedEventArgs e)
{
    string url = "http://www.nu.nl/feeds/rss/algemeen.rss";

    this.client.DownloadStringAsync(new Uri(url));
}

private void ClientDownloadStringCompleted(object sender, 
                      DownloadStringCompletedEventArgs e)
{
    try
    {
        Dispatcher.BeginInvoke(() => this.txbSummery.Text = e.Result ?? "");
    }
    catch (TargetInvocationException tiex)
    {
        throw tiex.InnerException;
    }
}

If the error occures again (I guess that will happen), please post a stacktrace and error message here.

Answer 4

@ Philip Daubmeier, this is the innerexception:

{System.Security.SecurityException ---> System.Security.SecurityException: Security error. at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult) at System.Net.Browser.BrowserHttpWebRequest.<>c_DisplayClass5.b_4(Object sendState) at System.Net.Browser.AsyncHelper.<>c_DisplayClass2.b_0(Object sendState) --- End of inner exception stack trace --- at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state) at System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult) at System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result) at System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result)} and

expanding it more gives me this innerexception:

{System.Security.SecurityException: Security error. at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult) at System.Net.Browser.BrowserHttpWebRequest.<>c_DisplayClass5.b_4(Object sendState) at System.Net.Browser.AsyncHelper.<>c_DisplayClass2.b_0(Object sendState)}

Answer 5

This is related to clientaccesspolicy.xml. Read more here: http://msdn.microsoft.com/en-us/library/cc645032(VS.95).aspx

"If the connection request was from a WebClient or an HTTP class to a cross-domain site, the Silverlight runtime tries to download the security policy file using the HTTP protocol. The Silverlight runtime first tries to download a Silverlight policy file with a name of "clientaccesspolicy.xml" at the root of the requested target domain using the HTTP protocol.

If the "clientaccesspolicy.xml" is either not found (the web request returns a 404 status code), returned with an unexpected mime-type, is not valid XML, or has an invalid root node, then the Silverlight runtime will issue a request for a for the Flash policy file with a name of "crossdomain.xml" at the root of the requested target domain, using the HTTP protocol.

HTTP redirects for the policy file are not allowed. A redirect for a policy file will result in a SecurityException of access denied."