Pufferfish OffsetSurface: it breaks the open brep into multiple planar trimmed surface and extrude them individually.
Because it is for surfaces, not breps, as the name implies 
Rhinocommon for R6 added offset brep method however brep offsets in general usually have many problems. Better to think about the construction of the geometry as a solid at the start of the process, rather than rely on one algo to solve the thickness for you afterwards. Anyway here is the method.
https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_Brep_CreateOffsetBrep.htm