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DropDownList SelectedIndexChanged not firing (AutoPostBack="true")

I have a dropdownList on my webpage inside update panel. When I select a different value from the drop-down list, nothing happens which means that the "SelectedIndexChanged" event is not firing.
ASPX Code:
<asp:UpdatePanel ID="UpdatePanel1" UpdateMode="Conditional" runat="server">
<ContentTemplate>
<table class="table table-mv-vouchers" style="margin:0px;" cellspacing="0" cellpadding="0">
<caption class="mv-clearfix">
<asp:DropDownList ID="ddlShort" Width="150" runat="server" AutoPostBack="True" ViewStateMode="Enabled" EnableViewState="true" OnSelectedIndexChanged="ddlShort_SelectedIndexChanged">
<asp:ListItem Text="By Estimate" Value="EstimateValue"></asp:ListItem>
<asp:ListItem Text="By Merchant" Value="MerchantName" Selected="True"></asp:ListItem>
<asp:ListItem Text="By Type" Value="MerchantCategory"></asp:ListItem>
<asp:ListItem Text="By Validity" Value="Validity"></asp:ListItem>
</asp:DropDownList>
</caption>
</table>
</ContentTemplate></asp:UpdatePanel>
Server Side Code:
protected void ddlShort_SelectedIndexChanged(object sender, EventArgs e)
{
string ByShort = ddlShort.SelectedValue;
if (ByShort != null)
{
DataSet dsAllMerchant = Main.Instance.serClient.GetMerchantList(null,true, ByShort, null,currentBaID,true);
DataTable newdata = this.GenerateData(dsAllMerchant.Tables[0]);
lvGiftVoucher.DataSource = newdata;
lvGiftVoucher.DataBind();
}
}

My problem was <caption class="mv-clearfix"> </caption> tag, I think this tag is not recognize. After deleting this tag dropdownlist is firing. Thanks all for your answering.

I think you should add a trigger to your UpdatePanel, like that:
<asp:updatepanel ...>
<triggers>
<asp:asyncpostbacktrigger controlid="DrpEmployeeType" eventname="SelectedIndexChanged" />
</triggers>
</asp:updatepanel>

Change UpdatePanel update mode to always
UpdateMode="Always"

Ensure that you have:
a ScriptManager on your page
an UpdatePanel which wraps the lvGiftVoucher markup
provided a unique Id to you update-panel
an AsyncPostbackTrigger for dropdown SelectedIndexChanged event
set UpdateMode to Conditional
AutoPostback = "True" for the dropdwonlist (you already did it)
Dropdown markup (as is, just remove the updatepanel wrap)
<table class="table table-mv-vouchers" style="margin:0px;" cellspacing="0" cellpadding="0">
<caption class="mv-clearfix">
<asp:DropDownList ID="ddlShort" Width="150" runat="server" AutoPostBack="True" ViewStateMode="Enabled" EnableViewState="true" OnSelectedIndexChanged="ddlShort_SelectedIndexChanged">
<asp:ListItem Text="By Estimate" Value="EstimateValue"></asp:ListItem>
<asp:ListItem Text="By Merchant" Value="MerchantName" Selected="True"></asp:ListItem>
<asp:ListItem Text="By Type" Value="MerchantCategory"></asp:ListItem>
<asp:ListItem Text="By Validity" Value="Validity"></asp:ListItem>
</asp:DropDownList>
</caption>
</table>
UpdatePanel mark-up
<asp:UpdatePanel runat="server" id="up1" UpdateMode="Conditional" ChildrenAsTrigger="False">
<Triggers>
<asp:AsyncPostbackTrigger ControlId="ddlShort" EventName="SelectedIndexChanged" />
</Triggers>
<ContentTemplate>
<!-- lvGiftVoucher markup here -->
</ContentTemplate>
</asp:UpdatePanel>

I agree with Sachu.....Server Side Coding, on "PageLoad Event" ....(!IsPostback) Event should happen inside & call "Drop downList"

is your webpage created with master page? If so then check that your form tag is in which page master page/webpage. If it is in master page then simple emit that one and use form tag in webpage.

try the following:
1. add a try-catch block and see if you are getting any exception.
2. if your update panel UpdateMode is conditional, then you have to manually call Update() method or the panel will not update.

