For the attached model, using the Selection Filter, one can select the sub object. However, the following code does not work for sub-object selection. How to fix the problem? Also, when specifying CRhinoGetObject::curve_object|CRhinoGetObject::edge_object for a box model, would Rhino selects a curve or edge from the model?
To select subcurve of a polycurve, press Ctrl+Shift and click with the mouse. This is standard subobject picking behavior in Rhino.
The ability to select subcurves of a polycurve without the use of the keyboard combination is unique to pre-selecting objects with the Selection Filter window open and the Sub-object and Curves/Edges options enabled.