DataTable dtA = new DataTable();
        dtA.Columns.Add("id", typeof(int));
        dtA.Columns.Add("price", typeof(string));
        dtA.Rows.Add(1, "111");
        dtA.Rows.Add(2, "222");
        dtA.Rows.Add(3, "333");
        dtA.Rows.Add(4, "444");

        DataTable dtB = dtA.Clone();
        dtB.Rows.Add(1, "121");
        dtB.Rows.Add(2, "221");

        dtB.Rows.Add(4, "331");
        dtB.Rows.Add(5, "331");
        dtB.Rows.Add(6, "331");

 DataTable dtC = dtA.Clone();
        dtC.Columns.Add("price_excel");

        var query = from a in dtA.AsEnumerable()
                    join b in dtB.AsEnumerable()
                    on a.Field<int>("id") equals b.Field<int>("id")  into g
                    from b in g.DefaultIfEmpty()
                    select new
                    {
                        id = a.Field<int>("id"),
                        price = a.Field<string>("price"),
                        price_excel = b == null ? "None" : b.Field<string>("price")
                    };

        query.ToList().ForEach(q => dtC.Rows.Add(q.id, q.price, q.price_excel));
        gwinfo.DataSource = dtC;
        gwinfo.DataBind();