How to revert back to 'default' XML serialization when implementing IXmlSerializable in a base class?

Question

I'm trying to serialize a class that inherits from a base class that implements IXmlSerializable.

The base class, called PropertyBag is a class that allows dynamic properties (credits to Marc Gravell).

I implemented IXmlSerializable so that the dynamic properties (stored in a Dictionary) are written as normal xml elements.

e.g. When serializing a class Person with a public property (non dynamic) Name and a dynamic property Age, I would like for it to generate the following XML:

<Person>
  <Name>Tim</Name>
  <DynamicProperties>
    <Country>
      <string>USA</string>
    </Country>
  </DynamicProperties>
<Person>

I can get the part to work with the following implementation of WriteXml in the base PropertyBag class:

public void WriteXml(System.Xml.XmlWriter writer)
    {
        writer.WriteStartElement("DynamicProperties");

        // serialize every dynamic property and add it to the parent writer
        foreach (KeyValuePair<string, object> kvp in properties)
        {
            writer.WriteStartElement(kvp.Key);

            StringBuilder itemXml = new StringBuilder();
            using (XmlWriter itemWriter = XmlWriter.Create(itemXml))
            {
                // serialize the item
                XmlSerializer xmlSer = new XmlSerializer(kvp.Value.GetType());
                xmlSer.Serialize(itemWriter, kvp.Value);                    

                // read in the serialized xml
                XmlDocument doc = new XmlDocument();
                doc.LoadXml(itemXml.ToString());

                // write to modified content to the parent writer
                writer.WriteRaw(doc.DocumentElement.OuterXml);
            }

            writer.WriteEndElement();
        }

        writer.WriteEndElement();
    }

However, when serializing the Person class, it no longer serializes the normal (non dynamic) properties unless I overwrite the WriteXml method in Person (which I do not want to do). Is there any way that in the base class I can automatically add the static properties? I know I can do this manually using reflection, but I was wondering if there is some built-in functionality in the .Net Framework?

Answer 1

I've spent quite a bit of time with XmlSerializer (and various other serialization APIs), and I'm pretty sure that simply: you can't. Implementing IXmlSerializable is all or nothing.

The closest I can think of is to cheat and move all the fixed properties to a sub-object; this would give you slightly different xml - something like:

<FixedProperties>
   <Name>Tim</Name>
</FixedProperties> 
<DynamicProperties>
  <Country>
    <string>USA</string>
  </Country>
</DynamicProperties>

but I expect it would work. You would have pass-thru properties on your base object:

[Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
public FixedProperties FixedProps {get;set;}
public string Name {
    get {return FixedProps.Name;}
    set {FixedProps.Name = value;}
}

Make sense? You could also mark Name as [XmlIgnore], but it seems pretty redundant. In your bespoke serialize method you'd use new XmlSerializer(typeof(FixedProperties))

Edit: Here's a working "serialize" example:

using System;
using System.ComponentModel;
using System.Xml.Serialization;

static class Program
{
    static void Main()
    {
        MyType obj = new MyType { Name = "Fred" };
        var ser = new XmlSerializer(obj.GetType());
        ser.Serialize(Console.Out, obj);
    }
}
public class MyType : IXmlSerializable
{
    public MyType()
    {
        FixedProperties = new MyTypeFixedProperties();
    }
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]
    public MyTypeFixedProperties FixedProperties { get; set; }
    [XmlIgnore]
    public string Name
    {
        get { return FixedProperties.Name; }
        set { FixedProperties.Name = value; }
    }

    System.Xml.Schema.XmlSchema IXmlSerializable.GetSchema()
    {
        return null;
    }

    void IXmlSerializable.ReadXml(System.Xml.XmlReader reader)
    {
        throw new System.NotImplementedException();
    }

    void IXmlSerializable.WriteXml(System.Xml.XmlWriter writer)
    {
        writer.WriteStartElement("DynamicProperties");
        writer.WriteElementString("Foo", "Bar");
        writer.WriteEndElement();
        fixedPropsSerializer.Serialize(writer, FixedProperties);
    }
    static readonly XmlSerializer fixedPropsSerializer
        = new XmlSerializer(typeof(MyTypeFixedProperties));

}
[XmlRoot("FixedProperties")]
public class MyTypeFixedProperties
{
    public string Name { get; set; }
}

Answer 2

Marc, your answer on putting the FixedProperties in a seperate collection got me thinking that instead of inheriting from PropertyBag, I should create a property of that type.

So I created a PropertyBagWrapper class that my Person class inherits from and it works.

[Serializable]
[TypeDescriptionProvider(typeof(PropertyBagDescriptionProvider))]    
public abstract class PropertyBagWrapper
{
    [Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]    
    public PropertyBag DynamicProperties { get; set; }

    public object this[string name]
    {
        get { return DynamicProperties[name]; }
        set { DynamicProperties[name] = value; }
    }
    protected PropertyBagWrapper()
    {
        DynamicProperties = new PropertyBag(this.GetType());
    }
}

[Serializable]    
public class Person : PropertyBagWrapper
{
    [Browsable(true)]
    public string Name { get; set; }
}

I won't repeat all the code for the PropertyBag and the custom classes needed for ICustomTypeDescriptor implementation, you can find that here.

I did move the TypeDescriptionProvider attribute from the PropertyBag class to the PropertyBagWrapper class.

The PropertyBag class still has the same implementation for WriteXml() method as posted in the question.