Events on Web Page - CodeProject

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Introduction

By default, the web page comes with four basic events. They are init, load, prerender and the unload events. Only the first three events can be captured by code. We can not capture unload event because it happens when the content has been rendered on the client browser. That explains why there is no value kept at the second column on the last row of the table above.

Background

For someone who has just started learning ASP.NET, it is important to know the order in which these events page are triggered, before they step forward to learn further.

Using the code

I created the following code to show how to trap each event by code and the order in which they are called:

<%@ Page Language="C#"%>

<script Runat="server">

//declare one integer variable and set the starting 
value to 1
public int sequence = 1;

//It is trapping init event page
void Page_Init(Object o,EventArgs e)
{
    lblPageInit.Text = sequence.ToString();
    sequence++; //increase the sequence 
value by one every time this event is called
}

//It is trapping load event page
void Page_Load(Object o,EventArgs e)
{
    lblPageLoad.Text = sequence.ToString();
    sequence++; //increase the sequence 
value by one every time this event is called
}

//It is trapping prerender event page
void Page_PreRender(Object o,EventArgs e)
{
    lblPagePreRender.Text = sequence.ToString();
    sequence++; //increase the sequence 
value by one every time this event is called
}

//It is trapping unload event page
void Page_Unload (Object sender , EventArgs e)
{
    lblPageUnload.Text = sequence.ToString();
    sequence++; //increase the sequence 
value by one every time this event is called
}

</script>

<html>
<head>
<style>
       .title 
{font-family:verdana;font-size:12pt;font-weight:bold;}
       .subtitle 
{font-family:verdana;font-size:10pt;font-weight:bold;font-style:italic;}
       .sequence 
{font-family:verdana;font-size:10pt;font-weight:normal;}
</style>
</head>
<body>
<form Runat="server">
<table border="0" width="500" cellpadding="1" cellspacing="1">
<tr>
    <td width="70%" height="30" align="center" bgColor="orange">
           <span class="title">Page Events</span>
    </td>
    <td width="30%" height="30" align="center" bgColor="orange">
           <span class="title">Sequence No.</span>
    </td>
</tr>
<tr>
    <td>
          <span class="subtitle">On Load</span>
    </td>
    <td align="center">
          <asp:Label id="lblPageLoad" Runat="server" class="sequence"/>
    </td>
</tr>
<tr>
    <td bgColor="#d9d9d9">
          <span class="subtitle

     
">OnInit</span></td><tdbgColor="#d9d9d9"align="center">
          <asp:Label id="lblPageInit" Runat="server" class="sequence"/>
    </td>
</tr>
<tr>
    <td>
          <span class="subtitle">On 
PreRender</span>
    </td>
    <td align="center">
          <asp:Label id="lblPagePreRender" 
Runat="server" 
class="sequence"/>
    </td>
</tr>
<tr>
    <td bgColor="#d9d9d9">
          <span class="subtitle">OnUnLoad</span>
    </td>
    <tdbgColor="#d9d9d9"align="center">
    <asp:Label id="lblPageUnload" Runat="server" class="sequence"/>
    </td>
</tr>
</table>
</form>
</body>
</html>

Points of Interest

By knowing which event is called first or next, it will give a new ASP.NET programmer an idea about where to keep a code as intended. Example: if they want you to do specific things when the web page is being loaded, you just need to keep your code inside Page_Load.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Gunawan Hadikusumo

Web Developer

Australia

Member

Born in Jakarta,Indonesia. Now he is in New Zealand and currently working for Macvad Limited.

"Programming is about practice and practice, then try to bring the best programming practice into the real world application / web development."

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Comments and Discussions

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My vote of 1

rameshsahu7

23:38 8 Jul '09

Poor
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to call page_UnLoad event server side in asp.cs

hafz

1:09 18 Apr '06

hi everyone there. i want to do certain task in the page_UnLoad() event . but i don't know how to call that method.
Sign In·View Thread·Permalink	2.00/5 (1 vote)

Re: to call page_UnLoad event server side in asp.cs

sgorbutica

23:52 17 Sep '06

klòkl
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Re: to call page_UnLoad event server side in asp.cs

Spiff Dog

10:04 5 Oct '06

Howdy!

You can control what happens in any public or protected method (as long as your class is derived from the parent that contains the method) by using method "overridding".

In C# you would do something like:



protected override void OnUnload(EventArgs e) {

	Trace.Warn("BEGIN - Template OnUnload");

	// Do Custom Processing Here.....

