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How to know which control has triggered the Post back event.

There may be scenarios where we require to know which control has triggered the postback event both at client-side and server-side.
 
For all the controls except button and Imagebutton
Below are two hidden fields added by Asp.net.
<input type=”hidden” name=”__EVENTTARGET” id=”__EVENTTARGET” value=”” />
<input type=”hidden” name=”__EVENTARGUMENT” id=”__EVENTARGUMENT” value=”” />
“__EVENTTARGET” value is set to the control id who has triggered the postback. This happens for all the controls except button and Imagebutton.

At client side, on body onbeforeunload event, check the value of “__EVENTTARGET” , this will return the control ID of the control which has triggered the postback.

document.getElementById(‘__EVENTTARGET’).value;

At server side, on Page_load check the control which has caused the postback.

string controlName = Request.Params.Get(“__EVENTTARGET”);

For Buttons and Imagebuttons

For Buttons and ImageButton, the “__EVENTTARGET” is set to empty string.
In this case, we need to check the data posted to server ( key/value pair).
In postbacks triggered by other controls except button and Image button, the key-value pair does not contains the buttonid and its value.
But if the postback is triggered by a button/imagebutton in this case the posted data has the buttonID and value it empty string.

At server side check the “__EVENTTARGET”, If its empty and check the control type to know which button has triggered the postback

foreach (string controlID in Request.Form)
{
  Control objControl = Page.FindControl(controlID);
  if (objControl is Button)
  {
    control = objControl;
    break;
  }
}

Explanation:

In ASP.net, all the controls (except buttons and imagebuttons) uses javascript function  __doPostBack to trigger postback.
If the web page has any control whose AutoPostBack Property is set to true then the below is the code snippet added to page by ASP.net

 

<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>

The Button controls and ImageButton controls does not call the __doPostBack function. Because of this, the _EVENTTARGET will always be empty.
When the user clicks on the “Submit” button, the content of the form is sent to the server. The form’s action attribute defines the name of the file to send the content to.
The form data (key-value pair) sent to the server has the button control id that has caused the triggered postback.

 

Tejaswini Das , ASP.Net Programmer, Mindfire Solutions.
Related Tags: ASP.NET
Author : Tejaswini Das

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