I am getting this error There is already an open DataReader associated with this Command which must be closed first.

1.The problem is genrated by the fact that your SqlDataAdapter object is using internally a DataReader object for reading the data, but you have just created a reader (by using ExecuteReader) before to read the data by using adapter.

2.The solution is to ExecuteRedear after the usage of the SqlDataAdapter, like in the next example:

public DataTable FillData(SaleOrderBEL SaleBEL)
{

    if (cn.State != ConnectionState.Open)
        cn.Open();
    cmd = new SqlCommand("SP_SalesOrder_FillData", cn);
    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.AddWithValue("@comp_id", SaleBEL.comp_id);
    cmd.Parameters.AddWithValue("@so_no", SaleBEL.so_no);
    cmd.Parameters.AddWithValue("@sod_deleted", SaleBEL.sod_deleted);
    if (dr.IsClosed == false) dr.Close();
    sda = new SqlDataAdapter(cmd);
    ds = new DataSet();
    sda.Fill(ds, "so_details");
    //
    dr = cmd.ExecuteReader(); //Here!!!
    while (dr.Read())
    {
        object[] obj = { dr["sod_order"].ToString(),"", dr["prod_code"].ToString(), dr["prod_desc"].ToString(), dr["sod_qty"].ToString(),
                               dr["sod_uom"].ToString(), dr["sod_uprice"].ToString(),  dr["sod_gstamt"].ToString(),
                               dr["sod_disc"].ToString(), dr["sod_netamount"].ToString(),"",dr["sod_qty_reserve"].ToString() };



        dt = ds.Tables[0];
        dt.Rows.Add(obj);
    }

    dr.Close();
    cn.Close();
    return dt;
}