visual studio doesn't see the dropdownlist for trigger - asp.net

Question

I have a VB.NET / ASP.NET web application. I added a dropdownlist into FooterTemplate of a Gridview. And I used ajax for postbacks. But when I add the dropdown to the triggers, vs gives me that error :
A control with ID 'DropDownList2' could not be found for the trigger in UpdatePanel 'UpdatePanel1'.
ASP.NET code is like that :
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:GridView ID="GridView2" runat="server" Width="329px" AutoGenerateColumns="False"
Font-Names="Arial" Font-Size="11pt"
AlternatingRowStyle-BackColor="#C2D69B" HeaderStyle-BackColor="green"
AllowPaging="True" ShowFooter="True" OnPageIndexChanging="OnPaging"
Style="margin-right: 0px" BackColor="White" BorderColor="#336666"
BorderStyle="Double" BorderWidth="3px" CellPadding="3"
GridLines="Horizontal">
<AlternatingRowStyle BackColor="#C2D69B" />
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Button ID="lnkRemove" runat="server" CommandArgument='<%# Eval("il_ad")%>'
OnClientClick="return confirm('SİLMEK İSTEDİĞİNİZDEN EMİN MİSİNİZ?')"
Text="SİL" OnClick="DeleteDURUM"></asp:Button>
</ItemTemplate>
<FooterTemplate>
<asp:Button ID="btnAdd" runat="server" Text="EKLE" OnClick="AddNewDURUM" />
</FooterTemplate>
</asp:TemplateField>
<asp:CommandField EditText="DEĞİŞTİR" ButtonType="Button"
CancelText="İPTAL" DeleteText="SİL" UpdateText="KAYDET" ControlStyle-Width="70PX"
CausesValidation="False">
<ControlStyle Width="70px" />
</asp:CommandField>
<asp:TemplateField ItemStyle-Width="30px" HeaderText="YTM No">
<ItemTemplate>
<asp:Label ID="lblytm_id" runat="server" Text='<%# Eval("ytm_id")%>'></asp:Label>
</ItemTemplate>
<ItemStyle Width="30px"></ItemStyle>
</asp:TemplateField>
<asp:TemplateField HeaderText="İl">
<ItemTemplate>
<asp:Label ID="il_ad" runat="server" Text='<%# Eval("il_ad")%>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="il_ad" runat="server" Text='<%# Eval("il_ad")%>'></asp:TextBox>
</EditItemTemplate>
<!-- Dropdownlist is here : -->
<FooterTemplate>
<asp:DropDownList ID="DropDownList2" runat="server" Width= "120px" DataSourceID="iller_yukle" AutoPostBack="True">
</asp:DropDownList>
</FooterTemplate>
</asp:TemplateField>
</Columns>
<FooterStyle BackColor="White" ForeColor="#333333" />
<HeaderStyle BackColor="#336666" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#336666" ForeColor="#000066" HorizontalAlign="Left" />
<RowStyle BackColor="White" ForeColor="#000066" />
<SelectedRowStyle BackColor="#339966" Font-Bold="True" ForeColor="White" />
<SortedAscendingCellStyle BackColor="#F7F7F7" />
<SortedAscendingHeaderStyle BackColor="#487575" />
<SortedDescendingCellStyle BackColor="#E5E5E5" />
<SortedDescendingHeaderStyle BackColor="#275353" />
</asp:GridView>
</div>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="GridView2" />
<asp:AsyncPostBackTrigger ControlID="DropDownList2" /> <!-- Error is here -->
</Triggers>
</asp:UpdatePanel>
where is the mistake, I couldn't figure it out, thanks..

Answer 1

The DropDownList only exists within the GridView and you cannot access controls within a GridView directly.
You would need to dynamically add the trigger to the UpdatePanel after you have bound the GridView.
To dynamically add it you would need to first Bind your gridview, then register the trigger
AsyncPostBackTrigger apt = new AsyncPostBackTrigger();
apt.ControlID = ((Button)this.GridView1.FooterRow.FindControl("btnTrigger")).UniqueID;
apt.EventName = "Click";
this.UpdatePanel1.Triggers.Add(apt);
In this example I am using a button's click event to trigger the UpdatePanel. You can adapt this to the DropDownList

Answer 2

Generally the mistake is that you don't need to specify triggers for controls inside an UpdatePanel till you don's set ChildrenAsTriggers="false" on that UpdatePanel