如何正确地释放Excel对象？

---

     ......
    xlbook.Close(False)
    xl.Quit()
    NAR(xlsheet)
    NAR(xlbook)
    NAR(xl)
    GC.Collect()
    GC.WaitForPendingFinalizers()
    ......
    Private Sub NAR(ByRef o As Object)
        Try
            System.Runtime.InteropServices.Marshal.ReleaseComObject(o)
        Catch
        Finally
            o = Nothing
        End Try
    End Sub