Mit der folgenden Funktion kann man ermitteln, welches Control einen Postback ausgelöst hat.

   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

               Response.Write(getPostBackControl.ID)

    End Sub

 Private Function getPostBackControl() As Control
         Dim ctlM As Control = Nothing
        Dim ctl_Name As String = Page.Request.Params("__EVENTTARGET")

        If ctl_Name IsNot Nothing AndAlso ctl_Name <> "" Then
            ctlM = Page.FindControl(ctl_Name)
        Else
            Dim cx As Control = Nothing
            Dim ctrlStr As String = ""
            For Each ctl As String In Page.Request.Form
                If ctl.EndsWith(".x") OrElse ctl.EndsWith(".y") Then
                    ctrlStr = ctl.Substring(0, ctl.Length - 2)
                    cx = Page.FindControl(ctrlStr)
                Else
                    cx = Page.FindControl(ctl)
                End If
                If TypeOf cx Is System.Web.UI.WebControls.Button _
                        OrElse TypeOf cx Is System.Web.UI.WebControls.ImageButton Then
                    ctlM = cx
                    Exit For
                End If
            Next
        End If
        Return ctlM

 End Function