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;

            }

        }