Id of Upserted Mongo Document in C#

It has been well documented how to get a return Id from an Insert to a MongoDb collection in C# (Example). But how do I get that for an upserted document? Is there a way, short of querying for the Id again, to use the SafeModeResult returned by an Update to find the upserted document Id?

My code:
var query = abc;
var update = 开发者_C百科xyz;
try
{
   db["Collection"].Update(query, update, UpdateFlags.Upsert);
}
catch
{
}

---

Solution found:

db["Collection"].FindAndModify(query, SortBy.Ascending(), update, true, true).Response.GetValue(1).AsBsonDocument.GetValue("_id")

returns the ObjectId.