To quote the ObjectARX docs: "The DBObject class is the base class for all objects that reside in an Database object." The Entity class directly inherits from DBObject. The Transaction.GetObject() method returns a DBObject type and you then cast it to other types. So if your ObjectId references an Entity, you can cast the DBObject to type Entity and then access the Entity members. I recommend you check out the ObjectARX arxmgd.chm file or use Reflector to get a better feel for the AutoCAD .NET inheritance chains.