C#

using System;
using System.Collections;
using System.Text;
using System.Net;
using System.Net.Sockets;

namespace Telnet_Test
{
	abstract class TelnetServer : TcpListener
	{
		/// <summary>
		/// Maximum number of users this server can hold (zero for infinite)
		/// </summary>
		int MaxUsers;

		/// <summary>
		/// User array (exception is thrown if this is more than MaxUsers)
		/// </summary>
		UserList Users;

		/// <summary>
		/// Starts a telnet server
		/// </summary>
		/// <param name="maxUsers">Maximum number of users this server can hold (zero for infinite)</param>
		public TelnetServer(int maxUsers) : base(IPAddress.Loopback, 23)
		{
			// Set max users
			MaxUsers = maxUsers;

			// Initialize users array
			Users = new UserList();
		}

		public void BeginAcceptSocket()
		{
			this.Start();

			// Start accepting connections (TcpListener.BeginAcceptSocket, if you ask)
			BeginAcceptSocket(
				new AsyncCallback(ClientConnected), null);
		}

		public void ClientConnected(IAsyncResult result)
		{
			// Get a temporary socket to process this user
			TelnetClient tmpClient = new TelnetClient(Users.GetNextID());

			// Accept the connection
			EndAcceptSocket(result);

// HERE: What's wrong? 

			if (Users.Count >= MaxUsers)
			{
				// Send error to client
				tmpClient.Socket.Send
					(Encoding.ASCII.GetBytes("ERROR:" + MaxUsersErrorMessage));

				// Close connection
				tmpClient.Socket.Disconnect(false);
				tmpClient = null;

				// TODO: Make a custom exception maybe?
				// Throw exception
				throw new Exception(MaxUsersErrorMessage);
			}

			// All processing complete: add to collection!
			Users.Add(tmpClient);

			byte[] pwn = { (byte)'h' };

			Console.Out.WriteLine("{0}", Users.At(tmpClient.ID));
			Users[Users.At(tmpClient.ID)].Socket.Send(pwn);
		}

		public string MaxUsersErrorMessage =
			"Maximum number of users reached!";
	}
}

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