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How to find child page control while using postback trigger

I am having one master page that has one update panel.
Content place holder is within the update panel
Now i have child page which has a File upload control
To make work File upload control, i have to put Postback trigger.
But question is where i can put that Postback trigger ??
If i put Postback trigger in Master page than it will give me error that control does not found
and i can't put Postback trigger because child page has not other update panel
What is the solution for this problem ?

Simply wrap the FileUpload with an UpdatePanel, which don't do anything and hasn't any side effect but will solve the problem.
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="btnSubmit" runat="server" Text="Button" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="btnSubmit" />
</Triggers>
</asp:UpdatePanel>

Differntiate between a dropdown list's postback and submit button's postback in ASP.NET

I am relatively new to ASP.NET & C#
I am creating a basic form with student name, program dropdown and semester number dropdown. I am using postback to populate semester number dropdown list. I am showing the data i acquired through the form into labels. I am using IsPostBack to populate the labels with data
My problem is that when I change program, it fires the postback to populate semester dropdown and since I am using that event to display data, I cant really select semester number.
How can I differentiate between submit button postback and dropdown postback?
Thanks!!!

Instead of coding inside the load event handler or using isPostBack. Just use buttons event handler. You can do that by double clicking on button in design view.
However better way of doing this altogether is use .Net Ajax Update Panel with script manager which is very simple to use.
Below is the simple example of using script manger and update panel, you can use it for drop down list so you don't have to postback for that. So postback should be only for button.
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<p runat="server">
<asp:Label ID="lblDOA" runat="server" Text="Appointment Date :" Height="21px"
Width="136px" Visible="true"></asp:Label>
<asp:TextBox ID="txtDOA" runat="server" CssClass="fieldz" AutoPostBack="true"
Visible="true" ViewStateMode="Enabled"></asp:TextBox>
</p>
</ContentTemplate>
</asp:UpdatePanel>
Instead of textbox in above code you can have dropdown list.

ajax modalpopup extender showing data from server on show

Perhaps my understanding of the Ajax Modal popup is not correct. What I would like to do is to retrieve some data from the server and show it on the modal pop up when the user clicks on a button on the page. The following code is in the aspx of the page.
<asp:Panel ID="pnlDetail" CssClass="modal" runat="server">
<div class="header">
Data
</div>
<div class="body">
<div class="header">
<asp:Label ID="lblInput" runat="server"></asp:Label>
</div>
</div>
</asp:Panel>
<ajaxToolkit:ModalPopupExtender ID="mpeDetail" PopupDragHandleControlID="pnlDetail" PopupControlID="pnlDetail" TargetControlID="hdnDetail"
BackgroundCssClass="modalBG" CancelControlID="ShowDetailClose" runat="server" />
In the button click event of a button on the page, I retrieve the data from the server and assign the value to the lblInput inside the pop up panel and call mpe.Show.. but it does not display the value. I'm assuming the data needs to be present on load of mpe but thats not what I have to do.
If MPE cannot achieve this, what is the alternative?

I'm using the dragpanel extender, because I had some problems with the modal popup too. But here is a possible solution with the modal popup:
http://blogs.microsoft.co.il/blogs/gilf/archive/2009/08/14/populating-a-modalpopupextender-dynamically.aspx

Ok, figured it out. Put the above panel inside an update panel.
<asp:UpdatePanel ID="updPnlDetail" runat="server" UpdateMode="Conditional">
<ContentTemplate>
..... <asp:panel .. > ajax mpe control etc (what I have in the question)..
</ContentTemplate>
</asp:UpdatePanel>
Then in the button click, call the following.
UserControl.LoadData(object dataContents); (user control has the popup data. It does not have to be user control. Bind your controls here with data).
updPnlDetail.Update();
mpe.Show();
The data will show. I guess the updatepanel's update method does a refresh.

