// Custom RO Serializer
public class ReadOnlyDictionarySerializer<TKey, TValue> : DictionarySerializerBase<ReadOnlyDictionary<TKey, TValue>, TKey, TValue>
{
    protected override ReadOnlyDictionary<TKey, TValue> CreateInstance() =>
        new ReadOnlyDictionary<TKey, TValue>(new Dictionary<TKey, TValue>());

    public override ReadOnlyDictionary<TKey, TValue> Deserialize(
        BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        var dict = BsonSerializer.Deserialize<IDictionary<TKey, TValue>>(context.Reader);
        return new ReadOnlyDictionary<TKey, TValue>(dict ?? new Dictionary<TKey, TValue>());
    }
}

// Wire it up Globally to both the concrete class as well as the interface
BsonSerializer.RegisterGenericSerializerDefinition(typeof(ReadOnlyDictionary<,>),
	typeof(ReadOnlyDictionarySerializer<,>));
BsonSerializer.RegisterGenericSerializerDefinition(typeof(IReadOnlyDictionary<,>),
	typeof(ReadOnlyDictionarySerializer<,>));