通过C#列出.dwg文件的属性时会抛出“公共语言运行时检测到无效程序”

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;

public class DWGPropertyReader
{
    [CommandMethod("ReadDWGProperties")]
    public void ReadDWGProperties()
    {
        Document doc = Application.DocumentManager.MdiActiveDocument;
        Database db = doc.Database;
        Editor ed = doc.Editor;

        using (Transaction tr = db.TransactionManager.StartTransaction())
        {
            BlockTable bt = tr.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;
            BlockTableRecord btr = tr.GetObject(bt[BlockTableRecord.ModelSpace], OpenMode.ForRead) as BlockTableRecord;

            foreach (ObjectId objId in btr)
            {
                Entity ent = tr.GetObject(objId, OpenMode.ForRead) as Entity;
                AttributeCollection attCol = ent.AttributeCollection;

                foreach (ObjectId attId in attCol)
                {
                    AttributeReference attRef = tr.GetObject(attId, OpenMode.ForRead) as AttributeReference;
                    ed.WriteMessage("Attribute: " + attRef.Tag + ", Value: " + attRef.TextString + "\n");
                }
            }

            tr.Commit();
        }
    }
}