This is also possible:
house.rooms.SelectMany(room => room.Doors.Where(door => door.Color == green),
(room, door) => new { r = room, d = door })
It’s this overload of SelectMany.
This is also possible:
house.rooms.SelectMany(room => room.Doors.Where(door => door.Color == green),
(room, door) => new { r = room, d = door })
It’s this overload of SelectMany.