When you are enumerating over DataTable with Linq to DataSet, you should call AsEnumerable()
on datatable and use Field<T>
extension to get column value:
var supvName = (from r in db.Supervisors.AsEnumerable()
select r.Field<string>("Names")).ToList();
BTW query variable r
will be of DataRow
type.
Your code can be simplified to:
var names = db.Supervisors.AsEnumerable().Select(r => r.Field<string>("Names"));
supv_cbox.DataSource = names.ToList();