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Silverlight 2 provides built-in support for sockets which lets servers push data to Silverlight clients. Using this feature, clients can avoid polling servers on a timed basis to ensure that clients are kept up-to-date. If you're new to the socket features built-into Silverlight 2 you'll want to read my previous articles to get additional details about how data can be pushed from a server to a client:

• Socket Support in Silverlight 2: Part I
• Socket Support in Silverlight 2: Part II

Silverlight 2 Beta 2 (and beyond) checks for a client access policy before accessing sockets located on host domain or cross-domain servers. Listing One is an example of a client access policy for sockets.

<?xml version="1.0" encoding ="utf-8"?>
<access-policy>
  <cross-domain-access>
    <policy>
      <allow-from>
        <domain uri="*" />
      </allow-from>
      <grant-to>
        <socket-resource port="4530" protocol="tcp" />
      </grant-to>
    </policy>
  </cross-domain-access>
</access-policy>

This XML code lets Silverlight access a TCP socket on port 4530. A range of ports can be specified in the port attribute if needed (ex: 4530-4532). Before Silverlight tries to call a server with a socket, it makes a call to the target server on port 943 to check the client access policy and see if the server allows socket connections. This helps minimize various types of hacker attacks. If a client access policy is available on the server and the policy allows access to the port the client is trying to call, processing of the socket code continues and Silverlight tries to connect. If not, the client will be unable to connect due to access being denied by Silverlight.

Listing Two is an example of creating a client access policy socket server that Silverlight can connect to on port 943:

using System;
using System.Collections.Generic;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.IO;
using System.Threading;
using System.Reflection;
using System.Configuration;

namespace PolicySocketServices
{
    class PolicySocketServer
    {
        TcpListener _Listener = null;
        TcpClient _Client = null;
        static ManualResetEvent _TcpClientConnected = new ManualResetEvent(false);
        const string _PolicyRequestString = "<policy-file-request/>";
        int _ReceivedLength = 0;
        byte[] _Policy = null;
        byte[] _ReceiveBuffer = null;

        private void InitializeData()
        {
            string policyFile = ConfigurationManager.AppSettings["PolicyFilePath"];
            using (FileStream fs = new FileStream(policyFile, FileMode.Open))
            {
                _Policy = new byte[fs.Length];
                fs.Read(_Policy, 0, _Policy.Length);
            }
            _ReceiveBuffer = new byte[_PolicyRequestString.Length];
        }
        public void StartSocketServer()
        {
            InitializeData();
            try
            {
                //Using TcpListener which is a wrapper around a Socket
                //Allowed port is 943 for Silverlight sockets policy data
                _Listener = new TcpListener(IPAddress.Any, 943);
                _Listener.Start();
                Console.WriteLine("Policy server listening...");
                while (true)
                {
                    _TcpClientConnected.Reset();
                    Console.WriteLine("Waiting for client connection...");
                    _Listener.BeginAcceptTcpClient(new AsyncCallback(OnBeginAccept), null);
                    _TcpClientConnected.WaitOne(); //Block until client connects
                }
            }
            catch (Exception exp)
            {
                LogError(exp);
            }
         }
        private void OnBeginAccept(IAsyncResult ar)
        {
            _Client = _Listener.EndAcceptTcpClient(ar);
            _Client.Client.BeginReceive(_ReceiveBuffer, 0, _PolicyRequestString.Length, SocketFlags.None,
                new AsyncCallback(OnReceiveComplete), null);
        }
        private void OnReceiveComplete(IAsyncResult ar)
        {
            try
            {
                _ReceivedLength += _Client.Client.EndReceive(ar);
                //See if there's more data that we need to grab
                if (_ReceivedLength < _PolicyRequestString.Length)
                {
                    //Need to grab more data so receive remaining data
                    _Client.Client.BeginReceive(_ReceiveBuffer, _ReceivedLength, 
                        _PolicyRequestString.Length - _ReceivedLength,
                        SocketFlags.None, new AsyncCallback(OnReceiveComplete), null);
                    return;
                }
                //Check that <policy-file-request/> was sent from client
                string request = System.Text.Encoding.UTF8.GetString(_ReceiveBuffer, 0, _ReceivedLength);
                if (StringComparer.InvariantCultureIgnoreCase.Compare(request, _PolicyRequestString) != 0)
                {
                    //Data received isn't valid so close
                    _Client.Client.Close();
                    return;
                }
                //Valid request received....send policy data
                _Client.Client.BeginSend(_Policy, 0, _Policy.Length, SocketFlags.None, 
                    new AsyncCallback(OnSendComplete), null);
            }
            catch (Exception exp)
            {
                _Client.Client.Close();
                LogError(exp);
            }
            _ReceivedLength = 0;
            _TcpClientConnected.Set(); //Allow waiting thread to proceed
        }
        private void OnSendComplete(IAsyncResult ar)
        {
            try
            {
                _Client.Client.EndSendFile(ar);
            }
            catch (Exception exp)
            {
                LogError(exp);
            }
            finally            
            {
                //Close client socket
                _Client.Client.Close();
             } 
        }
        private void LogError(Exception exp)
        {
            string appFullPath = Assembly.GetCallingAssembly().Location;
            string logPath = appFullPath.Substring(0, appFullPath.LastIndexOf("\\")) + ".log";
            StreamWriter writer = new StreamWriter(logPath, true);
            try
            {
                writer.WriteLine(logPath,
                    String.Format("Error in PolicySocketServer: "
                    + "{0} \r\n StackTrace: {1}", exp.Message, exp.StackTrace));
            }
            catch { }
            finally
            {
                writer.Close();
            }
        }
    }
}

Looking through the code you'll see that it uses the TcpListener class to listen for incoming client connections. Once a client connects the code checks the request for the following value:

<policy-file-request/>

Silverlight automatically sends this text to the policy file socket once it connects. If the request contains the proper value the code writes the contents of the client access policy back to the client stream (see the OnReceiveComplete() method). Once the policy file is received, Silverlight parses it, checks that it allows access to the desired port, and then accepts or rejects the socket call that the application is trying to make.

Figure 1 is an example of the Silverlight GameStream socket application I created to demonstrate the fundamentals of using sockets. You can download the complete application here.

[Click image to view at full size]

Figure 1:A Silverlight 2 interface that has data pushed to it from a socket server.

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