How to serialize FilterDefinition<T> and SortDefinition<T> to string

MongoSerializingExtensions.cs

public static class MongoSerializingExtensions {
        public static BsonDocument RenderToBsonDocument<T>(this FilterDefinition<T> filter) {
                var serializerRegistry = BsonSerializer.SerializerRegistry;
                var documentSerializer = serializerRegistry.GetSerializer<T>();
                return filter.Render(documentSerializer, serializerRegistry);
        }

        public static BsonDocument RenderToBsonDocument<T>(this SortDefinition<T> sort) {
                var serializerRegistry = BsonSerializer.SerializerRegistry;
                var documentSerializer = serializerRegistry.GetSerializer<T>();
                return sort.Render(documentSerializer, serializerRegistry);
        }
}