calling external websites from silverlight

问题:

I am writing a small silverlight app just to try silverlight. My idea is to make a small app that checks if websites are online. It works by the user inputting a URL and my app checks it uptime every 5 minutes.

But when ever I do a webrequest I get the security exception below. Reading http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest(VS.95).aspx it seems to indicate that silverlight doesn't allow crossdomain connection. So is there no way to make my idea work in silverlight?

Example code:

     WebRequest testHTTP = null;
            try
            {
                testHTTP = WebRequest.Create(serverToCheck);
            }
            catch (UriFormatException ufe)
            {
                try
                {
                    testHTTP = WebRequest.Create("http://" + serverToCheck);
                }
                catch (UriFormatException ufe1)
                {
                    MessageBox.Show("Invalid server address");

                }
            }
            if (testHTTP != null)
            {
                testHTTP.BeginGetResponse(new AsyncCallback(doCheck), testHTTP); 

                }

  void doCheck(IAsyncResult a)
    {
        try
        {
            HttpWebRequest req = (HttpWebRequest)a.AsyncState;
            HttpWebResponse res = (HttpWebResponse)req.EndGetResponse(a);

             Dispatcher.BeginInvoke(() => HTTPStatus.Content = "OK");

        }
        catch (Exception ex)
        {
  //handle exception
        }

    }

Exception: {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 monitor.Home.doCheck(IAsyncResult a)}

回答1:

You can't override the cross domain policy of the server in Silverlight 3. In Silverlight 4 you can with a trusted "out of browser" application.

Your best bet is to create a service that runs on the same domain as the one hosting the Silverlight application and have it do the checks.

回答2:

You can make a server-side page in ASP.Net that connects to any website and tells you whether it's up.
Since it would be hosted at the same domain as your Silverlight app, it would work fine. (ASP.Net has no such restrictions)

Note that it would then check whether the server can connect to the website rather than the client.

回答3:

The sites you are trying to access require that a cross domain policy file be in place to allow the access. Arbitary sites aren't likely to have these so what you are attempting to do is blocked.

回答4:

I guess the target URI needs a clientaccesspolicy.xml or crossdomain.xml file in the root. The solution could be to make a WCF service located on your server that handles the request.