Asynchronous Socket connection in Silverlight

Question

I have TCP Socket server and would like to make asynchronous Silverlight client for it, but I cannot find any useful information on that. To be exact, I need three steps: connect, send and receive.

No need for it to be very detailed, just the idea how to build it to connect to the server at least.

By the way, I use Silverlight 5.

Answer 1

Thank you very much, Karel Frajtak for the link to video about sockets in Silverlight! Everything is shown and explained in that video tutorial, so I recommend everyone who needs sockets in Silverlight to watch it! Here's the solution that I needed, with explanations what does what and my comments:

I assume that all the connection actions will be handled by separate class SocketConnection:

public class MessageEventArgs : EventArgs
{
    public string Message { get; set; }
}
public class SocketConnection
{
    /// <summary>
    /// Event handler shot when message is received
    /// </summary>
    public event EventHandler<MessageEventArgs> MessageReceived;

    /// <summary>
    /// Socket used for connection to the server, sending and receiving data
    /// </summary>
    Socket socket;

    /// <summary>
    /// Default buffer size that should be used with the same value in both server and client
    /// </summary>
    int bufferSize = 128;

    /// <summary>
    /// Buffer used to store bytes received
    /// </summary>
    byte[] buffer;

    /// <summary>
    /// Bytes received in current receiving operation
    /// </summary>
    int bytesReceived;

    public SocketConnection(string host = "localhost", int port = 4502)
    {
        // Initializing buffer for receiving data
        buffer = new byte[bufferSize];
        // Initializing socket to connect to the server with default parameters
        // *** If you need IPv6, set the AddressFamily.InterNetworkV6 ***
        // *** Silverlight supports only Stream or Unknown socket types (Silverlight 5) ***
        // *** The only defined protocol supported by Silverlight is TCP.
        //     Others can be only Unknown or Unspecified (Silverlight 5)***
        socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

        // Socket args that are needed for connection
        SocketAsyncEventArgs args = new SocketAsyncEventArgs()
        {
            // Server IP and port
            RemoteEndPoint = new DnsEndPoint(host, port),
            // If policy server is hosted as TCP socket, this has to be set to SocketClientAccessPolicyProtocol.Tcp
            // If policy is stored in HTTP server, this has to be set SocketClientAccessPolicyProtocol.Http
            SocketClientAccessPolicyProtocol = SocketClientAccessPolicyProtocol.Tcp
        };
        // Set the event handler for completed connection (nomatter if it is successful or not)
        args.Completed += OnConnected;
        // Start connecting to the server asynchronously
        socket.ConnectAsync(args);
    }

    /// <summary>
    /// Even handler shot when socket connection is completed
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    void OnConnected(object sender, SocketAsyncEventArgs e)
    {
        // If the connection is not successful, set socket object to null
        if (e.SocketError != SocketError.Success)
        {
            if (e.SocketError == SocketError.AccessDenied)
            {
                // Policy server is not running or cannot be reached
                throw new SocketException((int)SocketError.AccessDenied);
            }
            socket = null;
        }
        // Begin receiving data otherwise
        else
        {
            BeginRead();
        }
    }

    /// <summary>
    /// Method for receiving data from the server
    /// </summary>
    private void BeginRead()
    {
        // Receive data only if socket is connected
        if (socket != null && socket.Connected)
        {
            SocketAsyncEventArgs args = new SocketAsyncEventArgs();
            // Store the received buffer in a class variable
            args.SetBuffer(buffer, bytesReceived, buffer.Length - bytesReceived);
            // Set the event handler for received data
            args.Completed += OnReceived;

            // Start receiving data asynchronously
            socket.ReceiveAsync(args);
        }
    }

    /// <summary>
    /// Event handler shot when data is received from the server
    /// </summary>
    void OnReceived(object sender, SocketAsyncEventArgs e)
    {
        // Make sure that receiving was successful
        if (e.SocketError == SocketError.Success)
        {
            // Increase the count of received bytes in the current receiving operation
            bytesReceived += e.BytesTransferred;
        }
        // If the receiving was unsuccessful, throw an exception
        else
        {
            throw new SocketException();
        }
        // Check if the buffer is already full
        if (bytesReceived == buffer.Length)
        {
            // If the buffer is full, decode the string received from bytes
            // *** This should be your object deserialization, if you use anything but string ***
            string text = Encoding.UTF8.GetString(buffer, 0, buffer.Length);

