释放COM对象相关文档:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.releasecomobject.aspx
Marshal.FinalReleaseComObject 方法
http://msdn.microsoft.com/zh-cn/library/system.runtime.interopservices.marshal.finalreleasecomobject.aspx
Marshal.ReleaseComObject存安全隐患