Implementing an event which supports only a single event handler, in C#

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

/// <summary>
/// The following sample demonstrates how to implement a 
/// event which works like single cast, ie it will support 
/// only one handler to be subscribed to this event.
/// By default Events exhibit multicast behavior, ie when the 
/// event is raised all the clients who have subscribed to this 
/// event will receive the notification one after the other. 
/// But in this example only the client who has subscribed 
/// the event last will get the notification.
/// This case holds good when we have multiple threads subscribing 
/// for a notification, but it is enough if it is been processed 
/// by one thread, in this case the event may 
/// not be sent to all the thread. 
/// </summary>

/// <Author>
/// Madhu Raykar
/// </Author>
/// 
/// <Date>
/// 30 Nov 2005
/// </Date>

namespace SinglecastEvent
{
	public class frmSingleCast : System.Windows.Forms.Form
	{
        private System.Windows.Forms.GroupBox grpMessages;
        private System.Windows.Forms.GroupBox grpSubscribe;
        private System.Windows.Forms.GroupBox grpRaiseEvent;
        private System.Windows.Forms.TextBox txtMessages;
        private System.Windows.Forms.Button btnClient1;
        private System.Windows.Forms.Button btnClient2;
        private System.Windows.Forms.Button btnRaiseEvent;
		private System.ComponentModel.Container components = null;
        private EventRaiser eventRaiser = new EventRaiser();

		public frmSingleCast()
		{
			InitializeComponent();
		}

		#region Windows Form Designer generated code
        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        protected override void Dispose( bool disposing ) {
            if( disposing ) {
                if (components != null) {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        
        /// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
            this.grpMessages = new System.Windows.Forms.GroupBox();
            this.grpSubscribe = new System.Windows.Forms.GroupBox();
            this.grpRaiseEvent = new System.Windows.Forms.GroupBox();
            this.txtMessages = new System.Windows.Forms.TextBox();
            this.btnClient1 = new System.Windows.Forms.Button();
            this.btnClient2 = new System.Windows.Forms.Button();
            this.btnRaiseEvent = new System.Windows.Forms.Button();
            this.grpMessages.SuspendLayout();
            this.grpSubscribe.SuspendLayout();
            this.grpRaiseEvent.SuspendLayout();
            this.SuspendLayout();
            // 
            // grpMessages
            // 
            this.grpMessages.Controls.Add(this.txtMessages);
            this.grpMessages.Location = new System.Drawing.Point(8, 92);
            this.grpMessages.Name = "grpMessages";
            this.grpMessages.Size = new System.Drawing.Size(424, 104);
            this.grpMessages.TabIndex = 0;
            this.grpMessages.TabStop = false;
            this.grpMessages.Text = "Messages";
            // 
            // grpSubscribe
            // 
            this.grpSubscribe.Controls.Add(this.btnClient2);
            this.grpSubscribe.Controls.Add(this.btnClient1);
            this.grpSubscribe.Location = new System.Drawing.Point(8, 3);
            this.grpSubscribe.Name = "grpSubscribe";
            this.grpSubscribe.Size = new System.Drawing.Size(256, 80);
            this.grpSubscribe.TabIndex = 1;
            this.grpSubscribe.TabStop = false;
            this.grpSubscribe.Text = "Subscribe";
            // 
            // grpRaiseEvent
            // 
            this.grpRaiseEvent.Controls.Add(this.btnRaiseEvent);
            this.grpRaiseEvent.Location = new System.Drawing.Point(280, 3);
            this.grpRaiseEvent.Name = "grpRaiseEvent";
            this.grpRaiseEvent.Size = new System.Drawing.Size(152, 80);
            this.grpRaiseEvent.TabIndex = 2;
            this.grpRaiseEvent.TabStop = false;
            this.grpRaiseEvent.Text = "RaiseEvent";
            // 
            // txtMessages
            // 
            this.txtMessages.Location = new System.Drawing.Point(8, 16);
            this.txtMessages.Multiline = true;
            this.txtMessages.Name = "txtMessages";
            this.txtMessages.ReadOnly = true;
            this.txtMessages.ScrollBars = System.Windows.Forms.ScrollBars.Both;
            this.txtMessages.Size = new System.Drawing.Size(400, 80);
            this.txtMessages.TabIndex = 0;
            this.txtMessages.Text = "";
            // 
            // btnClient1
            // 
            this.btnClient1.Location = new System.Drawing.Point(18, 24);
            this.btnClient1.Name = "btnClient1";
            this.btnClient1.Size = new System.Drawing.Size(101, 40);
            this.btnClient1.TabIndex = 0;
            this.btnClient1.Text = "Client 1";
            this.btnClient1.Click += new System.EventHandler(this.btnClient1_Click);
            // 
            // btnClient2
            // 
            this.btnClient2.Location = new System.Drawing.Point(138, 24);
            this.btnClient2.Name = "btnClient2";
            this.btnClient2.Size = new System.Drawing.Size(101, 40);
            this.btnClient2.TabIndex = 1;
            this.btnClient2.Text = "Client 2";
            this.btnClient2.Click += new System.EventHandler(this.btnClient2_Click);
            // 
            // btnRaiseEvent
            // 
            this.btnRaiseEvent.Location = new System.Drawing.Point(24, 24);
            this.btnRaiseEvent.Name = "btnRaiseEvent";
            this.btnRaiseEvent.Size = new System.Drawing.Size(101, 40);
            this.btnRaiseEvent.TabIndex = 0;
            this.btnRaiseEvent.Text = "Raise Event";
            this.btnRaiseEvent.Click += new System.EventHandler(this.btnRaiseEvent_Click);
            // 
            // frmSingleCast
            // 
            this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
            this.ClientSize = new System.Drawing.Size(442, 208);
            this.Controls.Add(this.grpRaiseEvent);
            this.Controls.Add(this.grpSubscribe);
            this.Controls.Add(this.grpMessages);
            this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedDialog;
            this.MaximizeBox = false;
            this.Name = "frmSingleCast";
            this.Text = "Single Cast Event";
            this.grpMessages.ResumeLayout(false);
            this.grpSubscribe.ResumeLayout(false);
            this.grpRaiseEvent.ResumeLayout(false);
            this.ResumeLayout(false);

        }
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
 			Application.Run(new frmSingleCast());
        }


        private void btnClient1_Click(object sender, System.EventArgs e) {        
            txtMessages.Text += "Client 1 subscribed to event, other clients will stop listening to event. " + Environment.NewLine;
            // Simulate that client 1 is subscribing to the OnRaiseEvent of EventRaiser class
            eventRaiser.OnRaiseEvent += new EventHandler(Client1_OnRaiseEventHandler);
        }

        private void btnClient2_Click(object sender, System.EventArgs e) {
            txtMessages.Text += "Client 2 subscribed to event, other clients will stop listening to event." + Environment.NewLine; 
            // Simulate that client 2 is subscribing to the OnRaiseEvent of EventRaiser class
            eventRaiser.OnRaiseEvent += new EventHandler(Client2_OnRaiseEventHandler);
        }

        private void Client1_OnRaiseEventHandler(object sender, EventArgs e)
        {
            // This is client 1's handler for the event
            txtMessages.Text += "Client 1 : event received." + Environment.NewLine; 
        }
        
        private void Client2_OnRaiseEventHandler(object sender, EventArgs e) 
        {
            // This is client 2's handler for the event
            txtMessages.Text += "Client 2 : event received." + Environment.NewLine; 
        }

        private void btnRaiseEvent_Click(object sender, System.EventArgs e) {
            eventRaiser.RaiseEvent();
        }
    }
}