What is ObjectArx ?

ObjectArx

ObjectARX®, the AutoCAD® Runtime Extension programming environment, includes C++ libraries that are the building blocks you can use to develop AutoCAD applications, extend AutoCAD classes and protocol, and create new commands that operate in the same manner as built-in AutoCAD commands. You can add new classes and export them for use by other programs. You can also extend the ObjectARX protocol by adding functions at runtime to existing AutoCAD classes.
you can download it from http://www.autodesk.com/objectarx