check for duplicates in arraylist

List<Room> rooms = ...
string roomType = "lounge";
bool exists = rooms.Exists(delegate(Room room) { return room.Type == roomType; });

bool exists = rooms.Exists(room => room.Type == roomType);

bool exists = rooms.Any(room => room.Type == roomType);

if (!rooms.Any (r => r.RoomType == typeToFind /*kitchen, ...*/))
  //add it or whatever

if(rooms.ContainsKey(room.GetType()))
{
   // Can't add a second room of the same type
   ...
}
else
{
   rooms.Add(room.GetType(), room);
}

void AddRoom(Room r, IList<Room> rooms, IDictionary<string, bool> roomTypes)
{
   if (!roomTypes.Contains(r.RoomType))
   {
      rooms.Add(r);
      roomTypes.Add(r.RoomType, true);
   }
}

Dictionary<Type, List<Room>> rooms = new Dictionary<Type, List<Room>>;

void Main(){
  KitchenRoom kr = new KitchenRoom();
  DummyRoom dr = new DummyRoom();
  RoomType1 rt1 = new RoomType1();
  ...

  AddRoom(kr);
  AddRoom(dr);
  AddRoom(rt1);
  ...

}

void AddRoom(Room r){
  Type roomtype = r.GetType();
  if(!rooms.ContainsKey(roomtype){ //If the type is new, then add it with an empty list
   rooms.Add(roomtype, new List<Room>);
  }
  //And of course add the room.
  rooms[roomtype].Add(r);
}