很奇怪的错误提示：!dbobji.cpp@7227:eNotOpenForWrite

//这是另外一个类的，这两行应该是没问题的。
mDoc = Application. DocumentManager. MdiActiveDocument;
mDb = mDoc. Database;
public class MySymbolTableRecord
{
   protected ObjectId AddEntity (BlockTableRecord btRecord, Entity ent)
   {
    ObjectId ObjId;
    using (Transaction tran = mDb. TransactionManager. StartTransaction( ))
    {
     //调试的时候每次都是运行到这位置就出错了
     //内部错误： !dbobji.cpp@7227:eNotOpenForWrite
     ObjId = btRecord. AppendEntity(ent);
     tran. AddNewlyCreatedDBObject(ent, true);
     tran. Commit( );
     }
     return ObjId;
     }
     protected DBObject GetDBObject (ObjectId id, OpenMode mode)
      {
     DBObject dbObj;
     using (Transaction tran = mDb. TransactionManager. StartTransaction( ))
     {
     dbObj = tran. GetObject(id, mode);
     }
     return dbObj;
     }
}
public class Drawing : MySymbolTableRecord
{
     private BlockTableRecord mBlkTabRec;
     private Calculation mCalc;
     public Drawing ( )//构造函数
     {
     mBlkTabRec = GetDBObject(mDb. CurrentSpaceId, OpenMode. ForWrite) as BlockTableRecord;
     mCalc = new Calculation( );
     }
     public ObjectId AddLine (Point2d ptStart, Point2d ptEnd)//画直线
     {
     Line line = new Line(mCalc. P2ToP3(ptStart), mCalc. P2ToP3(ptEnd));
     return AddEntity(mBlkTabRec, line);
     }
   ｝