无法在app_code中设置类的Session变量

Question

Web服务器调用MagicServer执行某些计算。计算完成后MagicServer返回最终结果字符串但在进行中MagicServer通过事件发送消息。事件正在使用ClientEventSink在Web Server中处理。我可以在ClientEventSink中收到邮件，但由于它是Session，因此无法设置null。

app_code中的CS文件：MagicPerformer.cs

using System;
using System.Collections;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Tcp;
using System.Web;
using MagicAgent;

namespace MyWeb.Magic
{
    class ClientEventSink : MessageEventSink
    {
        protected override void ClientCallback(string str)
        {
            HttpContext.Current.Session["MagicServerMessage"] = str; // **Could not get SESSION here. Current null.**
        }

    }

    public class MagicPerformer
    {

      public string GetMagicString(int magicCount)
      {
        string magicString = string.Empty;

        IChannel preregistered = ChannelServices.GetChannel("magicSink");
        if (preregistered == null)
        {
            IDictionary props = new Hashtable();
            props["name"] = "magicSink";
            props["port"] = 0;
            props["rejectRemoteRequests"] = "false";

            BinaryServerFormatterSinkProvider serverProv = new BinaryServerFormatterSinkProvider();
            serverProv.TypeFilterLevel = System.Runtime.Serialization.Formatters.TypeFilterLevel.Full; 
            BinaryClientFormatterSinkProvider clientProv = new BinaryClientFormatterSinkProvider();

            TcpChannel tcpCh = new TcpChannel(props, clientProv, serverProv);
            ChannelServices.RegisterChannel(tcpCh, false);
        }

        MagicSearcher magician = (MagicSearcher)Activator.GetObject(typeof(MagicSearcher), "tcp://192.168.1.15:8010/magicsearcher");
        if (magician == null)
            throw new Exception("SERVER_NOT_RUNNING");

        RemotingConfiguration.RegisterWellKnownServiceType(typeof(ClientEventSink),"RemotingBroadcast",WellKnownObjectMode.SingleCall); 

        ClientEventSink sink = new ClientEventSink();
        ClientEventHandler clientEventHandler = new ClientEventHandler(sink.ServiceDispatchMethod);

        try
        {
            magician.ClientServiceEvent += clientEventHandler;
            magicString = magician.GetMagicStringFromServer(magicCount);
        }
        catch (System.Net.Sockets.SocketException se)
        {
            throw new Exception("CONNECTION_ERROR", se);
        }
        catch (Exception e)
        {

        }

        magician = null;
        return magicString;
     }

  }

}

如何从Session设置ClientEventSink，以便我可以在网页中显示进度消息。