Problem for get the hidden value from datagrid in WPF

Question

my problem for above WPF code, this code only useful when you bind single table to datagrid

 var data =(Voucher)dataGrid1.SelectedItem;
            var id = data.VoucherID;

this code not give value from selected datagrid row when multiple table bind to the datagrid?

anyone knows about this send me a proper code to fetch value

 thank u in advance