Hi,

You need to use "XmlInclude" to include the types of your Internal- and
Extend Data Object.

Below is an example:

using System;
using System.IO;
using System.Text;
using System.Xml;
using System.Collections;
using System.Xml.Serialization;

namespace testserialize
{
[Serializable]
public class BaseClass
{
 public BaseClass()
 {
 }
 public int x = 5;
}

[Serializable]
public class DerivedClass : BaseClass
{
 public DerivedClass()
 {
 }
 public int y = 8;
}

public class Foo
{
 public int Bar = 10;
}

/// <summary>
/// Summary description for Class1.
/// </summary>
[Serializable]
[XmlInclude(typeof(DerivedClass)), XmlInclude(typeof(BaseClass))]
public class Class1
{
 static void Main(string[] args)
 {
  XmlTextWriter writer = null;
  Class1 test = new Class1();
  try
  {
   if (!File.Exists("test.xml"))
   {
    File.Create("test.xml");
   }

   writer = new XmlTextWriter("test.xml", Encoding.Unicode);
   writer.Formatting = Formatting.Indented;
   XmlSerializer serializer = new XmlSerializer(typeof(Class1));
   serializer.Serialize(writer, test);
  }
  catch(Exception caught)
  {
   Console.WriteLine(caught.ToString());
  }
  finally
  {
   writer.Close();
  }
 }

 public Class1()
 {
  myArray = new ArrayList();
  myArray.Add(new BaseClass());
  myArray.Add(new DerivedClass());
 }

 [XmlArrayItem(typeof(DerivedClass), ElementName="Derived")]
 [XmlArrayItem(typeof(BaseClass), ElementName="Base")]
 public ArrayList myArray;
}
}

1) No ... the XmlSerializer is pretty picky about only serializing XML that
it can deserialize as well. In your case the XmlSerializer cannot determine
which type it serialized, because you're assigning the same element name to
two different classes.

2) Yes, you can by passing an XmlAttributeOverrides object to the
XmlSerializer constructor. See [0] and [1] for more details.