Explain update panel with example in ASP.NET AJAX?

The UpdatePanel control is probably the most important control in the ASP.NET AJAX package. It will AJAXify controls contained within it, allowing partial rendering of the area. 

The <asp:UpdatePanel> tag has two child tags - the ContentTemplate and the Triggers tags. The ContentTemplate tag is required, since it holds the content of the panel. The content can be anything that you would normally put on your page, from literal text to web controls. The Triggers tag allows you to define certain triggers which will make the panel update it's content. The following example will show the use of both child tags.

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>UpdatePanel</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:ScriptManager ID="ScriptManager1" runat="server" />
        <asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
        <Triggers>
            <asp:AsyncPostBackTrigger controlid="UpdateButton2" eventname="Click" />
        </Triggers>
            <ContentTemplate>
                <asp:Label runat="server" id="DateTimeLabel1" />
                <asp:Button runat="server" id="UpdateButton1" onclick="UpdateButton_Click" text="Update" />               
            </ContentTemplate>
        </asp:UpdatePanel>
        <asp:UpdatePanel runat="server" id="UpdatePanel1" updatemode="Conditional">           
            <ContentTemplate>
                <asp:Label runat="server" id="DateTimeLabel2" />
                <asp:Button runat="server" id="UpdateButton2" onclick="UpdateButton_Click" text="Update" />
            </ContentTemplate>
        </asp:UpdatePanel>
    </form>
</body>
</html>

Here is the Code Behind. Just add the following method to the file:

protected void UpdateButton_Click(object sender, EventArgs e)
{
    DateTimeLabel1.Text = DateTime.Now.ToString();
    DateTimeLabel2.Text = DateTime.Now.ToString();
}

So, what's this example all about? Try running it, and click the two buttons. You will notice that then first button updates only the first datestamp, while the second button updates both. Why? We have set the Panels to update conditionally, which means that their content is only updated if something insides them causes a postback, or if one of the defined triggers are fired.  
As you can see, the first UpdatePanel carries a trigger which references the second button. This will ensure that the first panel is updated even when a control on a different UpdatePanel is used. 
The AsyncPostBackTrigger tag is pretty simple - it only takes two attributes, the controlid, a reference to the control which can trigger it, and the eventname, which tells which eventtype can cause the trigger to fire. If you wish for the content of a UpdatePanel to be updated no matter what, you may change the updatemode property to Always. 
In general, you should only have UpdatePanels areound areas where you wish to do partial updates. Don't wrap your entire page within an UpdatePanel, and don't be afraid to use several panels, since this will give you more control of which areas update and when they do it.


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