无法更新AutoCAD中的属性

Question

以下代码不会更新属性，也无法确定我的属性未更新的原因。

案例陈述肯定受到打击，因此出于某种原因我的更改未提交给数据库。

Database db = new Database(false, true);
        using (Transaction tr = db.TransactionManager.StartTransaction())
        {
            db.ReadDwgFile(fileName, System.IO.FileShare.Read, false, "");
            db.CloseInput(true);
            BlockTable bt = tr.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;
            BlockTableRecord acBlkTblRec = tr.GetObject(bt[BlockTableRecord.PaperSpace], OpenMode.ForWrite) as BlockTableRecord;
            foreach (ObjectId id in acBlkTblRec)
            {
                DBObject obj = tr.GetObject(id, OpenMode.ForRead);
                BlockReference blkRef = obj as BlockReference;
                if (blkRef != null)
                {
                    AttributeCollection attCol = blkRef.AttributeCollection;
                    foreach (ObjectId objID in attCol)
                    {
                        DBObject dbObj = tr.GetObject(objID, OpenMode.ForWrite) as DBObject;

                        AttributeReference acAttRef = dbObj as AttributeReference;
                        switch (acAttRef.Tag)
                        {
                            case "DRAWINGTITLE1":
                                acAttRef.TextString = controllerName;
                                break;
                            case "DATE1":
                            case "DATE4":
                            case "XX_XX_XX":
                                acAttRef.TextString = DateTime.Now.ToShortDateString();
                                break;
                            case "DW1":
                            case "CK1":
                            case "XXX":
                                acAttRef.TextString = userSuffix;
                                break;
                            default:
                                break;
                        }
                    }
                }
            }
            tr.Commit();
        }