The same code is working fine in my PC....
Edited,
try this
add following code inside your Update panel
<Triggers>
<asp:PostBackTrigger ControlID="ddlshort" />
<asp:AsyncPostBackTrigger ControlID="ddlshort" EventName="SelectedIndexChanged" />
</Triggers>
hope this work....

reload on button click in

Here is my aspx:
<asp:Panel ID="pnl_updateClinicVisit" runat="server" CssClass="modalPopupClinicVisitEntry2" DefaultButton="bt_editClinicVisit_submit" Style="display:none">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button AutoPostBack="false" UseSubmitBehavior="false" ID="AddMedicationChange" ClientIdMode="Static" runat="server" Text="Add Med Change" OnClick="AddMedicationChange_Click" />
<asp:Panel ID="AddNewMedicationPanel" runat="server">
<asp:TextBox ID="NewDrugName" OnTextChanged="NewDrugName_TextChanged" runat="server"></asp:TextBox>
<asp:AutoCompleteExtender ID="NewDrugNameAutoCompleteExtender"
runat="server"
TargetControlID="NewDrugName"
MinimumPrefixLength="1"
EnableCaching="false"
CompletionSetCount="1"
CompletionInterval="500"
ServiceMethod="GetDrugs">
</asp:AutoCompleteExtender>
<asp:DropDownList OnSelectedIndexChanged="NewDrugChange_SelectedIndexChanged" ID="NewDrugChange" runat="server">
<asp:ListItem>Drug +</asp:ListItem>
<asp:ListItem>Drug -</asp:ListItem>
<asp:ListItem>Dose ↑</asp:ListItem>
<asp:ListItem>Dose ↓</asp:ListItem>
</asp:DropDownList>
<asp:Button AutoPostBack="false" UseSubmitBehavior="false" ID="SubmitMedChange" runat="server" Text="Add to Visit" OnClick="SubmitMedicationChange_Click" />
</asp:Panel>
<asp:ModalPopupExtender ID="updateClinicModalPopupExtender" runat="server" TargetControlID="bt_editClinicVisit_dummy"
PopupControlID="pnl_updateClinicVisit" CancelControlID="bt_editClinicVisit_cancel"
DropShadow="true" BackgroundCssClass="modalBackground" />
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>
For some reason my page is reloading when I click the "AddnewMdication" and "SubmitMedChange" buttons. When I have the AutoPostBack=false UseSubmitBehavior=false, the events fire and then the page reloads. If I don't have these attributes then the page reloads before the events even fire. How do I get AJAX functionality within this modal?

Try setting your Buttons as an AsyncPostBackTrigger. You can do this by adding in the Triggers section and declaring the UpdatePanel as a conditional update.
For example
<asp:UpdatePanel ID="UpdatePanel1" runat="server"
UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="AddMedicationChange" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="SubmitMedChange" EventName="Click" />
</Triggers>
<ContentTemplate>
<%-- Your code here --%>
</ContentTemplate>
</asp:UpdatePanel>
What this should do is to explicitly declare that those two buttons as being Async.
If this doesn't work, will you please post the relevant code to your two button OnClick events? AddMedicationChange_Click and SubmitMedicationChange_Click
If you try to interact with controls outside of the UpdatePanel in your code-behind during an asyncpostback, odd things can start to happen.

