参考网站：http://www.cnblogs.com/junny/archive/2012/10/30/2745978.html

public bool CheckUrlVisit(string url)

    {

        try

        {

            HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);

            HttpWebResponse resp = (HttpWebResponse)req.GetResponse();

            if (resp.StatusCode == HttpStatusCode.OK)

            {
　　　　　　　　　　resp.Close(); 

                return true;

            }

        }

        catch (WebException webex)

        {

            return false;

        }

        return false;

    }