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Way To Know Which Control Has Raised PostBack

, 13 Dec 2010
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This is the short code snippet with understanding to make you know about what is PostBack in ASP.NET and further how you can know about the Control that raised the PostBack by using inbuilt ASP.NET functionality and manually.

Introduction

The main aim of this article is to make you know among the several controls that resides in ASP.NET page, at the time of PostBack how to know which control has actually raised this event.

Background

Technical definition about PostBack: Most ASP.NET Controls are "Server Controls". This means that when an action occurs, the Page, which hosts all its controls inside <FORM... tags, makes a regular old HTTP POST, with itself being the target URL. This is referred to as a "POSTBACK". *

In simple words, we can differentiate PostBack and Redirection in this way.

Postback: It is the scenario in which ASP.NET page redirects to page itself.

Redirect: It is the scenario in which ASP.NET page is called from another page.

You can check whether the page is PostedBack or not with code side using IsPostBack Property which returns Bool.

Using the Code

The agenda of this article is to determine which control inside page has made page to be posted back to itself OR in other words which control has raised PostBack.

For the control which supports AutoPostBack property, ASP.NET Page has a inbuilt function which will determine the event raising control.

There are several controls that support AutoPostBack, among them some are:

  • Dropdown
  • CheckBoxList
  • RadioButtonList
  • TextBox, etc.

These controls calls _doPostback function in JavaScript generated by ASP.NET automatically.

Let's have a look at the _doPostback function.

function __doPostBack(eventTarget, eventArgument) {
            if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
                theForm.__EVENTTARGET.value = eventTarget;
                theForm.__EVENTARGUMENT.value = eventArgument;
                theForm.submit();
            }
        }

Note:

  • This function will not be visible until minimum of one control is raising the AutoPostBack inside page.
  • The _doPostBack event simply stores value in two hidden fields.
    1. eventTarget - Name of the control raised the postback
    2. eventArgument - The argument to be sent to server

Let's have a look at how those two hidden fields are on page.

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

Now if you want to get the ID of a raising control from the code side, you can write this simple snippet and find the values stored in hidden field:

if (IsPostBack)
{
    string ControlID = string.Empty;
    if (!String.IsNullOrEmpty(Request.Form["__EVENTTARGET"]))
    {
        ControlID = Request.Form["__EVENTTARGET"];
    }
}

OR alternatively, you can find full control object using:

if (IsPostBack)
{
    string ControlID = string.Empty;
    if (!String.IsNullOrEmpty(Request.Form["__EVENTTARGET"]))
    {
        ControlID = Page.Request.Params["__EVENTTARGET"];
        Control postbackControl = Page.FindControl(ControlID);
    }
}

_doPostBack function only will be called by AutoPostBack property supportive Controls now let's have a look of the other Control which don't support that inbuilt.

Controls that can be listed are:

  • Button
  • ImageButton, etc.

To make a functionality for Non Supportive AutopostBack Property Control, make one Hidden field on the page.

<asp:HiddenField ID="CustomHiddenField" runat="server" />

Call a custom Js function on the clientclick so that it will be stored in hidden field before page actually redirects to server.

<asp:Button ID="btn" runat="server" Text="Click me"
  OnClientClick = "SetSource(this.id)" />

function SetSource(SourceID)
    {
        var hidSourceID =
        document.getElementById("<%=CustomHiddenField.ClientID%>");
        hidSourceID.value = SourceID;
    }

Note: Always use "<%=CustomHiddenField.ClientID%>" convention because Hiddenfield ID can be renamed by ASP.NET.

And get a name once page is posted back by server using Codebehind.

if (IsPostBack)
{
    string CtrlID = string.Empty;
    if (!String.IsNullOrEmpty(Request.Form[CustomHiddenField.UniqueID]))
    {
        CtrlID = Request.Form[CustomHiddenField.UniqueID];
    }
}

Now below is a full code snippet for getting raising control among both type of controls.

protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        string ControlID = string.Empty;
        if (!String.IsNullOrEmpty(Request.Form["__EVENTTARGET"]))
        {
            ControlID = Request.Form["__EVENTTARGET"];
        }
        else
        {
            //Buttons and ImageButtons
            if (!String.IsNullOrEmpty(Request.Form[CustomHiddenField.UniqueID]))
            {
                ControlID = Request.Form[CustomHiddenField.UniqueID];
            }
        }
    }
} 

History

  • 14th December, 2010 - Truncated code with IsNullOrEmpty
  • 9th December, 2010 - Attached code and some typos corrected WTHO ThatRaja
  • 8th December, 2010 - Article published

Reference

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Hiren solanki
Software Developer
India India
He is a Smart IT devloper with Few years of Expeariance But having Great command on ASP.net,C#,SQL Query,SSRS,Crystal Reports
 
Apart from that He Loves multimedia work too, Master of Adobe photoshop, Illustrator, CSS , HTML and all things.
 
