I'm attempting to use the new 2.4.2 LINQ features in the aggregation pipeline, but my data structures use a lot of dictionaries that need to be unwound. It looks like the driver does not support $unwind generation for dictionaries. Aggregation stage {$unwind : "$TestDictionary" } words wonders in the shell, but calling the query.ToString() function below throws a System.NotSupportedException:

The collection selector's serializer must implement IBsonArraySerializer: aggregate([]).OfType().Where(to => (to.Name == "TestName")).SelectMany(to => to.TestDictionary)

So I need to pull the whole dictionary locally and then parse it from there. It's not ideal, but there's a workaround. Am I missing something in the attributes or data structure that would allow this to work? If truly not supported, is there a plan to support $unwind via LINQ for Dictionary objects?

Data storage object:

public class TestObject
{
    public string Name {get;set;};
    [BsonDictionaryOptions( DictionaryRepresentation.ArrayOfArrays )]
    public Dictionary<string,string> TestDictionary { get; set; } = new Dictionary<string, string>();
}

Aggregation query:

var query = MongoCollection.AsQueryable()
                .OfType<TestObject>()
                .Where( to => to.Name == "TestName" )
                .SelectMany( to => to.TestDictionary );

string queryText = query.ToString();