อันนี้ใน asmx ครับ
[WebMethod]
        [ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]
        public void getMenus(string deptType)
        {
            JavaScriptSerializer js = new JavaScriptSerializer();
            Context.Response.Clear();
            Context.Response.ContentType = "application/json";
            Context.Response.Write(js.Serialize(getMenusList(deptType)));
        }

        public static List<jmdMENUS> getMenusList(string deptType)
        {
            List<jmdMENUS> menuLists = new List<jmdMENUS>);
            try
            {
                using (masterEntities db = new masterEntities())
                {
                    var menus = from p in db.MENUBASE
                                where p.DEPTPERMIT.Contains(deptType.ToUpper())
                                select new {p.MENUCODE,p.MENUCAPTION };
                    foreach (var menu in menus)
                    {
                        menuLists.Add(new jmdMENUS() {
                            MENUCODE = menu.MENUCODE,
                            MENUCAPTION = menu.MENUCAPTION
                        });
                    }
                }
            }
            catch { throw; }
            return menuLists;
        }

เรียกใช้
var obj = JSON.stringify({ deptType: 'pcr' });
        $.ajax({
          type: "GET",
          url: "../mobileService/menuContent.asmx/getMenus",
          data: "deptType=pcr",
          contentType: "application/json; charset=utf-8",
          dataType: "json",
          success: function (data) {
              
          },
          error: function (e) {
               alert("ขัดข้อง!");
          }
          });

คือมัน error 500 (Internal Server Error) ตลอดเลยครับ แล้วก็ เข้า error function
ทดสอบตัว Web service ก็คืนค่าเป็น json ปกติครับ ไม่ทราบว่าจะแก้ยังไงดีครับ ติดมา 2 วันแล้ว
ขอบคุณครับ