ObserverPatternWithEventAndDelegate.cs

public delegate void StatusChangeEventHandler(MessageStatus newStatus); 

public enum MessageStatus { Approved , InApproval , Executed  }

public class Message
{
    private MessageStatus messageStatus;
    public MessageStatus MessageStatus
    {
        get { return messageStatus; }
        set 
        { 
            messageStatus = value;
            Notify();
        }
    }

    private void Notify()
    {
        if (_OnStatusChange != null)
            _OnStatusChange(messageStatus);
    }

    private event StatusChangeEventHandler _OnStatusChange;
    public event StatusChangeEventHandler OnStatusChange 
    {
        add 
        {
            _OnStatusChange += value; // to Subscribe
        }
        remove 
        {
            _OnStatusChange -= value; // to UnSubscribe
        }
    }
}

// using
class Program
{
    static void Main(string[] args)
    {
        Message msg = new Message();
        msg.MessageStatus = MessageStatus.InApproval;

        msg.OnStatusChange += msg_OnStatusChange; // Subscribe it

        msg.MessageStatus = MessageStatus.InApproval;
        msg.OnStatusChange += msg_OnStatusChange1;  // Subscribe it

        msg.MessageStatus = MessageStatus.Approved;
        msg.MessageStatus = MessageStatus.Executed;

        msg.OnStatusChange += msg_OnStatusChange2;
        msg.OnStatusChange -= msg_OnStatusChange1; // UnSubscribe -msg_OnStatusChange1

        Console.WriteLine();

        msg.MessageStatus = MessageStatus.InApproval;

        Console.ReadLine();
    }

    static void msg_OnStatusChange(MessageStatus newStatus)
    {
        Console.WriteLine("Status -{0} ", newStatus);
    }

    static void msg_OnStatusChange1(MessageStatus newStatus)
    {
        Console.WriteLine("Status -{0} ", newStatus);
    }

    static void msg_OnStatusChange2(MessageStatus newStatus)
    {
        Console.WriteLine("Status -{0} ", newStatus);
    }

    static void msg_OnStatusChange3(MessageStatus newStatus)
    {
        Console.WriteLine("Status -{0} ", newStatus);
    }
}