public class Item
    {
        private readonly string filename;
        private string code;
        public string Code
        {
            get
            {
                lock( this ) { return code; }
            }
            set
            {
                lock( this ) { code = value; }
            }
        }
        public Item( string filename )
        {
            this.filename = filename;
            if( !File.Exists(filename) )
            {
                code = "(no code)";
                return;
            }
            XmlDocument doc = new XmlDocument();
            doc.Load(filename);
            code = doc.SelectSingleNode("/item/code").InnerText;
        }
        public void Update()
        {
            string tempFullPath = Path.ChangeExtension(filename,".$$$");
            string bakFullPath = Path.ChangeExtension(filename,".bak");

            lock( this )
            {
                XmlTextWriter writer = new XmlTextWriter(tempFullPath,System.Text.Encoding.UTF8);
                try
                {
                    writer.WriteStartElement("item");
                    writer.WriteElementString("code",code);
                }
                finally
                {
                    writer.Close();
                }

                if( File.Exists(filename) )
                {
                    File.Delete(bakFullPath);
                    File.Move(filename,bakFullPath);
                }
                File.Move(tempFullPath,filename);
            }
        }
    }