abstract-clatroom.cs

public class Chatroom : AChatroom
{
    private Dictionary<string, User> _users = new Dictionary<string, User>();
    public override void Post(string fromUser, string toUser, string msg)
    {
        User participant = _users[toUser];

        if (participant != null)
        {
            participant.DM(fromUser, msg);
        }
    }

    public override void Register(User user)
    {
        if (!_users.ContainsValue(user))
        {
            _users[user.Name] = user;
        }

        user.Chatroom = this;
    }
}