What is the __doPostBack function and how can it be used?
This article would like to explain the functionality of the postback mechanism in ASP .NET and how the pages can be posted to the server from client code.
Let’s suppose we have a simple ASP .NET page which contains a LinkButton:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" > <head runat="server"> <title></title> </head> <body> <form id="form1" runat="server"> <div> <asp:LinkButton ID="lnkTest" runat="server" /> </div> </form> </body> </html>
When this page is render if we take a look at the source of the page (any modern browser has this feature, just press right click on the page and select View Source or View Page Source) it’s going to look like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head><title>
</title></head>
<body>
<form name="form1" method="post" action="Default.aspx" id="form1">
<div>
<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUJNTc5OTI5MzM1ZGRIdK4YuAGq1dppJ0EPmqso2jMx1g==" />
</div>
<script type="text/javascript">
//<![CDATA[
var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
theForm.__EVENTTARGET.value = eventTarget;
theForm.__EVENTARGUMENT.value = eventArgument;
theForm.submit();
}
}
//]]>
</script>
<div>
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWAgLrmfXzCgKSw44DvJoR+Ww1XMuknKIlXE7tgDogJ9c=" />
</div>
<div>
<a id="lnkTest" href="javascript:__doPostBack('lnkTest','')"></a>
</div>
</form>
</body>
</html>
First of all we can see that our LinkButton is rendered as an anchor element (<a> tag). Also the reference of that link points to a javascript function named __doPostBack, which is defined earlier in the page. This method has 2 parameters: eventTarget (this is the id of the control that determines the function to be executed) and eventArgument (this is an optional argument which can be set up if needed).
If we inspect the code of the __doPostBack function we can see that it first sets the values of two hidden fields created by ASP .NET named __EVENTTARGET and __EVENTARGUMENT with the two parameters passed to the function. After this the page is submitted back to the server.
When a page is posted back to the server ASP .NET inspects __EVENTTARGET and __EVENTARGUMENT values and this way it can decide which of the controls caused the page to be postedback and what is the event that has to be handled.
