Hopefully I'm not misunderstanding something simple, but I have noticed I get different _id values generated in the database depending on whether I use InsertOne or ReplaceOne.
I try to avoid any dependancy on MongoDB libraries etc, so the Id field in my objects is always a string. However, I want to use the ObjectId generation pattern. In much older versions of the driver, I was using the StringObjectIdGenerator, and am trying to do so now too.
I'm using custom conventions to do my mapping (nothing strange in it, just that I use my own attributes to identify, for example, the ID field), and the important code looks like this:
var cmm = memberMap.ClassMap;
cmm.MapIdMember(memberMap.MemberInfo)
.SetSerializer(new StringSerializer(BsonType.ObjectId).WithRepresentation(BsonType.String))
.SetIgnoreIfDefault(true)
.SetIdGenerator(StringObjectIdGenerator.Instance);

If I call InsertOne, I get this:
"_id" : "56a75e69e73e572dbc1b75c4"

and if I call ReplaceOne (with a new document/no id set) I get this:
"_id" : ObjectId("56a75e6a87dc53ded31754e8")