I can make it work. Please see " // HERE____" in your code. Now it compiles
and runs.

This line gives you the internal error:

  /** @attribute XmlElement(Type = Manager.class)   * */
  public Employee Manager;

Change it to:

   /** @attribute XmlElement(Manager.class, ElementName = "Manager") */
  public Employee Manager;

You have a few extra problems too I corrected for you.  =:o)

import System.*;
import System.Collections.*;
import System.IO.*;
import System.Xml.Serialization.*;

public class Group
{
   /** @attribute XmlElement(ElementName = "Members", Namespace =
http://www.cpandl.com) */
   public Employee Employees[];

   /** @attribute XmlElement(DataType = "double", ElementName = "Building")
*/
   public double GroupID;

   /** @attribute XmlElement(DataType = "hexBinary") */
   public ubyte HexBytes[];
   /** @attribute XmlElement(DataType = "boolean") */
   public boolean IsActive;

// HERE_____________
   /** @attribute XmlElement(Manager.class, ElementName = "Manager") */
   public Employee Manager;
// _________________

   /** @attribute XmlElement(int.class, ElementName = "ObjectNumber")
   * @attribute XmlElement(String.class, ElementName = "ObjectString") */
   public ArrayList ExtraInfo;
} //Group

public class Employee
{
   public String Name;
} //Employee

public class Manager extends Employee
{
   public int Level;
} //Manager

public class Runq
{
   public void SerializeObject(String filename)
   {

       // Create the XmlSerializer.
       XmlSerializer s = new XmlSerializer(Group.class.ToType());

       // To write the file, a TextWriter is required.
       TextWriter writer = new StreamWriter(filename);

       /* Create an instance of the group to serialize, and set its
properties. */
       Group group = new Group();
       group.GroupID = 10.089;
       group.IsActive = false;
       group.HexBytes = new ubyte[]{Convert.ToByte(100)};
       Employee x = new Employee();
       Employee y = new Employee();

// HERE_______
       x.Name = new String("Jack");
       y.Name = new String("Jill");
// ___________

       group.Employees = new Employee[]{x, y};
       Manager mgr = new Manager();

// HERE _____
       mgr.Name = new String("Sara");
// __________

       mgr.Level = 4;
       group.Manager = mgr;

       /* Add a number and a string to the ArrayList returned by the
ExtraInfo property. */
       group.ExtraInfo = new ArrayList();

// HERE________
       group.ExtraInfo.Add( (System.Int32)(42) );
       group.ExtraInfo.Add("Answer");
// ____________

       // Serialize the object, and close the TextWriter.

// HERE____
       try
       {
           s.Serialize(writer, group);
           writer.Close();
       }
       catch ( System.InvalidOperationException invalid )
       {
           System.Console.WriteLine( "Exception: "+invalid);
       }
// _______

   } //SerializeObject

   public void DeserializeObject(String filename)
   {
       FileStream fs = new FileStream(filename, FileMode.Open);
       XmlSerializer x = new XmlSerializer(Group.class.ToType());
       Group g = ((Group)(x.Deserialize(fs)));
       Console.WriteLine(g.Manager.Name);
       Console.WriteLine(g.GroupID);
       Console.WriteLine(g.HexBytes.get_Item( 0));
       for(int iCtr =0;iCtr < g.Employees.length;iCtr++)
       {
           Employee e = g.Employees[iCtr];

// HERE______
       Console.WriteLine(e.Name);
// __________

       }
   } //DeserializeObject

   public static void main(String[] args)
   {
       Runq test = new Runq();
       test.SerializeObject("FirstDoc.xml");
       test.DeserializeObject("FirstDoc.xml");
   }
}

Regards,
Lars-Inge T?nnessen
www.larsinge.com