Xml-serilizing with interfaces

A list of interfaces which is implemented by an abstract klass with two subclasses that need to be serialised. I have no idea how to do this since interfaces can not be serialised as I understand it.
My code generates the error message:

Cannot serialize interface ConsoleApplication4.Creature_interface.


..allthough Im not trying to serilize the interfaces themselves.

Which is the right way to deal with a problem like this? Is there any stright
way to deal with this problem or is there some kind of workaround needed?

Code:

---


using System;
using System.Collections.Generic;
using System.Text;
using UtilitiesLibrary;
using System.Xml.Serialization;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            Cat cat1 = new Cat("Knut", 6);
            Dog dog1 = new Dog("Bertil", 5);

            List<Creature_interface> Animallist = new List<Creature_interface>();
            Animallist.Add(cat1);
            Animallist.Add(dog1);

            UniversalSerializer.XMLSerialize("filename.xml", Animallist);
            Console.ReadLine();
           
        }
    }

    public interface Creature_interface
    {
        string Name { get;}
        int Age { get; }
    }

    public abstract class Creature : Creature_interface
    {   
        string _name;
        int _age;

        public Creature(string name, int age )
        {
            _name = name;
            _age = age;
        }

        public Creature()
        { }
       
        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }

        public int Age
        {
            get { return _age; }
            set { _age = value; }
        }

        public abstract void Speak();
    }

    [Serializable]
    public class Dog:Creature
    {   
        public Dog
            (string _name, int _age): base (_name, _age)
        { }

        public Dog { }

        public override void Speak()
        {
            Console.WriteLine("Bark Bark");
        }
    }

    [Serializable]
    public class Cat:Creature
    {
        public Cat
            (string _name, int _age): base (_name, _age)
        { }

        public Cat  { }



        public override void Speak()
        {
            Console.WriteLine("Mjau Mjau");
        }
    }

}

---

Code:

---

using System; 
using System.Collections.Generic; 
using System.Text; 
using System.IO; 
using System.Xml.Serialization; 
using System.Runtime.Serialization.Formatters.Binary; 

namespace UtilitiesLibrary 
{   
    public class UniversalSerializer 
    { 

        public static void XMLSerialize<T>(string filename, T objectToSerialize) 
        {   
            XmlSerializer Serializer = new XmlSerializer(typeof(List<Creature_interface>), new Type[] {typeof(Dog), typeof(Cat)});

            Stream fStream = new FileStream(filename, FileMode.Create, 
            FileAccess.Write, FileShare.None); 

            Serializer.Serialize(fStream, objectToSerialize); 
            fStream.Close(); 
        } 
//more methods... 
    } 
}

---

Thank you for any help.

since interface cannot be serialized, have you ever tried the base class - List<Creature>?? think it should work. furthermore, you must add a parameterless constructor to your Dog and Cat class.