Como $ lookup com pipeline e deixar parâmetros em C # (MongoDB.Driver 2.7.2)

BsonArray subpipeline = new BsonArray();

subpipeline.Add(
    new BsonDocument("$match",new BsonDocument(
        "$expr", new BsonDocument(
        "$and", new BsonArray { 
             new BsonDocument("$eq", new BsonArray{"$stock_item", "$$order_item"} ), 
             new BsonDocument("$gte", new BsonArray{"$instock", "$$order_qty"} )
             }  
        )
    ))
);

var lookup = new BsonDocument("$lookup", 
                 new BsonDocument("from", "warehouses")
                             .Add("let", 
                                 new BsonDocument("order_item", "$item")
                                             .Add("order_qty", "$ordered"))
                             .Add("pipeline", subpipeline)
                             .Add("as", "stockdata")
);

var results = collection.Aggregate()
                        .Match(new BsonDocument("_id", 1))
                        .AppendStage<BsonDocument>(lookup).ToEnumerable();

foreach (var x in results)
{
    Console.WriteLine(x.ToJson());
}