ajax +webservises 跨域问题

   function CkLoginId() {
            var Id = $("#TxtLoginId");
            $.ajax({
                url: "http://localhost:26850/ReportServises.asmx/GetLoginId?jsoncallback=?",
                dataType: "jsonp",
                data: { "loginId": Id.val() },
                success: OnSuccess,
                error: OnError
            });
        }


                [WebMethod]
        public void GetLoginId(string loginId)
        {
            string callback = HttpContext.Current.Request["jsoncallback"];

           // HttpContext.Current.Response.Write(callback +"({result:'" + loginId + "'})");
            string msg = JsonConvert.SerializeObject(new {Result = false, Msg = "请求失败！"});
            HttpContext.Current.Response.Write(callback + msg);

            HttpContext.Current.Response.End();
        }


                后台方法注释的是可以的，为什么序列化的就不行了。求大家解惑。。

msg是json字符串，明显和注释的不一样了，少了()
==>

 HttpContext.Current.Response.Write(callback +"("+ msg+")");