Сериализация: паттерн composite

Как нужно записывать в xml-файл объекты класса? Я сделал это так, но при добавлении листа (Leaf) появляется исключение:

InvalidOperationException: Тип TreeS.Leaf не ожидался. Используйте атрибут XmlInclude или SoapInclude для задания типов, которые не известны как статические.

Что нужно исправить?

using System;
using System.IO;
using System.Xml;
using System.Xml.Serialization;
using System.Collections.Generic;
namespace TreeS
{
    public abstract class Question
    {
        [XmlElement("Question")]
        public string name;
        public Question()
        {
        }
        public Question(string name)
        {
            this.name = name;
        }
        public abstract void Display();
        public abstract void Add(Question c);
        public abstract void Remove(Question c);
        public virtual void Print()
        {
            Console.WriteLine(name);
        }
    }
    public class Answer : Question
    {
        [XmlArray("Question1"), XmlArrayItem("Leaf")]
        private List<Question> children = new List<Question>();
        public List<Question> GetChildren { get => children; }
        public Answer()
        {
        }
        public Answer(string name)
            : base(name)
        {
        }
        public override void Add(Question component)
        {
            children.Add(component);
        }
        public override void Remove(Question component)
        {
            children.Remove(component);
        }
        public override void Display()
        {
            Console.WriteLine(name);
            foreach (Question component in children)
            {
                component.Display();
            }
        }
        public override void Print()
        {
            Console.WriteLine("Вопрос: " + name);
            if (children.Count != 0)
            {
                Console.WriteLine("Ответы:");
                for (int i = 0; i < children.Count; i++)
                {
                    children[i].Print();
                }
                Console.WriteLine('\n');
            }
        }
    }
    public class Leaf : Question
    {
        public Leaf()
        {
        }
        public Leaf(string name)
            : base(name)
        {
        }
        public override void Display()
        {
            Console.WriteLine(name);
        }
        public override void Add(Question component)
        {
            throw new NotImplementedException();
        }
        public override void Remove(Question component)
        {
            throw new NotImplementedException();
        }
    }
    class Program
    {
        public static void Main()
        {
            Question question = new Answer("123");
            Question question1 = new Answer("5");
            Question answer = new Leaf("qwe");
            Question answer1 = new Answer("asd");
            question.Add(question1);
            question.Add(answer);
            question.Add(answer1);
            question1.Add(answer);
            question.Print();
            CreateXML("tree.xml", question);
            Answer question2 = GetXML("tree.xml");
            Console.ReadKey();
        }
        private static void CreateXML(string filename, Question question)
        {
            XmlSerializer xmlSerializer = new XmlSerializer(typeof(Answer));
            StringWriter stringWriter = new StringWriter();
            TextWriter writer = new StreamWriter(filename);
            xmlSerializer.Serialize(writer, question);
            xmlSerializer.Serialize(stringWriter, question);
            writer.Close();
        }
        private static Answer GetXML(string filename)
        {
            XmlSerializer serializer = new XmlSerializer(typeof(Answer));
            FileStream fs = new FileStream(filename, FileMode.Open);
            Answer collection;
            collection = (Answer)serializer.Deserialize(fs);
            return collection;
        }
    }
}

Делал по примеру Компоновщик