I have this:

```
    public class Room
{
    public string RoomID { get; set; }
    public string RoomName { get; set; }
    public List<User> UsersInRoom { get; set; }

    //public IDuplexClient RoomCallbackChannel { get; set; }
}

```

As you can see there is an List that contains users in room.

But to actually make it work I need to add users, so I did this:

```
            User usr;
        Room roomi;
        if (userNameTest == null)
        {
            lock (_clients)
            {
                usr = new User { UserID = sessionID, CallbackChannel = client, UserName = userName };
                roomi = new Room();
                roomi.RoomID = sessionID;
                roomi.RoomName = room;
                roomi.UsersInRoom.Add(usr);
                //roomi.UsersInRoom.Add(usr);
                _rooms.Add(roomi);
                //_clients.Add(usr);
            }
        }

```

and at line:

```
roomi.UsersInRoom.Add(usr);

```

I get NullReferenceException.
What's going on ?