Dynamic serialization proposal

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.IO;

using System.Xml.Linq;
using System.Reflection;
using System.Runtime.Serialization;
namespace XmlSerializeTest
{
    public class DataClass
    {
        public DataClass Clone()
        {
            return (DataClass) this.MemberwiseClone();
        }
    }
    [DataContract]
    public class Foo : DataClass
    {
        private string test1 = "";
        private string test2 = "";
        [DataMember]
        public string Test1 { get { return test1; } set { test1 = value; } }
        [DataMember]
        public string Test2 { get { return test2; } set { test2 = value; } }

    }
    public class MeSerializer
    {
        Type targetType;
        public MeSerializer(Type targetType)
        {
            this.targetType = targetType;
            if (!targetType.IsDefined(typeof(DataContractAttribute), false))
                throw new Exception("No soup for you.");
        }

        public void WriteObjectDiff(Stream stream, object changed, object orig)
        {
            IEnumerable<PropertyInfo> properties =
                targetType.GetProperties().Where(p => p.IsDefined(typeof(DataMemberAttribute), false));

            var writer = new StreamWriter(stream);
            writer.Write("<" + targetType.Name + ">");

            foreach (PropertyInfo propInfo in properties)
            {
                string s_orig = propInfo.GetValue(orig, null).ToString();
                if (orig != null)
                {
                    string s_changed = propInfo.GetValue(changed, null).ToString();
                    if (s_orig.Equals(s_changed)) continue;
                }
                writer.Write("<" + propInfo.Name + ">" + propInfo.GetValue(orig, null) + "</" + propInfo.Name + ">");
            }
            writer.Write("</" + targetType.Name + ">");
            writer.Flush();
        }
    }
    public class Program
    {
        static void Main(string[] args)
        {
            Foo foo = new Foo();
            foo.Test1 = "bar";
            foo.Test2 = "ASDF";
            Foo foo2 = (Foo) foo.Clone();
            foo.Test2 = "ASDG";

            MeSerializer ser = new MeSerializer(foo.GetType());

            var output = new MemoryStream();
            ser.WriteObjectDiff(output, foo2, foo);
            output.Seek(0, SeekOrigin.Begin);
            string s = Convert.ToString(XElement.Parse(Encoding.ASCII.GetString(output.GetBuffer()).Replace("\0", "")));
            Console.WriteLine(s);
            Console.ReadKey();
        }
    }
}