Pass the page to the function and the control that caused the postback will be returned from the function

    Public Function GetPostBackControl(ByVal page As System.Web.UI.Page) As System.Web.UI.Control  
        ' Find which control caused the postback  
        Dim control As Control = Nothing  
        Dim ctrlname As String = page.Request.Params("__EVENTTARGET")  
        If Not (ctrlname Is Nothing) AndAlso Not (ctrlname = String.Empty) Then  
            control = page.FindControl(ctrlname)  
        Else  
            Dim ctrlStr As String = String.Empty  
            Dim c As Control = Nothing  
           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)  
                   c = page.FindControl(ctrlStr)  
               Else  
                   c = page.FindControl(ctl)  
               End If  
               If TypeOf c Is System.Web.UI.WebControls.Button OrElse TypeOf c Is System.Web.UI.WebControls.ImageButton Then  
                   control = c  
                   ' break  
               End If  
           Next  
       End If  
       Return control  
   End Function