PromptSelectionOptions pso = new()
{
SingleOnly = true,
ForceSubSelections = true,
MessageForAdding = "....",
SinglePickInSpace = true,
RejectObjectsOnLockedLayers = true
};
PromptSelectionResult psr = ed.GetSelection(pso);
f (psr.Status != PromptStatus.OK) return;
SelectedObject so = psr.Value[0];
ObjectId solidId = so.ObjectId;
SelectedSubObject[] ssubs = so.GetSubentities();
SubentityType subentityType = ssubs[0].FullSubentityPath.SubentId.Type;
if (subentityType != SubentityType.Face) return;
SubentityId id = ssubs[0].FullSubentityPath.SubentId;