怎么判断某个.asmx链接有效？

try            {                System.Net.WebRequest myRequest = System.Net.WebRequest.Create(                "http://192.168.0.6/rootserver/DirService.asmx ");                System.Net.WebResponse myResponse = myRequest.GetResponse();                return "ok ";            }            catch (Exception ex)            {                return ex.Message.ToString();            }