Unwind then Group aggregation in MongoDB C#
When you let Unwind infer the type parameters it will use the collection type for TResult and BsonDocument for TNewResult.
If you want to use a specific type and not BsonDocument you need to add these type parameters:
var pipeline = usersCollection.Aggregate() .Unwind<OriginalType, NewResultType>(....As always, you need to make sure the operation actually returns something that can be of that type.