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Are there well-known patterns for asynchronous network code in C #?

I recently wrote a fast and dirty proxy server with a proxy concept in C # as part of trying to get a Java web application to communicate with an outdated VB6 application located on another server. This is ridiculously simple:

The proxy server and clients use the same message format; in the code, I use the class ProxyMessageto represent both requests from clients and responses generated by the server:

public class ProxyMessage
{
   int Length; // message length (not including the length bytes themselves)
   string Body; // an XML string containing a request/response

   // writes this message instance in the proper network format to stream 
   // (helper for response messages)
   WriteToStream(Stream stream) { ... }
}

Messages are as simple as they can be: body length + message body.

I have a separate class ProxyClientthat represents a connection to a client. It handles all interactions between the proxy and one client.

, , ? , , , , , . TcpClient.BeginRead .

, BeginRead, . , , " ", , , (, , ?).

private enum BufferStates 
{ 
    GetMessageLength, 
    GetMessageBody 
}
// The read buffer. Initially 4 bytes because we are initially
// waiting to receive the message length (a 32-bit int) from the client 
// on first connecting. By constraining the buffer length to exactly 4 bytes,
// we make the buffer management a bit simpler, because
// we don't have to worry about cases where the buffer might contain
// the message length plus a few bytes of the message body.
// Additional bytes will simply be buffered by the OS until we request them.
byte[] _buffer = new byte[4];

// A count of how many bytes read so far in a particular BufferState.
int _totalBytesRead = 0;

// The state of the our buffer processing. Initially, we want
// to read in the message length, as it the first thing
// a client will send
BufferStates _bufferState = BufferStates.GetMessageLength;

// ...ADDITIONAL CODE OMITTED FOR BREVITY...

// This is called every time we receive data from
// the client.

private void ReadCallback(IAsyncResult ar)
{
    try
    {
        int bytesRead = _tcpClient.GetStream().EndRead(ar);

        if (bytesRead == 0)
        {
            // No more data/socket was closed.
            this.Dispose();
            return;
        }

        // The state passed to BeginRead is used to hold a ProxyMessage
        // instance that we use to build to up the message 
        // as it arrives.
        ProxyMessage message = (ProxyMessage)ar.AsyncState;

        if(message == null)
            message = new ProxyMessage();

        switch (_bufferState)
        {
            case BufferStates.GetMessageLength:

                _totalBytesRead += bytesRead;

                // if we have the message length (a 32-bit int)
                // read it in from the buffer, grow the buffer
                // to fit the incoming message, and change
                // state so that the next read will start appending
                // bytes to the message body

                if (_totalBytesRead == 4)
                {
                    int length = BitConverter.ToInt32(_buffer, 0);
                    message.Length = length;
                    _totalBytesRead = 0;
                    _buffer = new byte[message.Length];
                    _bufferState = BufferStates.GetMessageBody;
                }

                break;

            case BufferStates.GetMessageBody:

                string bodySegment = Encoding.ASCII.GetString(_buffer, _totalBytesRead, bytesRead);
                _totalBytesRead += bytesRead;

                message.Body += bodySegment;

                if (_totalBytesRead >= message.Length)
                {
                    // Got a complete message.
                    // Notify anyone interested.

                    // Pass a response ProxyMessage object to 
                    // with the event so that receivers of OnReceiveMessage
                    // can send a response back to the client after processing
                    // the request.
                    ProxyMessage response = new ProxyMessage();
                    OnReceiveMessage(this, new ProxyMessageEventArgs(message, response));
                    // Send the response to the client
                    response.WriteToStream(_tcpClient.GetStream());

                    // Re-initialize our state so that we're
                    // ready to receive additional requests...
                    message = new ProxyMessage();
                    _totalBytesRead = 0;
                    _buffer = new byte[4]; //message length is 32-bit int (4 bytes)
                    _bufferState = BufferStates.GetMessageLength;
                }

                break;
        }

        // Wait for more data...
        _tcpClient.GetStream().BeginRead(_buffer, 0, _buffer.Length, this.ReadCallback, message);
    }
    catch
    {
        // do nothing
    }

}

, MessageBuffer . MessageBuffer , BufferState, , , ProxyClient -, .

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4

. ( ).

Stream ( NetworkStream, TcpClient ). . , . (4 ), , (4 + ). , MessageReceived . .

public class MessageStream : IMessageStream, IDisposable
{
    public MessageStream(Stream stream)
    {
        if(stream == null)
            throw new ArgumentNullException("stream", "Stream must not be null");

        if(!stream.CanWrite || !stream.CanRead)
            throw new ArgumentException("Stream must be readable and writable", "stream");

        this.Stream = stream;
        this.readBuffer = new byte[512];
        messageBuffer = new List<byte>();
        stream.BeginRead(readBuffer, 0, readBuffer.Length, new AsyncCallback(ReadCallback), null);
    }

    // These belong to the ReadCallback thread only.
    private byte[] readBuffer;
    private List<byte> messageBuffer;

    private void ReadCallback(IAsyncResult result)
    {
        int bytesRead = Stream.EndRead(result);
        messageBuffer.AddRange(readBuffer.Take(bytesRead));

        if(messageBuffer.Count >= 4)
        {
            int length = BitConverter.ToInt32(messageBuffer.Take(4).ToArray(), 0);  // 4 bytes per int32

            // Keep buffering until we get a full message.

            if(messageBuffer.Count >= length + 4)
            {
                messageBuffer.Skip(4);
                OnMessageReceived(new MessageEventArgs(messageBuffer.Take(length)));
                messageBuffer.Skip(length);
            }
        }

        // FIXME below is kinda hacky (I don't know the proper way of doing things...)

        // Don't bother reading again.  We don't have stream access.
        if(disposed)
            return;

        try
        {
            Stream.BeginRead(readBuffer, 0, readBuffer.Length, new AsyncCallback(ReadCallback), null);
        }
        catch(ObjectDisposedException)
        {
            // DO NOTHING
            // Ends read loop.
        }
    }

    public Stream Stream
    {
        get;
        private set;
    }

    public event EventHandler<MessageEventArgs> MessageReceived;

    protected virtual void OnMessageReceived(MessageEventArgs e)
    {
        var messageReceived = MessageReceived;

        if(messageReceived != null)
            messageReceived(this, e);
    }

    public virtual void SendMessage(Message message)
    {
        // Have fun ...
    }

    // Dispose stuff here
}
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, , , . , ​​ , , .

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yield return . AsyncEnumerator, .

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There is nothing wrong with how you did it. For me, however, I like to separate the receipt of data from its processing, which seems to be what you think of your proposed MessageBuffer class. I discussed this in detail here .

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