一起谈.NET技术，模拟IIS向Silverlight输出策略文件 狼人:

　　问题

　　最近的Silverlight开发中，由于部分需求对实时性和数据量下载速度有要求，部分WCF服务配置成了netTcpBinding，这种方式跟普通的service.svc寄宿IIS不同的是，Silverlight需要的策略文件需要放置在本机IIS的根下，也就是wwwroot文件夹下，以满足Silverlight在以TCP协议调用本机WCF服务时请求策略文件。（注：Silverlight通过TCP协议调用WCF服务时，会以http方式请求主机的一个策略文件，地址是http://localhost/clientaccesspolicy.xml)

　　这其实是个不太好的选择，程序运行的所需的环境被分成了两部分，同事的机器上并未安装IIS，为了大家开发简便，不用在额外安装IIS，也为了让程序更加独立，我就想能不能写代码监控80端口模拟IIS向Silverlight输出这个策略文件。

　　解决方法

　　有了这个想法之后，首先想到的是通过Socket进行监听，因为此前在MSDN上看到过这种方式，但很无奈，将代码转移过来之后，并未成功。相信做过Silverlight在Socket方面应用的朋友对下面这个PolicyServer类很熟悉吧。

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;

namespace PolicyServer
{
// Encapsulate and manage state for a single connection from a client
class PolicyConnection
{
    private Socket m_connection;

    // buffer to receive the request from the client
    private byte[] m_buffer;
    private int m_received;

    // the policy to return to the client
    private byte[] m_policy;

    // the request that we're expecting from the client
    private static string s_policyRequestString = "<policy-file-request/>";



    public PolicyConnection(Socket client, byte[] policy)
    {
        m_connection = client;
        m_policy = policy;

        m_buffer = new byte[s_policyRequestString.Length];
        m_received = 0;

        try
        {
            // receive the request from the client
            m_connection.BeginReceive(m_buffer, 0, s_policyRequestString.Length, SocketFlags.None, new AsyncCallback(OnReceive), null);
        }
        catch (SocketException)
        {
            m_connection.Close();
        }
    }

    // Called when we receive data from the client
    private void OnReceive(IAsyncResult res)
    {
        try
        {
            m_received += m_connection.EndReceive(res);

            // if we haven't gotten enough for a full request yet, receive again
            if (m_received < s_policyRequestString.Length)
            {
                m_connection.BeginReceive(m_buffer, m_received, s_policyRequestString.Length - m_received, SocketFlags.None, new AsyncCallback(OnReceive), null);
                return;
            }

            // make sure the request is valid
            string request = System.Text.Encoding.UTF8.GetString(m_buffer, 0, m_received);
            if (StringComparer.InvariantCultureIgnoreCase.Compare(request, s_policyRequestString) != 0)
            {
                m_connection.Close();
                return;
            }

            // send the policy
            m_connection.BeginSend(m_policy, 0, m_policy.Length, SocketFlags.None, new AsyncCallback(OnSend), null);
        }
        catch (SocketException)
        {
            m_connection.Close();
        }
    }

    // called after sending the policy to the client; close the connection.
    public void OnSend(IAsyncResult res)
    {
        try
        {
            m_connection.EndSend(res);
        }
        finally
        {
            m_connection.Close();
        }
    }
}

// Listens for connections on port 943 and dispatches requests to a PolicyConnection
class PolicyServer
{
    private Socket m_listener;
    private byte[] m_policy;

    // pass in the path of an XML file containing the socket policy
    public PolicyServer(string policyFile)
    {
        // Load the policy file
        FileStream policyStream = new FileStream(policyFile, FileMode.Open);

        m_policy = new byte[policyStream.Length];
        policyStream.Read(m_policy, 0, m_policy.Length);

        policyStream.Close();

        m_listener = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);

        m_listener.SetSocketOption(SocketOptionLevel.IPv6, (SocketOptionName)27, 0);

        m_listener.Bind(new IPEndPoint(IPAddress.IPv6Any, 943));
        m_listener.Listen(10);

        m_listener.BeginAccept(new AsyncCallback(OnConnection), null);
    }

     public void OnConnection(IAsyncResult res)
    {
        Socket client = null;

        try
        {
            client = m_listener.EndAccept(res);
        }
        catch (SocketException)
        {
            return;
        }

        // handle this policy request with a PolicyConnection
        PolicyConnection pc = new PolicyConnection(client, m_policy);

        // look for more connections
        m_listener.BeginAccept(new AsyncCallback(OnConnection), null);
    }

    public void Close()
    {
        m_listener.Close();
    }
}
public class Program
{
    static void Main(string[] args)
    {
        if (args.Length == 0)
        {
            Console.WriteLine("usage: PolicyServer.exe PolicyFile.xml");
            return;
        }

        PolicyServer ps = new PolicyServer(args[0]);
        System.Threading.Thread.Sleep(System.Threading.Timeout.Infinite);
    }
}
}

　　此路不通之后，又想起使用HttpListener类，看看是否能够监听http请求，果然能够截获HTTP的请求。

HttpListener listener = new HttpListener();
listener.Prefixes.Add(http://localhost/); 

listener.Start();Console.WriteLine("开始监听…");

HttpListenerContext context = listener.GetContext();
HttpListenerRequest request = context.Request;
HttpListenerResponse response = context.Response;

　　但是这种方式有个明显的缺点，就是线程是阻塞的。于是，又想到使用线程池。

System.Threading.ThreadPool.QueueUserWorkItem(new System.Threading.WaitCallback(Listen));
private static void Listen(object state)
{
            while (httpListener.IsListening)
            {
                httpListener.BeginGetContext(new AsyncCallback(ListenerCallback), httpListener);
                listenForNextRequest.WaitOne();
            }
}

　　这样的话，每接收一个请求便会异步处理这个请求。在请求的处理上，接收请求后需要向外输出策略文件流，供silverlight端接收验证。

using (System.Net.HttpListenerResponse response = context.Response)
            {
                System.Threading.Thread.Sleep(1000);

                string responseString = "<?xml version=\"1.0\" encoding=\"utf-8\"?> "
                                        + " <access-policy> "
                                        + "    <cross-domain-access> "
                                           + "    <policy> "
                                              + "    <allow-from http-request-headers=\"*\">"
                                                 + "    <domain uri=\"*\" /> "
                                                 + " </allow-from> "
                                                 + " <grant-to> "
                                                   + "  <socket-resource port=\"4502-4534\" protocol=\"tcp\" /> "
                                                + "  </grant-to> "
                                             + "  </policy> "
                                        + "    </cross-domain-access>"
                                        + " </access-policy>";
                byte[] buffer = System.Text.Encoding.UTF8.GetBytes(responseString);
                response.ContentLength64 = buffer.LongLength;
                response.OutputStream.Write(buffer, 0, buffer.Length);
            } 

　　启动这个模拟服务，将clientaccesspolicy从wwwroot中移除后再运行一下程序，OK，我们不再需要将策略文件放到IIS下了。

　　提醒 

　　如果你的机器装了IIS，请还是放一个策略文件到wwwroot吧，否则就停掉IIS再使用这个类，因为IIS和这个类只能有一方监听80端口。

　　本文中的这个类参考了：http://forums.silverlight.net/forums/p/113106/255614.aspx