Asp.net Postback Mechanism(doPostBack function)

Let us understand how asp.net postback works

What is __doPostBack function in javascript and what it does? In asp.net all the web server controls except Button and ImageButton (discussed at the end) use javascript __doPostBack function to trigger postback. This behavior is known as asp.net postback mechanism.

To better understand this function lets go step by step. Lets take any web server control except Button and ImageButton on a sample aspx page.

///ASPX

<asp:LinkButton ID="LinkButton1" runat="server" OnClick="LinkButton1_Click" Text="LinkButton" />

///Code behind

protected void LinkButton1_Click(object sender, EventArgs e) { }

Run the page in browser and view it source.

///JavaScript

function __doPostBack(eventTarget, eventArgument) {

    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {

        theForm.__EVENTTARGET.value = eventTarget;

        theForm.__EVENTARGUMENT.value = eventArgument;

        theForm.submit();

    }

}

///HTML

<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />

<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />



<a id="LinkButton1" href="javascript:__doPostBack('LinkButton1','')">LinkButton</a>

As you can see _doPostBack function takes two arguments viz. eventTarget and eventArgument. Moreover two hidden fields, “__EVENTTARGET” and “__EVENTARGUMENT,” are automatically declared which are used to store values of ‘eventTarget’ and ‘eventArgument’ respectively.

When LinkButton1 is clicked, __doPostBack function in javascript is called passing it respective arguments, in this case eventTarget is LinkButton1 and eventArgument is empty, these values are stored in respective hidden fields i.e. they are added to form parameter collection and finally the form is submitted. On server, ASP.NET retrieves and uses the values from form collection and understands which control caused the postback.

Ok if you have some confusion on these values are retrieved, lets do it ourselves to clear the things out. Take another control on web form, say another link button ‘LinkButton2’.

Insert break point on Page_Load event and hit F5. Click on LinkButton1, check the values that are retrieved in Page_Load event.

As you see __EVENTTARGET key returns the value of the control that caused postback, here LinkButton1. Now click on LinkButton2 and check the value you get, it will be “LinkButton2”. This is how the asp.net postback mechanism works through use of javascript __doPostBack function.

Button and ImageButton

These are the only two controls which cause postback in asp.net. Rest (as explained above) use javascript __doPostBack function and post the form (ASP.NET postback mechanism). In other words these two controls use client browser’s submit behavior and hence they do not use __doPostBack function.

However you can make Button to use asp.net postback mechanism by making use of Button.UseSubmitBehavior property (this property is not available for ImageButton control). UseSubmitBehavior property is of type bool and decides whether Button should use client browser’s submit behavior (Default. value = true) or asp.net postback mechanism (value = false).

Lets take two Button control on page:

<asp:Button ID="Button1" runat="server" Text="Uses browser submit behavior" UseSubmitBehavior="true" />

<asp:Button ID="Button2" runat="server" Text="Uses asp.net postback mechanism" UseSubmitBehavior="false" />

Note that you don’t have to specify UseSubmitBehavior=”true” explicitly as it is the default value/behavior. I have added it just for the sake of understanding. Now run this page and view its source in browser, you will these buttons rendered as follows:

<input type="submit" name="Button1" value="Uses browser submit behavior" id="Button1" />

<input type="button" name="Button2" value="Uses asp.net postback mechanism" onclick="javascript:__doPostBack('Button2','')" id="Button2" />

Button2 for which we set UseSubmitBehavior property to false, will now use _doPostBack function to submit the form.

Hope this helps…Happy Coding!!!!