MongoDB .NET/C# Polymorphism BsonKnownTypes

Program.cs

using System;
using MongoDB.Bson;
using MongoDB.Driver;
using MongoDB.Bson.Serialization;
using MongoDB.Bson.Serialization.Serializers;
using MongoDB.Bson.Serialization.Attributes;
using System.Collections.Generic;

namespace Application
{
    [BsonKnownTypes(typeof(Cat), typeof(Dog))]
    public class Pet 
    {
        public string Name {get; set;}
    }
    public class Cat : Pet 
    {
        public string Toy {get; set;}
    }
    public class Dog : Pet 
    {
        public string Toy {get; set;}
    }
    public class House
    {
        public ObjectId Id {get; set;}
        public List<Pet> Pets {get; set;}
    }

    class Program
    {
        static void Main(string[] args)
        {
            var mongoURL = new MongoUrl("mongodb://localhost:27017/test");
            var client = new MongoClient(mongoURL);
            var database = client.GetDatabase("test");            
            var collection = database.GetCollection<House>("myhouse");

            /* Part A: Serialize */
            var mypets = new List<Pet>();
            mypets.Add(new Cat{ Name="Izzy", Toy="scratchy"} );
            mypets.Add(new Dog{ Name="Dotty", Toy="plushy"} );
            collection.InsertOne(new House { Pets=mypets});

            /* Part B: Deserialize */
            var document = collection.Find<House>(new BsonDocument()).FirstOrDefault();
            Console.WriteLine(document.ToJson());

        }
    }
}