Getting Ajax autocompleteextender selected value

1) create data source as  List<string>();

 List<string> custList = new List<string>();
            string custItem = string.Empty;
            while (dr.Read())
            {
                custItem=AutoCompleteExtender.CreateAutoCompleteItem(dr[0].ToString(),dr[1].ToString());
                custList.Add(custItem);

            }

2)In the Asp.net page

 <script type="text/javascript">
    function ace1_itemSelected(sender, e)
    {
        var hdCustID = $get('<%= hdEmpID.ClientID %>');
        hdCustID.value = e.get_value();
    }
    </script>
 <asp:TextBox ID="txtEmpName" AutoPostBack="true" AutoComplete="off" runat="server"
                    OnTextChanged="txtEmpName_TextChanged" />
                <cc1:AutoCompleteExtender ID="ace1" TargetControlID="txtEmpName" ServiceMethod="GetSuggestions"
                    MinimumPrefixLength="1" OnClientItemSelected="ace1_itemSelected" FirstRowSelected="true"
                    runat="server" />
                <asp:HiddenField ID="hdEmpID" runat="server" />

3) the the code behind 

   String selectedEmpID =  hdEmpID.Value;

Thats it! 

Ref: http://forums.asp.net/t/1608031.aspx/1?AutoCompleteExtender+Selected+Value+Feature