Using IHttpAsyncHandler and XMLHttpRequest to “push” messages to the client

//
// This code is released under Common Public License Version 1.0 (CPL)
// Full license text is available at http://www.opensource.org/licenses/cpl1.0.php
//
// Author  : Karel Boek
// Date    : Sept 29, 2009
// Contact : karel.boek@raskenlund.com
//

using System;
using System.Collections.Generic;
using System.Web;

namespace SandBox.CometSample
{
    /// <summary>
    /// An IHttpAsyncHandler to "push" messages to the intended recipients
    /// </summary>
    public class MyAsyncHandler : IHttpAsyncHandler
    {
        /// <summary>
        /// The queue holds a list of asynchronous results with information about registered sessions
        /// </summary>
        public static List<MyAsyncResult> Queue;

        /// <summary>
        /// Static constructor
        /// </summary>
        static MyAsyncHandler()
        {
            // Initialize the queue
            Queue = new List<MyAsyncResult>();
        }

        #region IHttpAsyncHandler Members

        public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {
            // Fetch the session id from the request
            var sessionId   = context.Request["sessionId"];
            
            // Check if the session is already registered
            if (Queue.Find(q => q.SessionId == sessionId) != null)
            {
                var index = Queue.IndexOf(Queue.Find(q => q.SessionId == sessionId));

                // The session has already been registered, just refresh the HttpContext and the AsyncCallback
                Queue[index].Context  = context;
                Queue[index].Callback = cb;

                return Queue[index];
            }

            // Create a new AsyncResult that holds the information about the session
            var asyncResult = new MyAsyncResult(context, cb, sessionId);

            // This session has not been registered yet, add it to the queue
            Queue.Add(asyncResult);

            return asyncResult;
        }

        public void EndProcessRequest(IAsyncResult result)
        {
            var rslt  = (MyAsyncResult) result;
            
            // send the message to the recipient using the recipients HttpContext.Response object
            rslt.Context.Response.Write(rslt.Message);

            // reset the message object
            rslt.Message = string.Empty;
        }

        #endregion

        #region IHttpHandler Members

        public bool IsReusable
        {
            get { return true; }
        }

        /// <summary>
        /// In an asychronous solution, this message shouldn't be called
        /// </summary>
        /// <param name="context"></param>
        public void ProcessRequest(HttpContext context)
        {
            throw new NotImplementedException();
        }

        #endregion
    }
}