Xml Serialize override problem

The code is below.. I want to be able to override two
classes to the same element name... I've commented out
the code which I cant get to work... the base of the code
is an extention of one of the .Net examples...
using System.Xml;
using System.Xml.Seri alization;
using System.Collecti ons;
using System.IO;
using System.Runtime. Serialization;
using System;

namespace ConsoleApplicat ion1
{
[System.Xml.Seri alization.XmlTy peAttribute("li brary")]
[System.Xml.Seri alization.XmlRo otAttribute
(IsNullable=fal se)]

public class library
{
[System.Xml.Seri alization.XmlEl ementAttribute
(Type=typeof(Or der))]
public ArrayList Orders;

public class Order
{
[System.Xml.Seri alization.XmlEl ementAttribute
("Books",Type=t ypeof(Book))]

[System.Xml.Seri alization.XmlEl ementAttribute
(Type=typeof(Ex pandedBook))]
//[System.Xml.Seri alization.XmlEl ementAttribute
(Type=typeof(Ex pandedBook2))]

public ArrayList Books;
}

public class Book
{
[System.Xml.Seri alization.XmlEl ementAttribute( )]
public string ISBN;
}

public class ExpandedBook:Bo ok
{
[System.Xml.Seri alization.XmlEl ementAttribute( )]
public bool NewEdition;
}

public class ExpandedBook2:B ook
{
}
}

public class Run
{
static void Main(string[] args)
{
Run ser = new Run();
ser.SerializeOb ject("c:\\class .xml");
}

public void SerializeObject (string filename)
{
XmlAttributes attrs = new XmlAttributes() ;
XmlElementAttri bute attra = new XmlElementAttri bute();
attra.ElementNa me = "BookImNew" ;
attra.Type = typeof(library. ExpandedBook);
attrs.XmlElemen ts.Add(attra);

//THIS IS THE MAIN PROBLEM
//Specify the same element name for a derived class
//XmlElementAttri bute attrb = new XmlElementAttri bute
();
//attrb.ElementNa me = "BookImNew" ;
//attrb.Type = typeof(library. ExpandedBook2);
//attrs.XmlElemen ts.Add(attrb);

XmlAttributeOve rrides attrOverrides = new
XmlAttributeOve rrides();
attrOverrides.A dd(typeof(libra ry.Order), "Books",
attrs);
XmlSerializer s =
new XmlSerializer(t ypeof(library), attrOverrides);

TextWriter writer = new StreamWriter(fi lename);

library newlib = new library();

// Creates the object that will be serialized.
library.Order myOrders = new library.Order() ;

myOrders.Books = new ArrayList ( ) ;

// Creates an object of the derived type.
library.Expande dBook a = new library.Expande dBook();

a.ISBN= "123456789" ;
a.NewEdition = true;
myOrders.Books. Add(a);

// Creates an object of the derived type.
//library.Expande dBook2 b = new library.Expande dBook2
();
//b.ISBN= "123456789" ;
//myOrders.Books. Add(b);

newlib.Orders = new ArrayList ( ) ;
newlib.Orders.A dd(myOrders);

// Serializes the object.
s.Serialize(wri ter,newlib);
writer.Close();
}
}

}