Hi,
running that code :

await this.Collection.ReplaceOneAsync(x => x.Name == city.Name x.CountryCode == city.CountryCode, newObject, new UpdateOptions()

{ IsUpsert = true }

);

if the object doesn't exist it create the new one but with an empty id.
Here what is stored

/* 1 */

{ "_id" : ObjectId("000000000000000000000000"), "CreateAt" : [ NumberLong(635700317931446025), 0 ], "UpdateAt" : [ NumberLong(635700317931446025), 0 ], "Name" : "My value", "CountryCode" : "it", "Culture" : "en-US" }