refresh update panel from cascading drop down list - asp.net

Question

I am developing a webpage that will utilize two cascading drop down lists and an update panel that contains a nested gridview.
I have managed to get the cascading drop down lists to work correctly, and I have also managed to get my nested gridview working.
The next step is to populate the gridview based on the selection of the second cascading drop down list, which is where the update panel comes into play. I figured that once a selection has been made to the second drop down list, I could then refresh the update panel and load my nested gridviews. Looking through the documentation, I realized that I needed a trigger of some sort to force the update panel to reload, but I must not be doing something correctly...
Within my update panel, I use the trigger parameter, where cddMachine is the name of the second drop down list.
<Triggers>
<asp:AsyncPortBackTrigger ControlID="cddMachine" EventName="SelectedIndexChanged" />
</Triggers>
Upon running my code, the following error is thrown:
Control with ID 'cddMachine' being registered through
RegisterAsyncPostBackControl or RegisterPostBackControl must implement either
INamingContainer, IPostBackDataHandler, or IPostBackEventHandler
After searching for a while, I have been unable to find any clear information as to how one would go about refreshing an update panel by means of a cascading drop down list. I'm sure I'm overlooking something simple, but I am not sure what it is that I need to look for.
EDIT - Included code
lockout.aspx
<%# Page Title="" Language="vb" AutoEventWireup="true" MasterPageFile="~/Site.Master" CodeBehind="Lockout.aspx.vb" Inherits="LockoutTagout.Lockout" %>
<%# Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">
<asp:ScriptManager ID="asm" runat="server" />
<div>
Complex:
<asp:DropDownList ID="ComplexList" runat="server" />
Machine:
<asp:DropDownList ID="MachineList" runat="server" AutoPostBack="true" />
<asp:CascadingDropDown ID="cddComplex" runat="server"
ServicePath="~/lockoutService.asmx" ServiceMethod="GetComplex"
TargetControlID="ComplexList" Category="Complex"
PromptText="Select Complex" />
<asp:CascadingDropDown ID="cddMachine" runat="server"
ServicePath="~/lockoutService.asmx" ServiceMethod="GetMachine"
TargetControlID="MachineList" ParentControlID="ComplexList"
Category="Machine" PromptText="Select Machine" />
</div>
<div>
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:SqlDataSource ID="SqlDataSource1" runat="server"
ConnectionString="<%$ ConnectionStrings:LOTOConnectionString %>"
SelectCommand="SELECT * FROM [DEVICES] WHERE machine_id = #machine_id">
<SelectParameters>
<asp:ControlParameter Name="machine_id" ControlID="cddMachine" PropertyName="SelectedValue" />
</SelectParameters>
</asp:SqlDataSource>
<asp:GridView ID="GridView1" GridLines="None" runat="server" AllowSorting="True" AutoGenerateColumns="False" DataKeyNames="devices_id" DataSourceID="SqlDataSource1" AlternatingRowStyle-BackColor="White">
<Columns>
<asp:BoundField DataField="devices_id" HeaderText="ID" InsertVisible="False" ReadOnly="True" SortExpression="devices_id" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="description" HeaderText="Description" SortExpression="description" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="location" HeaderText="Location" SortExpression="location" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="verification" HeaderText="Verification" SortExpression="verification" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="rack_num" HeaderText="Rack_#" SortExpression="rack_num" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="section_num" HeaderText="Section_#" SortExpression="section_num" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="color_or_number" HeaderText="Key_Identifier" SortExpression="color_or_number" />
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="normal_position" HeaderText="Normal_Position" SortExpression="normal_position" />
