updatepanel asynpostbacktrigger does not throw defined event? - asp.net

Question

I have the following code.
What i want is when the ddlProvince dropdown is changed, to throw the event SelectedIndexChanged, however that method is never accessed.
<asp:UpdatePanel ID="pnlCountries" runat="server">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ddlProvince" EventName="SelectedIndexChanged" />
</Triggers>
<ContentTemplate>
<tr>
<td><asp:Literal ID="Literal37" Text="<%$Resources:glossary,country %>" runat="server"/></td>
<td>
<asp:DropDownList ID="ddlCountries" CssClass="textbox" runat="server">
</asp:DropDownList>
<br />
<cc1:cascadingdropdown ID="cddCountries" runat="server" Category="Country" Enabled="True" LoadingText="<%$Resources:Glossary,loading %>" PromptText="<%$Resources:Glossary,country_choose %>"
ServiceMethod="GetCountries" TargetControlID="ddlCountries">
</cc1:cascadingdropdown>
<asp:RequiredFieldValidator CssClass="errortext" Text="<%$Resources:Glossary,required %>" SetFocusOnError="true" ID="rfvcboScenario" runat="server" InitialValue="" ControlToValidate="ddlCountries" Display="Dynamic" />
</td>
</tr>
<tr>
<td><strong><asp:Literal ID="Literal9" Text="<%$Resources:Glossary,province %>" runat="server" /> *</strong></td>
<td>
<asp:DropDownList ID="ddlProvince" CssClass="textbox" runat="server">
</asp:DropDownList>
<asp:RequiredFieldValidator CssClass="errortext" Text="<%$Resources:Glossary,required %>" SetFocusOnError="true" ID="RequiredFieldValidator1" runat="server" InitialValue="" ControlToValidate="ddlProvince" Display="Dynamic" />
<cc1:CascadingDropDown ID="cddProvince" runat="server" TargetControlID="ddlProvince" ParentControlID="ddlCountries"
Category="Province" LoadingText="<%$Resources:Glossary,loading %>" prompttext="<%$Resources:Glossary,province_select %>" ServiceMethod="GetProvincesForCountry" >
</cc1:CascadingDropDown>
</td>
</tr>
</ContentTemplate>
</asp:UpdatePanel>
Currently this codeline is never hit:
Protected Sub ddlProvince_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ddlProvince.SelectedIndexChanged
ReportError("ddlProvince_SelectedIndexChanged", "")
End Sub
update:
previously I had the Autopostback="true" attribute on the ddlProvince control, however, that caused a full postback (issue also described here: Drop Down List (in Update Panel) causing FULL PostBack!)
What am I missing?

Answer 1

EDIT:
You need to set AutoPostBack="true" for the dropdownlist. Change this:
<asp:DropDownList ID="ddlProvince" CssClass="textbox" runat="server">
</asp:DropDownList>
to this:
<asp:DropDownList ID="ddlProvince" CssClass="textbox" runat="server" AutoPostBack="true" >
</asp:DropDownList>

Answer 2

Possibly you haven't set the OnSelectedIndexChanged event in the Markup as seen above.
Three properties you should set: OnSelectedIndexChanged, AutoPostback & EnableViewState
<asp:DropDownList ID="ddlProvince" runat="server"
AutoPostBack="true" EnableViewState="true"
OnSelectedIndexChanged="ddlProvince_SelectedIndexChanged">
</asp:DropDownList>
Incase you are binding your dropdownlist in page_Load event, place it inside !IsPostback condition check:
protected void page_Load ( object sender, EventArgs e )
{
if(!IsPostBack)
{
//DropDownList data bind and all...
}
}