	// Do Custom Processing Here.....

	base.OnUnload (e);

	Trace.Warn("END - Template OnUnload");

}

In regards to this article, there are actually a lot more events that you can override. The most useful, non-mentioned events include:

> Render
> OnError - especially useful for a global error handler.

-- Spiff

-------------------------

Spiffdog Design

It's ok.. he's no ordinary dog...

help! what's wrong with my code?

Your details have been updated

21:31 28 Aug '05

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Xml;
using System.Text;

namespace WebUI.kp.mulu
{
///

/// ShowKP 的摘要说明。
///

public class ShowKP : System.Web.UI.Page
{
private void Page_Load(object sender, System.EventArgs e)
{
if(!Page.IsPostBack)
{
// 在此处放置用户代码以初始化页面
string kpid = "121";
string startTime = DateTime.Now.ToString();
string xmlPath = Session["rootPath"].ToString();
string strFileName = xmlPath + "\\time.xml";
XmlTextWriter _XmlTextWriter = new XmlTextWriter(strFileName,Encoding.UTF8);
_XmlTextWriter.Formatting = Formatting.Indented;
_XmlTextWriter.Indentation = 4;
_XmlTextWriter.WriteStartDocument();
_XmlTextWriter.WriteStartElement("time");
_XmlTextWriter.WriteStartElement("kptime");
_XmlTextWriter.WriteElementString("kpid",kpid);
_XmlTextWriter.WriteElementString("starttime",startTime);
_XmlTextWriter.WriteElementString("endtime",startTime);
_XmlTextWriter.WriteEndElement();
_XmlTextWriter.WriteEndElement();
_XmlTextWriter.WriteEndDocument();
_XmlTextWriter.Flush();
_XmlTextWriter.Close();

// XmlDocument doc = new XmlDocument();
// string xmlName = Request["xmlName"];
// doc.Load(xmlPath + @"\" + xmlName);
// Response.Write(doc.OuterXml.Replace("?>","?>"));

}
}

private void Page_Unload (object sender , System.EventArgs e)
{
string endTime = DateTime.Now.ToString();
string filePath = Session["rootPath"].ToString();
string strFileName = filePath + "\\time.xml";
XmlDocument xdoc = new XmlDocument();
xdoc.Load(strFileName);
XmlNode xNode = xdoc.SelectSingleNode("//endtime");
xNode.InnerText = endTime;
xdoc.Save(strFileName);
}

#region Web 窗体设计器生成的代码
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

///

/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
///

private void InitializeComponent()
{
this.Load += new System.EventHandler(this.Page_Load);
this.Unload += new System.EventHandler(this.Page_Unload);
}
#endregion
}
}

Hi Gun

charjo

10:53 17 May '05

Gunawan, what's your email address? We were together at Primanet. Send me email if you read this. chairudin.harjo@gmail.com Thanks.
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Good and Tidy Example for beginner

Veronika Steffanca

7:59 26 Jan '05

Well, there may be others like this but this is one is more tidy and easy to follow. Good for begginer like me. Thanks
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Already been documented

Jacob Slusser

5:15 25 Jan '05

I rarely ever give negative reviews, but I felt this article was a poor attempt to demonstrate what has already been very well documented.

A quick Google search revealed many related articles, all of which are more comprehensive:

http://blog.daveranck.com/archive/2004/07/19/217.aspx[^]
http://www.15seconds.com/issue/020102.htm[^]
http://aspalliance.com/articleViewer.aspx?aId=134&pId=[^]
http://www.dnzone.com/ShowDetail.asp?NewsId=26[^]
http://www.expresscomputeronline.com/20031215/techspace01.shtml[^]

Please don't be discourage from writing publications, but make sure you do your research first.

Re: Already been documented

Caelib

3:32 28 Jan '05

Microsoft has provided extremely thorough documentation on the control execution lifecycle. The MSDN article can be found here: [^]
Sign In·View Thread·Permalink	5.00/5 (1 vote)

Re: Already been documented

Anonymous

7:23 31 Jan '05

I just did a quick glance at the other articles you linked (and the MSDN one), and IMO - they can go a bit overkill on information, epsically if you are just a beginner and want a very quick idea of - which of these get fired in what order?

While once you know that, and may want to dig deeper into things, the other articles are more usefull, but for what this one is trying to offer, it does it quite well and I would of found it to be more usefull for my needs a few months ago than the other articles you linked.

Information-Overload of being 'comphrensive' can prevent you from being able to see the forest because of the trees'.

While it may not be the most indepth article on the topic, I think it does what it aimed to do quite nicely.