Using IXmlSerializable interface on complex object graph

Question

If using custom XML Serialization (IXmlSerialiable), on a complex object that contains properties with constituent complex objects which do NOT use custom IXmlSerializable interface, how do you specify, in the IXmlSerializable.ReadXml(XmlReader reader) method, that you want the deserializer to use the ordinary deserialization on those child properties?

NOTE: similar to this question

Answer 1

The IXmlSerializable is a bit tedious to implement since it's pretty much an all or nothing approach given that you cannot select child types for normal XML serialization. However, if I understood you correctly you can achieve what you want by manually creating XmlSerializer for the types that do not implement IXmlSerializable.

For example, if we start with two classes, Default that does not implement IXmlSerializable and Custom which does implement it.

public class Default // Uses default XML Serialization
{
    public int Count { get; set; }
}

public class Custom : IXmlSerializable
{
    public int Count { get; set; }

    public XmlSchema GetSchema() { throw new NotImplementedException(); }

    public void ReadXml(XmlReader reader)
    {
        reader.ReadToDescendant("Count");
        this.Count = reader.ReadElementContentAsInt();
    }

    public void WriteXml(XmlWriter writer)
    {
        writer.WriteStartElement("Custom");
        writer.WriteElementString("Count", this.Count.ToString());
        writer.WriteEndElement();
    }
}

Then we create a third class Parent that has a child of each of the previous instances and implements IXmlSerializable in a way that calls ReadXml/WriteXml methods for the child that supports it and create default XML serializer for the other child.

public class Parent : IXmlSerializable
{
    public Parent()
    {
        this.Default = new Default { Count = 1 };
        this.Custom = new Custom { Count = 2 };
    }

    public Default Default { get; set; }
    public Custom Custom { get; set; }

    public XmlSchema GetSchema() { throw new NotImplementedException(); }

    public void ReadXml(XmlReader reader)
    {
        reader.ReadToFollowing("Custom");
        this.Custom = new Custom();
        this.Custom.ReadXml(reader);

        reader.ReadToFollowing("Default");
        var serializer = new XmlSerializer(typeof(Default));
        this.Default = (Default)serializer.Deserialize(reader);
    }

    public void WriteXml(XmlWriter writer)
    {
        this.Custom.WriteXml(writer);

        var ns = new XmlSerializerNamespaces();
        ns.Add("", "");
        new XmlSerializer(typeof(Default)).Serialize(writer, this.Default, ns);
    }
}

To make the example complete, a sample program that serializes and deserializes a Parent instance:

static void Main()
{
    var sb = new StringBuilder();
    var serializer = new XmlSerializer(typeof(Parent));

    serializer.Serialize(new StringWriter(sb), new Parent());

    Console.WriteLine(sb);

    var parent = (Parent)serializer.Deserialize(new StringReader(sb.ToString()));

    Console.WriteLine("Parent.Custom.Count: {0}", parent.Custom.Count);
    Console.WriteLine("Parent.Default.Count: {0}", parent.Default.Count);
}