Modalpopupextender not firing

I am doing research to the Modalpopupextender from the Ajax Toolkit.
This code doesn't work, it doesn't open the pop-up. The only thing what happens when I click on the button is: Refreshing the page.
What am I doing wrong? Also tried this with an Updatepanel... Did 3 hours of research before posting this question, please don't blame it on me...
I also used the Toolkitscriptmanager, but that didn't solve it.
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:Button ID="btnTest" runat="server" Text="Button" />
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnTest" PopupControlID="Panel"></asp:ModalPopupExtender>
<asp:Panel ID="Panel" runat="server">
<h1>Hello World!</h1>
</asp:Panel>

Use PostBackTrigger for the button and then check it out .:)
Example:
<asp:UpdatePanel ID="updatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="btnTest" runat="server" Text="Button" />
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnTest" PopupControlID="Panel"></asp:ModalPopupExtender>
<asp:Panel ID="Panel" runat="server">
<h1>Hello World!</h1>
</asp:Panel>
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="btnTest"/>
</Triggers>
</asp:UpdatePanel>

Updating one UpdatePanel from 2nd UpdatePanel

Is this a known behavior? and if it is how do I make it work?
I have 2 update panels. Each one has a DropDownList control inside of
it.
I have 1 button that sets-up (binds) the first DropDownList. This
works.
When I change the 1st DropDownList, it should update the 2nd
DropDownList.
For some reason, the 2nd dropdown list never gets updated.
Here is the pseudo--code:
1st Update Panel & DropDownList:
<asp:Button ID="btnSet" runat="server"></asp:Button>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="DropDownList1" runat="server"
onselectedindexchanged="DropDownList1_SelectedIndexChanged" AutoPostBack="true"></asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnSet" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
<asp:UpdatePanel ID="UpdatePanel2" runat="server">
<ContentTemplate>
<asp:DropDownList ID="DropDownList2" runat="server"></asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="DropDownList1" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
Code Behind:
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
BindSecondDropDownList();
}
Help me figure this one out.

You can merge them all in one updatepanel, dont forget btnSet.OnClick event
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="btnSet" runat="server"></asp:Button>
<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
AutoPostBack="true">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server">
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>

This is called cascading dropdown lists, and there's a known technique for it, and a control provided by ASP.Net Ajax...
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/CascadingDropDown/CascadingDropDown.aspx

RadioButtonList OnSelectedIndexChanged Only firing ONCE

I have a radiobuttonlist that is hardcoded into my asp.net page like so:
<asp:RadioButtonList runat="server" ID="rblOrientation" RepeatDirection="Horizontal"
OnSelectedIndexChanged="rblOrientation_onSelectedIndexChanged" AutoPostBack="true">
<asp:ListItem Text="Portrait" Value="P" Selected="True"></asp:ListItem>
<asp:ListItem Text="Landscape" Value="L"></asp:ListItem>
</asp:RadioButtonList>
The code behind is as follows:
protected void rblOrientation_onSelectedIndexChanged(object sender, EventArgs e)
{
ddlPaperSize_onSelectedIndexChanged(sender, e);
}
Basically what happens is it calls on another function which updates a bunch of values that
are within an update panel. This is also hardcoded in and looks like this:
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:Label ID="lblPaperWidth" runat="server" />
<span id="txtUOMWidth" runat="server" /> ×
<asp:Label ID="lblPaperHeight" runat="server" />
<span id="txtUOMHeight" runat="server" />
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="rblOrientation" />
<asp:AsyncPostBackTrigger ControlID="ddlPaperSize" />
<asp:AsyncPostBackTrigger ControlID="ddlScale" />
</Triggers>
</asp:UpdatePanel>
Everything works fine the first time I change the value of the radiobuttonlist, however, if I try a second time it does not work. Does anyone know why this might be? Any advice would be great, thanks!
note: From the testing I have done, this is the only control where the OnSelectedIndexChanged is not acting how I expected it to. The event for the rest of the controls fires correctly

Try this code, which works fine for me.
<asp:ScriptManager ID="smMain" runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always" ChildrenAsTriggers="true" >
<ContentTemplate>
<asp:RadioButtonList runat="server" ID="rblOrientation" RepeatDirection="Horizontal"
OnSelectedIndexChanged="rblOrientation_onSelectedIndexChanged" AutoPostBack="true">
<asp:ListItem Text="Portrait" Value="P" Selected="True"></asp:ListItem>
<asp:ListItem Text="Landscape" Value="L"></asp:ListItem>
</asp:RadioButtonList>
</ContentTemplate>
</asp:UpdatePanel>