AutoCAD .Net禁止图元被删除的方法

static EraseOverrule eraseRule = null;

public class EraseOverrule : ObjectOverrule
{
 public override void Erase(DBObject dbObject, bool erasing)
 {
  throw new Autodesk.AutoCAD.Runtime.Exception(
    Autodesk.AutoCAD.Runtime.ErrorStatus.NotApplicable);
  //base.Erase(dbObject, erasing);
 }
}

[CommandMethod("AddEraseOverrule")]
public static void AddEraseOverrule()
{
 if (eraseRule == null)
 {
  eraseRule = new EraseOverrule();
  Overrule.AddOverrule(RXObject.GetClass(typeof(Entity)),
        eraseRule, true);
  Overrule.Overruling = true;

  eraseRule.SetIdFilter(CreateEntities());
 }
}

[CommandMethod("RemoveEraseOverrule")]
public static void RemoveEraseOverrule()
{
 if (eraseRule != null)
 {
  Overrule.Overruling = false;
  Overrule.RemoveOverrule(RXObject.GetClass(typeof(Entity)),
        eraseRule);
  eraseRule.Dispose();
  eraseRule = null;
 }
}

private static ObjectId[] CreateEntities()
{
 Document doc = Application.DocumentManager.MdiActiveDocument;
 Database db = doc.Database;

 ObjectId[] oids = new ObjectId[2];
 using (Transaction tr = db.TransactionManager.StartTransaction())
 {
  BlockTableRecord space = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) as BlockTableRecord;

  //
  Line line = new Line();
  line.StartPoint = new Point3d(0, 0, 0);
  line.EndPoint = new Point3d(100, 100, 0);
  space.AppendEntity(line);
  tr.AddNewlyCreatedDBObject(line, true);

  oids[0] = line.ObjectId;

  //
  Circle circle = new Circle();
  circle.Center = new Point3d(0, 0, 0);
  circle.Radius = 50;
  space.AppendEntity(circle);
  tr.AddNewlyCreatedDBObject(circle, true);

  oids[1] = circle.ObjectId;

  tr.Commit();
 }

 return oids;
}