In <ARX>\samples\entity\polysamp you find poly.cpp/h where a custom entity is derived from AcDbCurve.
I would also recomment to derive from AcDbCurve rather than directly from AcDbEntity.
Thomas Brammer ● Software Developer ● imos AG ● LinkedIn ● ![](http://download.autodesk.com/us/expert_elite/ADSK_Expert_Elite_Icon_S_Color_Blk.png)
If an answer solves your problem please [ACCEPT SOLUTION]. Otherwise explain why not.