Reference pickedObj = uidoc.Selection.PickObject(Autodesk.Revit.UI.Selection.ObjectType.Element);
if (pickedObj != null)
{
ElementId elementId = pickedObj.ElementId;
Element element = doc.GetElement(elementId);
familyInstance = element as FamilyInstance;
ICollection<ElementId> subElementsSet = familyInstance.GetSubComponentIds();
}