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

Question

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 1

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.

Answer 2

Here's @bugventure 's code in VB.NET... With the modification of adding a Page reference as a parameter so I could use it in a common library.

Public Function GetAsyncPostBackControlID(ByRef page As Page) As String
        Dim smUniqueId As String = ScriptManager.GetCurrent(page).UniqueID
        Dim smFieldValue As String = page.Request.Form(smUniqueId)

        If Not String.IsNullOrEmpty(smFieldValue) AndAlso smFieldValue.Contains("|") Then
            Return smFieldValue.Split("|")(1)
        End If

        Return String.Empty
End Function

Usage:

' Call from a control or Page
Dim postbackControlID As String = GetAsyncPostBackControlID(Page)

' Call from a page
Dim postbackControlID As String = GetAsyncPostBackControlID(Me)