using System;

using System.Xml;

using System.Xml.Linq;

using System.Xml.Schema;

using System.Xml.Serialization;

public class Program

{

    public static void Main()

    {

        var house = new House

        {

            Address = new FlatAddress

                              {

                                 Street = "SomeStreet",

                                 HouseNo = "123",

                                 State = "Texas"

                              }

        };

        var serializer = new XmlSerializer(typeof(House));

        serializer.Serialize(Console.Out, house);

    }

}

​

public sealed class House

{

    public FlatAddress Address { get; set; }

}

​

public sealed class FlatAddress : IXmlSerializable

{

    public string Street { set; get; }

    public string HouseNo { set; get; }

    public string State { set; get; }

    public XmlSchema GetSchema()

    {

        return default;

    }

    public void ReadXml(XmlReader reader)

    {

        while (reader.Read())

        {

            switch (reader.Name)

            {

                case nameof(Street):

                    this.Street = reader.ReadContentAsString();

                    break;

                case nameof(HouseNo):

                    this.HouseNo = reader.ReadContentAsString();

                    break;

                case nameof(State):

                    this.State = reader.ReadContentAsString();

                    break;

            }

        }

    }

    public void WriteXml(XmlWriter writer)

    {

        writer.WriteElementString(nameof(Street), this.Street);

        var houseNoXml = new XElement(nameof(HouseNo), this.HouseNo);

        writer.WriteRaw(houseNoXml.ToString());

        var stateXml = new XElement(nameof(State), this.State);

        writer.WriteRaw(stateXml.ToString());

    }

}