c# Mongodb創建自增列

MongoCollection<BsonDocument> tblCount;

if (!db.CollectionExists(tblCountName))
           {
               db.CreateCollection(tblCountName);
           }
           tblCount = db.GetCollection(tblCountName);

public long Get()
        {          

            FindAndModifyArgs arg = new FindAndModifyArgs();
            arg.Query = Query.EQ("_id", "Inc");
            arg.Update = Update.Inc("seq", 1L);
            arg.Upsert = true;
            arg.VersionReturned = FindAndModifyDocumentVersion.Modified;

            var result = tblCount.FindAndModify(arg);
            return result.ModifiedDocument["seq"].AsInt64;
        }