SharePoint eventhandler 如何判断一个文档是否被人修改

 public override void ItemUpdating(SPItemEventProperties properties)
        {
            try
            {
                this.EventFiringEnabled = false;
                if ((string)properties.BeforeProperties["vti_contenttag"] != (string)properties.AfterProperties["vti_contenttag"])
                {
                        //do some work
                }
            }
            catch (Exception ex)
            {
                LogError(ex);
            }
            finally
            {
                this.EventFiringEnabled = true;
            }
        }