Custom Discriminator Convention

using System.Linq;
using System.Threading;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Driver.Builders;
using MongoDB.Driver.Linq;
using NUnit.Framework;
using System.Collections.Generic;
using System;
using MongoDB.Bson;
using MongoDB.Bson.Serialization;
using System.Linq.Expressions;
using MongoDB.Bson.Serialization.Conventions;

namespace MongoDB.Groups.Repro
{
    public static class Program
    {
        public interface IFoo
        {
            string Description { get; set; }
        }

        [Serializable]
        public class Foo : IFoo
        {
            public string Description { get; set; }
        }

        [Serializable]
        public class Bar
        {
            public ObjectId Id { get; set; }

            public IFoo MyFoo { get; set; }
        }

        public static void Main()
        {
            BsonSerializer.RegisterDiscriminatorConvention(typeof(IFoo), new FooDiscriminatorConvention());

            var server = MongoServer.Create();
            var database = server.GetDatabase("test");
            var collection = database.GetCollection<Bar>("bars");

            collection.RemoveAll();

            var docWithDiscriminator = new BsonDocument
            {
                new BsonElement("MyFoo", new BsonDocument("Description", "Blah").Add("_t", "Foo"))
            };
            var docWithOutDiscriminator = new BsonDocument
            {
                new BsonElement("MyFoo", new BsonDocument("Description", "Blah"))
            };

            collection.InsertBatch(new[] { docWithDiscriminator, docWithOutDiscriminator });

            var bars = collection.FindAll();
        }

        private class FooDiscriminatorConvention : IDiscriminatorConvention
        {
            public string ElementName
            {
                get { return "_t"; }
            }

            public Type GetActualType(MongoDB.Bson.IO.BsonReader bsonReader, Type nominalType)
            {
                var bookmark = bsonReader.GetBookmark();
                bsonReader.ReadStartDocument();
                if (bsonReader.FindElement(ElementName))
                {
                    var value = bsonReader.ReadString();
                    if (value != "Foo")
                    {
                        throw new NotSupportedException();
                    }
                }

                bsonReader.ReturnToBookmark(bookmark);
                return typeof(Foo);
            }

            public BsonValue GetDiscriminator(Type nominalType, Type actualType)
            {
                return actualType.Name;
            }
        }
    }
}