access session variable in web service

 [System.Web.Services.WebMethod(EnableSession = true)]
        public static string[] Save_Data(string args)
        {
            string[] KeyValues = null;
            string[] arguments = args.Split('&');
            string income = arguments[0];
            string bonus = arguments[1];
            string Investment = arguments[2];
  string LeadId = Convert.ToString(HttpContext.Current.Session["LeadId"]);
            string Result = CommonClass.Common.Instance.AddModifyIncomeDetails(income, bonus, Investment);
            return KeyValues;
        }