            // If the event was set from somewhere, shoot it
            // *** In most cases event is set from UI thread to handle the
            //     received string or object and show the result to user ***
            if (MessageReceived != null)
            {
                // Shoot the event, if it's set
                MessageReceived(this, new MessageEventArgs()
                {
                    Message = text
                });
            }

            // Set the bytes received count to 0, for other data receiving event to fill the buffer from begining
            bytesReceived = 0;
        }
        // Begin the data receiving again
        BeginRead();
    }

    /// <summary>
    /// Sample method to send data to the server
    /// </summary>
    /// <param name="text">Text you would like the server to receive</param>
    public void SendText(string text)
    {
        // Check if the socket is connected to the server
        if (socket != null && socket.Connected)
        {
            // Encode the string to be sent to bytes
            // *** This is where your object serialization should be done ***
            byte[] buffer = Encoding.UTF8.GetBytes(text);
            // Check if the buffer is not empty
            if (buffer.Length != 0)
            {
                SocketAsyncEventArgs args = new SocketAsyncEventArgs();
                // Set the buffer to be sent
                args.SetBuffer(buffer, 0, buffer.Length);

                // Start sending buffer to the server asynchronously
                socket.SendAsync(args);
            }
        }
    }
}

For socket connection in Silverlight to be successful, there HAS to be

• cross-domain policy socket server, hosted on port 943

OR

• cross-domain policy file hosted on HTTP server (port 80).

If you choose to use socket server, here's a working sample source code:

Sample taken from here and it says there that

the policy server <...> must be run on port 943

and says nothing about possibility to host it on HTTP server, just because the article is too old.

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 _connection;
        private byte[] _buffer; // buffer to receive the request from the client       
        private int _received;
        private byte[] _policy; // the policy to return to the client

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

        public PolicyConnection(Socket client, byte[] policy)
        {
            _connection = client;
            _policy = policy;

            _buffer = new byte[_policyRequestString.Length];
            _received = 0;

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

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

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

                // make sure the request is valid
                string request = System.Text.Encoding.UTF8.GetString(_buffer, 0, _received);
                if (StringComparer.InvariantCultureIgnoreCase.Compare(request, _policyRequestString) != 0)
                {
                    _connection.Close();
                    return;
                }

                // send the policy
                Console.Write("Sending policy...\n");
                _connection.BeginSend(_policy, 0, _policy.Length, SocketFlags.None,
                    new AsyncCallback(OnSend), null);
            }
            catch (SocketException)
            {
                _connection.Close();
            }
        }

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

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

        // pass in the path of an XML file containing the socket policy
        public PolicyServer(string policyFilePath)
        {

            // Load the policy file
            FileStream policyStream = new FileStream(policyFilePath, FileMode.Open);

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


            // Create the Listening Socket
            _listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream,
                ProtocolType.Tcp);

            _listener.SetSocketOption(SocketOptionLevel.Tcp, (SocketOptionName)
                SocketOptionName.NoDelay, 0);

            _listener.Bind(new IPEndPoint(IPAddress.Any, 943));
            _listener.Listen(10);

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

        // Called when we receive a connection from a client
        public void OnConnection(IAsyncResult res)
        {
            Socket client = null;

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

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

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

        public void Close()
        {
            _listener.Close();
        }
    }

    public class Program
    {
        static void Main()
        {
            Console.Write("Starting...\n");
            PolicyServer ps =
                new PolicyServer(@".\clientaccesspolicy.xml");
            System.Threading.Thread.Sleep(System.Threading.Timeout.Infinite);
        }
    }
}

That's about it. Run the cross-domain policy server first and your client only then, to make sure the connection will succeed.

If you need an asynchronous WPF socket server sample, just ask and give me link to the question! I have one built with async-await keywords, to be as simple as possible.

Good luck in everything!