Autocad API,读取块属性强制转换错误

我收到以下错误

“无法强制转换类型为的对象
autodesk.autocad.databaseservices。要键入的多段线 autodesk.autocad.databaseservices.blockreference

同时读取带有大量绘图内容的块属性。

代码:

Dim db As Database = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Database

Using tr As Transaction = db.TransactionManager.StartTransaction

Dim ed As Editor = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor
Dim blockids As ObjectIdCollection = New ObjectIdCollection
Dim bt As BlockTable = tr.GetObject(db.BlockTableId, OpenMode.ForRead)
Dim btr As BlockTableRecord = tr.GetObject(bt(BlockTableRecord.ModelSpace), OpenMode.ForRead)

For Each objid As ObjectId In btr

    If btr.IsAnonymous = False Or btr.IsLayout = False Then

        Dim blkref As BlockReference = tr.GetObject(objid, OpenMode.ForRead)

        Dim attcol As AttributeCollection = blkref.AttributeCollection

        For Each attid As ObjectId In attcol

            Dim attref As AttributeReference = tr.GetObject(attid, OpenMode.ForRead)

            ed.WriteMessage(attref.Tag.ToString & vbNewLine)

            ed.WriteMessage(attref.TextString.ToString & vbNewLine)

        Next

    End If

Next

tr.Commit()

End Using

你能帮忙吗!!!