Оказалось все достаточно просто, хотя не понятно поначалу:
public class Command : IExternalCommand
{
RebarHostData rebarHostData;
public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
{
UIApplication uiapp = commandData.Application;
UIDocument uidoc = uiapp.ActiveUIDocument;
Application app = uiapp.Application;
Document doc = uidoc.Document;
Selection sel = uidoc.Selection;
//Выбираем элемент
Reference reference = uidoc.Selection.PickObject(ObjectType.Element, "Select an Element");
Element e = doc.GetElement(reference);
//Выбираем грань на этом элементе
Reference faceRef = uidoc.Selection.PickObject(ObjectType.Face, "Select a Face of the Element");
rebarHostData = RebarHostData.GetRebarHostData(e);
RebarCoverType coverData = rebarHostData.GetCoverType(faceRef);
TaskDialog.Show("Revit", $"GetCoverType: {coverData.Name}\n");
return Result.Succeeded;
}
}