user-mediator.cs

public class User
{
    private Chatroom _chatroom;
    private string _name;
    public User(string name) => this._name = name;

    public string Name => _name;

    public Chatroom Chatroom
    {
        set { _chatroom = value; }
        get => _chatroom;
    }
    public void Post(string to, string message) => 
        _chatroom.Post(_name, to, message);

    public virtual void DM(string from, string message) => 
        Console.WriteLine("{0} to {1}: '{2}'", from, Name, message);

}