The simplest approach would be to keep the object and xml
representations in sync. You can still promote properties to the parent
by using a facade property and marking it as XmlIgnore. More complex
examples involve proxies (see the dictionary example), or custom
IXmlSerializable.

Marc

using System;
using System.IO;
using System.Xml.Serialization;
static class Program
{
    static void Main()
    {
        Venue venue = new Venue
        {
            Name = "The place",
            Address = new Address
            {
                Line1 = "Foo", Line2 = "Bar", PostCode="Zip"
            }
        };

        XmlSerializer ser = new XmlSerializer(typeof(Venue));
        StringWriter writer = new StringWriter();
        ser.Serialize(writer, venue);
        string xml = writer.ToString();
    }
}
[Serializable]
public sealed class Venue
{
    [XmlAttribute]
    public string Name { get; set; }
    public Address Address { get; set; }
    // PostCode is a facade property
    [XmlIgnore]
    public string PostCode
    {
        get
        {
            return Address == null ? null : Address.PostCode;
        }
    }
}
[Serializable]
public sealed class Address
{
    [XmlAttribute]
    public string Line1 { get; set; }
    [XmlAttribute]
    public string Line2 { get; set; }
    [XmlAttribute]
    public string PostCode { get; set; }

}