Problem with this code to test the Credentials?

I wrote this method to test for credentials but I dunno why when it goes to the GetResponse method is actually goes and runs the webservice. I just want it to test if the credentials for that web service are correct or not.

   private bool TestCredentials(string sURL, ref  string sUsername, ref  string sPassword)
  开发者_开发问答  {

        bool retVal = false;
        CredentialCache myCache = new CredentialCache();
        myCache.Add(new Uri(sURL), "Full", new NetworkCredential(sUsername, sPassword, "our_domain"));

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(sURL);
        request.Credentials = myCache;
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        if (response.StatusCode == HttpStatusCode.OK)
            retVal = true;
        return retVal;

You can't know if the credentials are good without some kind of communication with the server. You could create a dummy URL to execute a test request against. IE, an URL you can request that doesn't actually do anything on the server.