using System;
using System.Collections.Generic;
using System.Xml.Serialization;

namespace ConsoleApplication30
{
    class Program
    {
        static void Main(string[] args)
        {
            SerializeSample();
            DeserializeSample();
            Console.ReadKey();
        }

        static void DeserializeSample()
        {
            const string xmlSample = @"<request> 
    <protocol-version>4.00</protocol-version> 
    <request-type>10</request-type> 
    <terminal-id>123</terminal-id> 
    <extra name=""login"">login</extra> 
    <extra name=""password-md5"">password-md5</extra> 
    <extra name=""client-software"">Dealer v3.22</extra>
</request>";
            var result = XmlSerializeHelper.Deserialize<Request>(xmlSample);
            Console.Write(result.ToString());
        } 

        static void SerializeSample()
        {
            var req = new Request
            {
                ProtocolVersion = "4.00",
                RequstType = 10,
                TerminalId = 123,
                Extras = new List<ExtraItem>
                {
                    new ExtraItem
                    {
                        Name = "login",
                        Value = "login"
                    },
                    new ExtraItem
                    {
                        Name = "password-md5",
                        Value = "password-md5"
                    },
                    new ExtraItem
                    {
                        Name = "client-software",
                        Value = "Dealer v3.22"
                    },
                }
            };
            var result = XmlSerializeHelper.Serialize(req, true);
            Console.Write(result);
        }
    }

    [XmlRoot("request")]
    public class Request
    {
        [XmlElement("protocol-version")]
        public string ProtocolVersion { set; get; }
        [XmlElement("request-type")]
        public int RequstType { set; get; }
        [XmlElement("terminal-id")]
        public int TerminalId { set; get; }
        [XmlElement("extra")]
        public List<ExtraItem> Extras { set; get; }
    }

    public class ExtraItem
    {
        [XmlAttribute("name")]
        public string Name { set; get; }
        [XmlText]
        public string Value { set; get; }
    }
}

using System.IO;
using System.Text;
using System.Xml;
using System.Xml.Serialization;

namespace ConsoleApplication30
{
    public static class XmlSerializeHelper
    {
        public class StringWriterUtf8 : StringWriter
        {
            public override Encoding Encoding
            {
                get
                {
                    return Encoding.UTF8;
                }
            }
        }
        
        public static string Serialize<T>(T value, bool emptyNs = false)
        {
            if (value.Equals(null)) return null;
            XmlSerializerNamespaces xmlNamespaces;
            if (!emptyNs)
            {
                xmlNamespaces = new XmlSerializerNamespaces();
                xmlNamespaces.Add("x", @"http://www.w3.org/2001/XMLSchema-instance");
            }
            else
            {
                xmlNamespaces = new XmlSerializerNamespaces(new[] {XmlQualifiedName.Empty});
            }

            var serializer = XmlSerializer.FromTypes(new[] { typeof(T) })[0];
            var settings = new XmlWriterSettings
            {
                Encoding = new UnicodeEncoding(false, false),
                Indent = true,
                IndentChars = "\t",
                NewLineChars= "\r\n",
                OmitXmlDeclaration = emptyNs
            };
            using (var textWriter = new StringWriterUtf8())
            {
                using (var xmlWriter = XmlWriter.Create(textWriter, settings))
                {
                    serializer.Serialize(xmlWriter, value, xmlNamespaces);
                }
                return textWriter.ToString();
            }
        }

        public static T Deserialize<T>(string xml)
        {

            if (string.IsNullOrEmpty(xml))
            {
                return default(T);
            }

            var serializer = XmlSerializer.FromTypes(new[] { typeof(T) })[0]; 

            var settings = new XmlReaderSettings();

            using (var textReader = new StringReader(xml))
            {
                using (var xmlReader = XmlReader.Create(textReader, settings))
                {
                    return (T)serializer.Deserialize(xmlReader);
                }
            }
        }

        public static T Deserialize<T>(XmlReader xmlReader)
        {
            var serializer = XmlSerializer.FromTypes(new[] { typeof(T) })[0];
            return (T)serializer.Deserialize(xmlReader);
        }
    }
}