1. You need to target the appropriate ObjectARX development SDK from Autodesk for 2015, not 2011, and recomplie. You can obtain it via the following link:
http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=773204[
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IAW Autodesk FAQ:
"6. Do I have to rebuild my older AutoCAD applications to work on the latest version of AutoCAD?
Applications developed using ObjectARX 2015 will work fine on AutoCAD 2016 without any changes. However, applications developed for versions older than 2015 must be recompiled with current ObjectARX libraries to be compatible with the current version of AutoCAD. Applications built with ObjectARX for the current version of AutoCAD (2016) will not work on older AutoCAD releases. Note that the DWG format has not been changed in this release."
http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=773128[
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