var dataResult = from
                                 datatable
                             in
                                 dt.AsEnumerable()
                             where
                                datatable.Field<string>("StaffNo") == TransF_Supervisor
                             select
                                datatable.Field<string>("StaffNo");

            DataTable dt = new DataTable();
            var dataResult = from datatable in dt.AsEnumerable()
                             where datatable.Field<string>("StaffNo") == "0001"
                             select datatable;

------解决方案--------------------
如果只是判断的话，可以这样：
bool b=dt.AsEnumerable().Any(d=>d.Field<string>("StaffNo") == "0001");

------解决方案--------------------
1:Linq 可以判断是否有，并且取得其值
var dataResult =( from datatable  in dt.AsEnumerable()
                where datatable.Field<string>("StaffNo") == TransF_Supervisor
                select datatable.Field<string>("StaffNo")).FirstOrDefault();

2:lamda 仅仅判断是否存在
var reslut = dt.AsEnumerable().Any(c => c.Field<string>("StaffNo") == TransF_Supervisor
);