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Get control name in Page_Load event which make the post back


      Few day back i need post back control name in the Page_Load event. I did lots of search on net. I found few intersting fact about that and i am sharing that.

There are two types of controls which make post back in ASP.NET. One button type control like image button, button (whose type is “submit”), and another type use javascript function “_doPostBack” for the post back.

If post back control is button type then it will be added in the Request.Form collection means if there are two button in a page named button1 and button2 and if button1 make post back then only button1 will be in Request.Form Collection not button2 (but Request.Form collection can contains other server controls also like if page contains few textbox, dropdown list etc.) and if post back made by the button2 then only button2 will be available in Request.Form collection not button1(with other server control as I discuss earlier).

So if you want to catch which button type control made a post back in page load event, you have to just iterate the Request.Form collection.

 I’ll show you demo latter in this article.

Another category of server control who make post back use the client side javascript function _doPostBack like if we made autopostback true for dropdown list, radio button etc. 

If you look view source you will found _doPostBack javascript function.

function __doPostBack(eventTarget, eventArgument)

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

    theForm.__EVENTTARGET.value = eventTarget;

    theForm.__EVENTARGUMENT.value = eventArgument;

As you can see this function take two arguments “eventTarget” and “eventArgument”. “eventTarget” used for the control ID who is responsible for the postback and “eventArgument” used for the additional information about the control.

If you look at the view source you will also found these two hidden fields.

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

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

Lets add one dropdown server control in page and make autopostback true also add some dummy data. If you look at the view source you will found

<select name="DropDownList1" onchange="javascript:setTimeout('__doPostBack(\'DropDownList1\',\'\')', 0)" id="DropDownList1">

            <option value="1">abc</option>

            <option value="2">xyz</option>


ASP.NET engine automatically add onchange event and call the _doPostBack function and pass the appropriate parameter.

_doPostBack function first set the value of those hidden field and then submit the form. So if you want to know whether this dropdown list make post back or not you have to just check the value of “__EVENTTARGET” hidden field from the form parameter collection. I’ll show you code as well.

In my example I am adding two buttons, one image button, one dropdown list, one checkbox and two image buttons. I’ll try to print the control name which makes the post back. All I’ll try to find in page load event. So be ready for the ride.

partial class _Default : System.Web.UI.Page

  void Page_Load(object sender, EventArgs e)

    if (IsPostBack) Response.Write(getPostBackControlName());

  string getPostBackControlName()

    Control control = null;

    // first we will check the "__EVENTTARGET" because if post back made by the
    // controls

    // which used "_doPostBack" function also available in Request.Form
    // collection.

    string ctrlname = Page.Request.Params["__EVENTTARGET"];

    if (ctrlname != null && ctrlname != String.Empty)

      control = Page.FindControl(ctrlname);


    // if __EVENTTARGET is null, the control is a button type and we need to

    // iterate over the form collection to find it


      string ctrlStr = String.Empty;

      Control c = null;

      foreach (string ctl in Page.Request.Form)

        // handle ImageButton they having an additional "quasi-property" in
        // their Id which identifies

        // mouse x and y coordinates

        if (ctl.EndsWith(".x") || ctl.EndsWith(".y"))

          ctrlStr = ctl.Substring(0, ctl.Length - 2);

          c = Page.FindControl(ctrlStr);



          c = Page.FindControl(ctl);

        if (c is System.Web.UI.WebControls.Button ||

            c is System.Web.UI.WebControls.ImageButton)

          control = c;


    return control.ID;

I Hope that will help you out in some situation. If you have any issue you can post in this blog.


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