<asp:TemplateField HeaderText="P&P Numbers" ControlStyle-Width="100px">
<ItemTemplate>
<asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:LOTOConnectionString %>" SelectCommand="SELECT * FROM [PNP] WHERE ([devices_id] = #devices_id)">
<SelectParameters>
<asp:Parameter Name="devices_id" Type="Int32" />
</SelectParameters>
</asp:SqlDataSource>
<asp:GridView ID="GridView2" ShowHeader="false" GridLines="None" runat="server" AutoGenerateColumns="False" DataKeyNames="pnp_id" DataSourceID="SqlDataSource2">
<Columns>
<asp:BoundField ItemStyle-HorizontalAlign="Center" DataField="pnp_num" HeaderText="pnp_num" SortExpression="pnp_num" />
</Columns>
</asp:GridView>
</ItemTemplate>
</asp:TemplateField>
</Columns>
<HeaderStyle BackColor="#a52138" ForeColor="White" Font-Size="Medium" />
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
</div>
lockout.aspx.vb
Public Class Lockout
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load, MachineList.SelectedIndexChanged
End Sub
Protected Sub GridView1_RowDataBound(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewRowEventArgs) Handles GridView1.RowDataBound
If e.Row.RowType = DataControlRowType.DataRow Then
Dim s As SqlDataSource = CType(e.Row.FindControl("SqlDataSource2"), SqlDataSource)
s.SelectParameters(0).DefaultValue = e.Row.Cells(0).Text
End If
End Sub
End Class
lockoutService.asmx.vb
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel
Imports AjaxControlToolkit
Imports System.Data.SqlClient
' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()> _
<System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
<System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<ToolboxItem(False)> _
Public Class lockoutService
Inherits System.Web.Services.WebService
<WebMethod()> _
Public Function GetComplex(ByVal knownCategoryValues As String, ByVal category As String) As CascadingDropDownNameValue()
Dim conn As New SqlConnection("Data Source=WIWRMS-SQLD2\SQLD2;Initial Catalog=LOTO;Persist Security Info=True;User ID=LOTO_ADMIN;Password=lotoadmin")
conn.Open()
Dim comm As New SqlCommand( _
"SELECT * FROM complex", conn)
Dim dr As SqlDataReader = comm.ExecuteReader()
Dim l As New List(Of CascadingDropDownNameValue)
While (dr.Read())
l.Add(New CascadingDropDownNameValue(dr("complex_name").ToString(), dr("complex_id").ToString()))
End While
conn.Close()
Return l.ToArray()
End Function
<WebMethod()> _
Public Function GetMachine(ByVal knownCategoryValues As String, ByVal category As String) As CascadingDropDownNameValue()
Dim complex_id As Integer
Dim kv As StringDictionary = CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues)
If Not kv.ContainsKey("complex") Or Not Int32.TryParse(kv("complex"), complex_id) Then
Throw New ArgumentException("Couldn't find complex.")
End If
Dim conn As New SqlConnection("Data Source=WIWRMS-SQLD2\SQLD2;Initial Catalog=LOTO;Persist Security Info=True;User ID=LOTO_ADMIN;Password=lotoadmin")
conn.Open()
Dim comm As New SqlCommand( _
"SELECT * FROM machine WHERE complex_id = #complex_id", conn)
comm.Parameters.AddWithValue("#complex_id", complex_id)
Dim dr As SqlDataReader = comm.ExecuteReader()
Dim l As New List(Of CascadingDropDownNameValue)
While (dr.Read())
l.Add(New CascadingDropDownNameValue(dr("machine_name").ToString(), dr("machine_id").ToString()))
End While
conn.Close()
Return l.ToArray()
End Function
End Class

Answer 1

A CascadingDropDown is really just an extender on a DropDownList, so you should be able to remove the trigger, set your final DropDownList's AutoPostBack property to true, then handle its SelectedIndexChanged event. That should automatically trigger the PartialPostBack so that you can update your GridView accordingly.
Additional details for fix:
I'm thinking you may want to change the following in your DataSource markup: <asp:ControlParameter Name="machine_id" ControlID="cddMachine" PropertyName="SelectedValue" /> change ControlID to MachineList.