AJAX UpdatePanel help needed

I have the following ASPX code:
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel runat="server" ID="UpdatePanel" UpdateMode="Conditional">
<ContentTemplate>
<asp:Button runat="server" ID="UpdateButton1" OnClick="NextImg_Click" Text="Update" />
<asp:Repeater runat="server" ID="urlsUpdateRepeater">
<ItemTemplate>
<!-- THIS WOULD BE A LOOP FROM HERE -->
<!-- OPENS RESULT ITEM DIV CONTAINER -->
<div id="result_item">
<a href="<%# Eval("myUrl") %>" target="_blank">
<%# Eval("urlPageTitle")%></a>
</ItemTemplate>
</asp:Repeater>
</ContentTemplate>
</asp:UpdatePanel>
I have a NextImg_Click() event which works fine.
I use this code for DataBind... what is the Update method?
urlsUpdateRepeater.DataSource = resultsCollection;
urlsUpdateRepeater.DataBind();
Everything would appear to be in order. But every time the Update button is clicked it re-renders the whole page instead of the partial-postback UpdatePanel only.
It is driving me completely mad as I can't see anything wrong with the code. Is there some simple thing I'm missing?! Please help!
The search and data is displayed correctly (inside the panel) it just will not do a partial postback.
Appreciate your help with my noob problems!

Because the button is inside your UpdatePanel's ContentTemplate, it is unnecessary to take any extra action to get a partial page postback.
Try removing the line from your Page_Load() method.

Taken from MSDN:
Use the RegisterAsyncPostBackControl
method to register controls outside an
UpdatePanel control as triggers for
asynchronous postbacks, and to
potentially update the content of an
update panel. To update an UpdatePanel
control programmatically, call the
Update method.
So, you're control (UpdateButton1) is inside the UpdatePanel, no need for the ScriptManager1.RegisterAsyncPostBackControl call - ditch it and your problem is solved.

The problem was that my <form> tag was nested further into the document than it's corresponding end tag (with Warning!)...
Upon moving my form tag - it worked!
Entirely my fault, thanks guys.

How do you use an UpdatePanel properly?

I have an UpdatePanel with some checkboxes in it. I check them, and hit my Save button, but that causes the UpdatePanel to postback (refresh) and sets them all back to blank. The re-drawing method runs before the button code.
What is the correct way to have an UpdatePanel with checkboxes in that you can manipulate?

Example of code:
<asp:UpdatePanel runat="server" UpdateMode="Conditional" ID="updatePanel1">
<ContentTemplate>
<asp:CheckBox runat="server" ID="myCheckBox" Caption="CheckBox"/>
<asp:Button runat="server" ID="saveButton"
Caption="Save" OnClick="SaveButtonClick"/>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="saveButton" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
Make sure that:
UpdateMode of UpdatePanel is Conditional
SaveButton contained in Triggers-section as ControlID of AsyncPostBackTrigger

Your code behind should look like:
if(!page.ispostback)
{
re-drawing();
}
As when you hit Save button your re-drawing() method is called and it again refreshes your checkboxes. Asynchronous postback behaves and hit to page method the same as full postback, but refreshes the values in any updatepanels.
Also check this URL
http://ajax.net-tutorials.com/controls/updatepanel-control/

Make sure the Save button is inside the Update Panel, for a start, and if not, that is designated as a Trigger for the Update Panel, in the <Triggers> section of the Update Panel.
<asp:UpdatePanel ID="MyControlPanel" runat="server" UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="SaveButton" />
</Triggers>
<ContentTemplate> ...
Can you show some code for your UpdatePanel?

If you use server controls to render checkboxes you should add EnableViewState="true" attribute to these controls and update panel.
If you have checkboxes that are not server controls, then remove them from update panel, include only server controls inside. This leads to keeping several updates panel on a page that's usually not a big issue.

Add a ScriptManager object to your page if you do not have one. Set EnablePartialRendering="true". Put your UpdatePanel anywhere else on the page and place the content you want ajaxified within a tag within your UpdatePanel.