Работа с xml c#

пытаюсь создать xml файл вот так:

[XmlRoot(ElementName = "payment")]
        public class Payment
        {
            [XmlAttribute(AttributeName = "trn")]
            public string Trn { get; set; }
            [XmlAttribute(AttributeName = "fsum")]
            public string Fsum { get; set; }
            [XmlAttribute(AttributeName = "tsum")]
            public string Tsum { get; set; }
            [XmlAttribute(AttributeName = "prv_id")]
            public string Prv_id { get; set; }
            [XmlAttribute(AttributeName = "account")]
            public string Account { get; set; }
            [XmlAttribute(AttributeName = "receipt")]
            public string Receipt { get; set; }
            [XmlAttribute(AttributeName = "time")]
            public string Time { get; set; }
        }
        [XmlRoot(ElementName = "auth")]
        public class Auth
        {
            [XmlElement(ElementName = "payment")]
            public Payment Payment { get; set; }
            [XmlAttribute(AttributeName = "count")]
            public string Count { get; set; }
            [XmlAttribute(AttributeName = "toAmount")]
            public string ToAmount { get; set; }
        }
        [XmlRoot(ElementName = "request")]
        public class Request
        {
            [XmlElement(ElementName = "requestNum")]
            public string RequestNum { get; set; }
            [XmlElement(ElementName = "terminalNum")]
            public string TerminalNum { get; set; }
            [XmlElement(ElementName = "login")]
            public string Login { get; set; }
            [XmlElement(ElementName = "password")]
            public string Password { get; set; }
            [XmlElement(ElementName = "clientType")]
            public string ClientType { get; set; }
            [XmlElement(ElementName = "auth")]
            public Auth Auth { get; set; }
        }
private void button3_Click(object sender, EventArgs e)
        {
            Request req = new Request();
            req.RequestNum = "PAY";
            req.TerminalNum = "123456";
            req.Login = "5454";
            req.Password = "2525";
            req.ClientType = "xml";
            req.Auth.ToAmount = "";
            req.Auth.ToAmount = "25";
            req.Auth.Payment.Account = "";
            req.Auth.Payment.Fsum = "";
            req.Auth.Payment.Prv_id = "";
            req.Auth.Payment.Receipt="";
            req.Auth.Payment.Time="";
            req.Auth.Payment.Trn = "";
            req.Auth.Payment.Tsum = "";
            XmlSerializer formatter = new XmlSerializer(typeof(Request));
            using (FileStream fs = new FileStream(@"C:\\1.xml", FileMode.OpenOrCreate))
            {
                // сериализация (сохранение объекта в поток) 
                formatter.Serialize(fs, req);
            }
            using (FileStream fs = new FileStream(@"C:\\1.xml", FileMode.OpenOrCreate))
            {
                // десериализация (создание объекта из потока) 
                Request req2 = (Request)formatter.Deserialize(fs);
            }
            XPathDocument xpathDoc = new XPathDocument(@"C:\\1.xml");
            XslCompiledTransform xslTransform = new XslCompiledTransform();
            xslTransform.Load(@"C:\\remove-namespace.xslt");
            using (XmlTextWriter xmlWriter = new XmlTextWriter(@"C:\\1.xml", null))
            {
                xslTransform.Transform(xpathDoc, null, xmlWriter);
            }
        }

Но выдается ошибка, подскажите пожалуйста что я делаю ни так???