Something

        public static void Ecrire_Log(string IP, string UserID, string Guid, string Fonction, string Information)
        {
            XDocument doc = XDocument.Parse(File.ReadAllText("C:\\xmllog.xml"));

            /*
            CFER_Log log = new CFER_Log() { IP = IP,
                                            UserID = UserID, 
                                            Guid = Guid, 
                                            Fonction = Fonction, 
                                            Information = Information };*/

            XElement XML_log = new XElement("logs",
                                   new XElement("IP", IP),
                                   new XElement("UserID", UserID),
                                   new XElement("Guid", Guid),
                                   new XElement("Fonction", Fonction),
                                   new XElement("Information", Information));

            //XElement XML_log = new XElement("logs",IP, UserID, Guid, Fonction, Information);
       
            doc.Add(XML_log.Value.Replace(@"<logs>\n","").Replace(@"\n</logs>",""));
            doc.Save("C:\\xmllog.xml");
            //XML_log.Save("c:\\xmllog.xml");
        }