If you look closely at the __doPostBack function you will notice that the second argument is called the eventArgument.  You can allow controls to pass arguments to the doPostBack function.  Check out the code below.

Listing 6 – Passing arguments to the __doPostBack function

<input type="button" id="Button2" value="Press me" onclick="DoPostBack()" />
<script language="javascript" type="text/javascript">
 
function DoPostBack() 
{
  __doPostBack('Button2','My Argument');     
}
 
</script>
string passedArgument = Request.Params.Get("__EVENTARGUMENT");

Analysis

The “Button2” when clicked fires the DoPostBack function which in turn calls the __doPostBack.  The __doPostBack function contains two arguments, eventTarget and eventArgument.  The eventTarget is “Button2” and the eventArgument is “My Argument.”  Later, in the C# code behind, I have accessed the eventArgument using the Request.Params collection.  The passedArgument variable will contain the value “My Argument.”