Hey, I'm new here and maybe it's too late for you, but could be useful for the other guys. Here is what I did, and worked nice:
AutoCAD.Interop.AcadApplication app = (AutoCAD.Interop.AcadApplication)Autodesk.AutoCAD.ApplicationServices.Application.AcadApplication;
// your code to draw geometries, or something else
app.ZoomExtents();
To make it work, u have to add the following dll:
...\ObjectARX 2008\inc\Autodesk.AutoCAD.Interop.dll
Regards,
