public class HomeController : Controller

    {

        public ActionResult Index()

        {

            var url = "http://www.ly.com?ak=abcdefg&user=sbd";

            var queryString = "ak=hello&user=test&passpord=111";

            url = this.DealQueryString(url, queryString);

            return Content(url);

        }

        private string DealQueryString(string url, string queryString)

        {

            var arr = queryString.Split('&');

            foreach (var item in arr)

            {

                var arrSub = item.Split('=');

                url = this.DealQueryString(url, arrSub[], arrSub[]);

            }

            return url;

        }

        private string DealQueryString(string url, string key, string val)

        {

            var exp = key + "=([^&]*)";

            var r = new Regex(exp);

            if (r.IsMatch(url))

            {

                url = r.Replace(url, key + "=" + val);

            }

            else

            {

                url += url.Contains("?") ? "&" : "?";

                url += key + "=" + val;

            }

            //url = Regex.Replace(url, exp, para + "=" + val);

            return url;

        }

    }