Mediator Design Patterns in c#

What is mediator pattern?

According to the GOF the "Mediator Pattern defines an object that encapsulates how a set of objects interact."
Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it let's you vary their interaction independently.
When to use?

Consider a situation where we have many classes encapsulating specific logic inside them (like invoicing, inventory management, payroll).
     * 
     * Chat application using Mediator Pattern

Let's build a small chat-based application.

Note:

This will not be a real-time chat application.
***** A real chat application requires sockets or other network-based programming along with a Mediator pattern.

     * ***A Mediator Pattern or any other design pattern we have already discussed are not rules, and its not even mandatory that it be used in every application. 
     * Try to understand your problem first and then see which design pattern satisfies your requirements.

class Program
    {
        static void Main(string[] args)
        {
            ClsAbstractMediator objMediator = new ClsMediator();
            ClsUser userA = new ClsMobileUser("userA");
            ClsUser userB = new ClsMobileUser("userB");
            ClsUser userC = new ClsDesktopUser("userC");
            ClsUser userD = new ClsDesktopUser("userD");
            objMediator.AddUser(userA);
            objMediator.AddUser(userB);
            objMediator.AddUser(userC);
            objMediator.AddUser(userD);
            userA.Send("userB", "Hi userB, can you help me to understand mediator pattern");
            userB.Send("userA", "sorry man, even i m trying to understand the same.Try to consult userC");
            userA.Send("userB", "OK\n");
            userA.Send("userC", "hey userC can you help me.");
            userC.Send("userA", "sorry bro, m busy right now, working on an article,will ping in some time ;)");
            userC.Send("userA", "As usual :P");
        }
    }
    public abstract class ClsUser
    {
        internal string userName { get; set; }
        internal ClsAbstractMediator objMediator;
        public ClsUser(string userName)
        {
            this.userName = userName;
        }
        public abstract void Send(string toUser, string message);
        public abstract void Receive(string fromUser, string message);
    }
    public abstract class ClsAbstractMediator
    {
        public abstract void AddUser(clsUser user);
        public abstract void RemoveUser(string userName);
        public abstract void SendMessage(string fromUser, string toUser, string message);
    }
    public class ClsDesktopUser : ClsUser
    {
        public override void Send(string toUser, string message)
        {
            objMediator.SendMessage(toUser, this.userName, message);
        }

        public override void Receive(string fromUser, string message)
        {
            Console.WriteLine(this.userName + ":" + message);
        }
    }


    public class ClsDesktopUser : ClsUser
    {
        public override void Receive(string fromUser, string message)
        {
            Console.WriteLine(this.userName + ":" + message);
        }
        public override void Send(string toUser, string message)
        {
            objMediator.SendMessage(toUser, this.userName, message);
        }
    }
    public class ClsMobileUser : ClsUser
    {
        public override void Receive(string fromUser, string message)
        {
            Console.WriteLine(this.userName + ":" + message);
        }
        public override void Send(string toUser, string message)
        {
            objMediator.SendMessage(this.userName, toUser, message);
        }
    }