Data Controls :: Cascading DropDownList Not Working When DateValueField Is Set

I have 2 dropdownlist in my page

1--ddlTarde
2--ddlTradeClass

and 2 table in database

1-Trade table

ID Tname
1 Industry
2 Cloth

2-Tradeclass

ID Class Tname
1 Electronic Industry
2 Car Industry
3 Scarf Cloth
4 Pants Cloth

I bind ddlTrade from Tradetable and I want when users click on Item from ddlTrade it bind data from ddlTradeclass that TNAME=ddlTrade.selectedItem in ddltradeClass for this I used below code

for binding ddltrade

BindDropDownList(DDLtrade, "tradename", "Name", "id");
protected void BindDropDownList(DropDownList DDL, string spname, string dataTextfield, string dataValuefield) {
SqlCommand _cmd = new SqlCommand(spname, _cn);
_cmd.CommandType = CommandType.StoredProcedure;
_cn.Open();
SqlDataReader _dr = _cmd.ExecuteReader();

[Code] ....

NOW my problem is that when I select Item from ddlTrade it didn't show any thing in ddlTradeClass but when I delete this code

DDL.DataValueField=dataValuefield;

from BindDropDownList Metod and change like below:

protected void BindDropDownList(DropDownList DDL, string spname, string dataTextfield)
{
SqlCommand _cmd = new SqlCommand(spname, _cn);
_cmd.CommandType = CommandType.StoredProcedure;
_cn.Open();

[Code] ....

It worked correctly I mean when I click Item from ddlTrade in ddlTradeClass show data from TradeClass Table

so here what should I do ? I need DataValueField in ddlbinding ... Why this problem happen?