If Session("idstruttura") IsNot Nothing OrElse Session("idstruttura") <> String.Empty Then
.......
        Else
            REM serve a pulire la cache
            Response.Cache.SetExpires(DateTime.UtcNow.AddMinutes(-1))
            Response.Cache.SetCacheability(HttpCacheability.NoCache)
            Response.Cache.SetNoStore()
             Session.Abandon()
            Response.Redirect("~/default.aspx")
        End If