Given an entity defined as such:

public class Foo
{
public ObjectId Id

{ get; set; }
public string Bar { get; set; }

}

I want the ability to "upsert" a document with an object having that type, where a new record would have an auto-generated "_id". After executing the following:

var foo = new Foo

{ Bar = "hello world" }

;
collection.ReplaceOne(x => x.Id.Equals(foo.Id), foo, new UpdateOptions

{ IsUpsert = true }

);

The mongo shell, however, reports the following:
> db.foo.find()

{ "_id" : ObjectId("000000000000000000000000"), "Bar" : "hello world" }

The _id inserted is equivalent to ObjectId.Empty. Even applying the [BsonIgnoreIfDefault] attribute atop the Id property has no effect.

Expected Behavior:

The .ReplaceOne() and .ReplaceOneAsync() methods would recognize that the Id has not been explicitly initialized and will "insert" with an auto-generated _id (when used in conjunction with the "upsert" flag) instead of performing, what it appears to be, an "update".