string t_qty = "";

    SqlCommand cmd1 = new SqlCommand("select QTY from STOCK_ARRIVE where BARCODE = @barcode", d.con);
     cmd1.Parameters.AddWithValue("@barcode", txt_barcode.Text);
    d.con.Open();
    SqlDataReader dr = cmd1.ExecuteReader();


    while (dr.Read())
    {
        t_qty = dr["QTY"].ToString();

    }


        d.con.Close();


        try
        {

            int less = 0;
            less = int.Parse(t_qty) - int.Parse(txt_qty.Text);

            SqlCommand cmd2 = new SqlCommand("update STOCK_ARRIVE set QTY=@pq where BARCODE=@barcode AND S_VC=@vc", d.con);
            cmd2.Parameters.AddWithValue("@pq", less);

            cmd2.Parameters.AddWithValue("@barcode", txt_barcode.Text);
            cmd2.Parameters.AddWithValue("@vc", txt_vc.Text);
            d.con.Open();
            cmd2.ExecuteNonQuery();
            d.con.Close();

        }
        catch (Exception ex)
        {

            Response.Write(ex);
        }


        Response.Write("<script>alert('Data has been saved')</script>");