data saving on view state list

private List<AcoTransaction> AcoTrans
   {
       get
       {
           if (ViewState["AcoTransaction"] != null)
           {
               ViewState["AcoTransaction"] = new List<AcoTransaction>();
           }
           return (List<AcoTransaction>)ViewState["AcoTransaction"];
       }
       set
       { ViewState["AcoTransaction"] = value; }
   }