Data inserted in Temporary variable getting lost in Web Method

It is  working on button click , becuase i when store the values from datalist into my database , the values got inserted .. but when i am trying to store those value in temporary datatable its not working

[WebMethod]
    public static ProductDetails[] DisplayMessage(string name, string price,string obj, string id)
    {

        string emailid = HttpContext.Current.User.Identity.Name; ;
        string date = DateTime.Now.ToString();
        string s = HttpContext.Current.User.Identity.Name;
        string[] split = s.Split('@');
        string username = split[0];
       
         DataTable shop = new DataTable("Table"); // create a dataTable name shop ["Table"] always be same.
            // declare the columns name according to your need.
            shop.Columns.Add(new DataColumn("prodname", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("prodprice", Type.GetType("System.Double")));
            shop.Columns.Add(new DataColumn("produrl", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("userid", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("quantity", Type.GetType("System.Int32")));
            shop.Columns.Add(new DataColumn("productid", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("emailid", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("username", Type.GetType("System.String")));
            shop.Columns.Add(new DataColumn("date", Type.GetType("System.DateTime")));
            shop.Columns.Add(new DataColumn("subtotal", Type.GetType("System.Double")));

         



        DataRow new_row;
       
            new_row = shop.NewRow();
            new_row["prodname"] = name;
            new_row["prodprice"] = price;
            new_row["produrl"] = obj ;
            new_row["userid"] = UserId;
            new_row["quantity"] = 1;
            new_row["productid"] = id;
            new_row["emailid"] =emailid;
            new_row["username"] = username;
            new_row["date"] = date;
            new_row["subtotal"] = price ;
            //add the row to the table
            shop.Rows.Add(new_row);
        
        //reload the cart into session
        List<ProductDetails> details = new List<ProductDetails>();
        foreach (DataRow dtrow in shop.Rows)
        {
            ProductDetails x = new ProductDetails();
            x.prodname = dtrow["prodname"].ToString();
            x.quantity = dtrow["quantity"].ToString();
            x.subtotal = dtrow["subtotal"].ToString();
            details.Add(x);
        }

        return details.ToArray();
     
    }
   public class ProductDetails
   {
       public string prodname { get; set; }
       public string quantity { get; set; }
       public string subtotal { get; set; }
   }