thomasvdb thomasvdb - 3 months ago 33
ASP.NET (C#) Question

Get ASP.NET control which fired a postback within a AJAX UpdatePanel

Related to this question: On postback, how can I check which control cause postback in Page_Init event

If the control is wrapped in an ASP.NET AJAX UpdatePanel, the variable "control" is empty because it has a different ID after the AJAX PostBack.
Is there a solution to get the control which fired a postback within an ASP.NET Ajax UpdatePanel?

public static string GetPostBackControlName( Page page ) {
Control control = null;

/**
* First we will check the "__EVENTTARGET" because if the postback is made
* by controls which used the _doPostBack function, it will be available in the Request.Form collection.
*/
string ctrlname = page.Request.Params["__EVENTTARGET"];

if ( !String.IsNullOrEmpty( ctrlname ) ) {
control = page.FindControl( ctrlname );
} else {
/**
* If __EVENTTARGER is null, the control is a button-type and
* need to iterate over the form collection to find it.
*/
Control c = null;
string ctrlStr = null;

foreach ( string ctl in page.Request.Form ) {
if ( ctl.EndsWith( ".x" ) || ctl.EndsWith( ".y" ) ) {
/**
* ImageButtons have an additional "quasi-property" in their ID which identifies
* the mouse-coordinates (X and Y).
*/
ctrlStr = ctl.Substring( 0, ctl.Length - 2 );
c = page.FindControl( ctrlStr );
} else {
c = page.FindControl( ctl );
}

if ( c is Button || c is ImageButton ) {
control = c;
break;
}
}
}

if ( control != null ) {
return control.ID;
}

return string.Empty;
}

Answer

Try the following method:

public string GetAsyncPostBackControlID()
{
    string smUniqueId = ScriptManager.GetCurrent(Page).UniqueID;
    string smFieldValue = Request.Form[smUniqueId];

    if (!String.IsNullOrEmpty(smFieldValue) && smFieldValue.Contains('|'))
    {
        return smFieldValue.Split('|')[1];
    }

    return String.Empty;
}

The above method uses the hidden field of the ScriptManager on page. Its value can be accessed on the server by searching for a form key with the UniqueID of the ScriptManager. The value in the hidden field is in the format [UpdatePanel UniqueID]|[Postback Control ID]. Knowing this information we can retrieve the ID of the control that initiated the asynchronous postback. And it works for submit buttons too.