WPF Hidden cell Content Read Problem

When ever i Have fill the datagrid with

 var v = from t in am.Stocks
                    join t1 in am.Items
                    on t.Item_ID equals t1.Item_ID

                    select new { t.Stock_ID, t.Quantity, t.Item_ID };

            dataGrid1.ItemsSource = am.Stocks.ToList();

then I hide the Stock_ID in datagrid then i cant acess the Stock ID using following Code

   var v=(Stock)datagrid1.SelectedItem;
 
   var id=v.Stock_ID;

  Any Other Methode For This To Read Hidden value

When The More than one table is Present  in Datagrid?