He is Currently working in Microsoft Dynamics CRM and Having Nice Expearince with CRM. CRM Rocks!!!

Comments and Discussions

 
GeneralMy vote of 5 Pinmemberdida9-Dec-10 21:48 
GeneralRe: My vote of 5 PinmemberHiren Solanki9-Dec-10 21:49 
GeneralMy vote of 5 Pinmembervirang9-Dec-10 18:56 
GeneralRe: My vote of 5 PinmemberHiren Solanki9-Dec-10 18:57 
GeneralMy vote of 5 PinmemberSChristmas9-Dec-10 4:54 
GeneralRe: My vote of 5 PinmemberHiren Solanki9-Dec-10 18:23 
GeneralA couple of suggestions PinmemberPete Sutcliffe9-Dec-10 4:38 
GeneralRe: A couple of suggestions. PinmemberHiren Solanki9-Dec-10 18:55 
Pete Sutcliffe wrote:
Firstly Buttons contain a property "UseSubmitBehavior" setting this to false will cause the button to use the __doPostBack function.

 
I am not aware about it till, But will have look on it further for sure.
 
Pete Sutcliffe wrote:
Secondly it might be useful to explain why such functionality is useful?

 
It's useful when you have many controls residing on your page and sometime it's very need to know to have determination of raising control before it makes round trip to server.
 
Pete Sutcliffe wrote:
There's probably a good reason why Microsoft didn't include this functionality out of the box.

 
Microsoft have provided _doPostBack out-of-box, They might have provided to make use of it only I think.
 
Pete Sutcliffe wrote:
I can't help but think if you need this you're probably doing something wrong

 
After making all the functionality if you come with some problem That only be solved by this, Then there's no need to reversing all the functionality again rather then using this simple solution.
 
The most probable reason for me to publish this article is that I have seen many question on internet asking for the same, Here's a solution for that all.
 
Thank you for your kind suggestion.
Regards,
Hiren.
Microsoft Dynamics CRM
 
modified on Friday, December 10, 2010 2:22 AM

GeneralMy vote of 5 Pinmembermarciomacedo9-Dec-10 1:44 
GeneralRe: My vote of 5 PinmemberHiren Solanki9-Dec-10 1:50 
GeneralMy vote of 5 Pinmembermadhukk9-Dec-10 0:15 
GeneralRe: My vote of 5 PinmemberHiren Solanki9-Dec-10 0:18 
GeneralMy vote of 1 PinmemberSelvin9-Dec-10 0:09 
GeneralReason for me to downvote you. PinmemberHiren Solanki9-Dec-10 0:23 
GeneralRe: My vote of 1 PinmemberHiren Solanki9-Dec-10 0:26 
GeneralMy vote of 5 PinmemberSandeep Ramani8-Dec-10 19:42 
GeneralRe: My vote of 5 PinmemberHiren Solanki8-Dec-10 19:51 
GeneralMy vote of 5 Pinmemberthatraja8-Dec-10 18:33 
GeneralRe: My vote of 5 [modified] PinmemberHiren Solanki8-Dec-10 18:46 
GeneralRe: My vote of 5 Pinmemberthatraja8-Dec-10 18:58 
Generalnice PinmemberPranay Rana8-Dec-10 8:56 
GeneralRe: nice PinmemberHiren Solanki8-Dec-10 18:20 
GeneralMy vote of 5 PinmemberMarcelo Ricardo de Oliveira8-Dec-10 6:43 
GeneralRe: My vote of 5 PinmemberHiren Solanki8-Dec-10 18:20 
GeneralMy vote of 5 Pinmemberlinuxjr8-Dec-10 5:56 

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