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;
}