MongoDB .NET not generating _id on upsert
And of course I find the answer immediately after posting the question. From this answer, the solution is to add a [BsonIgnoreIfDefault] attribute to the ID. In the example from the question it would be:
public class MongoObject{ [BsonId(IdGenerator = typeof(StringObjectIdGenerator))] [BsonRepresentation(BsonType.ObjectId)] [BsonIgnoreIfDefault] // <--- this is what was missing public string MongoID { get; set; } public int Index { get; set; }}