how to validate a URL in c#

  Hi, I need to write a tool that will report broken URL's.    The URL should only reports broken if the user see's a 404 Error in the browser.  I believe there might be tricks to handle web servers that do URL re-writing.   Here's what I have.   As you can see only some URL validate incorrectly.  Thanks you for any help.

           
  string url = "";

            // TEST CASES 
            url = "http://newsroom.lds.org/ldsnewsroom/eng/news-releases-stories/local-churches-teach-how-to-plan-for-disasters";   //Prints "BROKEN", although this is getting re-written to good url below.
            //url = "http://beta-newsroom.lds.org/article/local-churches-teach-how-to-plan-for-disasters";  // Prints "GOOD"
            //url = "http://";     //Prints "BROKEN"
            //url = "google.com";     //Prints "BROKEN" althought this should be good.
            //url = "www.google.com";     //Prints "BROKEN" althought this should be good.
            //url = "http://www.google.com";     //Prints "GOOD"

            try
            {

                if (url != "")
                {
                    WebRequest Irequest = WebRequest.Create(url);
                    WebResponse Iresponse = Irequest.GetResponse();
                    if (Iresponse != null)
                    {
                        _txbl.Text = "GOOD";
                    }
                }
            }
            catch (Exception ex)
            {
                _txbl.Text = "BROKEN";
            }