c# - C# 中是否有众所周知的异步网络代码模式？

Question

我最近用 C# 编写了一个简单的概念验证代理服务器，作为让 Java Web 应用程序与驻留在另一台服务器上的遗留 VB6 应用程序通信的努力的一部分。这非常简单:

代理服务器和客户端都使用相同的消息格式；在代码中，我使用 ProxyMessage 类来表示来自客户端的请求和服务器生成的响应:

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) { ... }
}

消息尽可能简单:正文长度 + 消息正文。

我有一个单独的 ProxyClient 类，代表与客户端的连接。它处理代理和单个客户端之间的所有交互。

我想知道的是，它们是用于简化与异步套接字编程相关的样板代码的设计模式还是最佳实践？例如，您需要小心管理读取缓冲区，以免意外丢失字节，并且需要跟踪当前消息的处理进度。在我当前的代码中，我在 TcpClient.BeginRead 的回调函数中完成所有这些工作，并在一些实例变量的帮助下管理缓冲区的状态和当前消息处理状态。

下面是我传递给 BeginRead 的回调函数的代码，以及上下文的相关实例变量。该代码似乎“按原样”运行良好，但我想知道是否可以对其进行一些重构以使其更清晰(或者可能已经是？)。

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's 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
    }

}

到目前为止，我唯一真正的想法是将与缓冲区相关的内容提取到一个单独的 MessageBuffer 类中，然后让我的读取回调在新字节到达时向其附加新字节。 MessageBuffer 会担心当前的 BufferState 并在收到完整消息时触发事件，ProxyClient 可以进一步传播该消息直到主代理服务器代码，请求可以在其中处理。

Answer 1

我不得不克服类似的问题。这是我的解决方案(已修改以适合您自己的示例)。

我们围绕 Stream 创建了一个包装器(NetworkStream 的父类(super class)，它是 TcpClient 或其他类的父类(super class))。它监视读取。当一些数据被读取时，它被缓冲。当我们收到长度指示符(4 字节)时，我们检查是否有完整的消息(4 字节 + 消息正文长度)。当我们这样做时，我们会引发带有消息正文的 MessageReceived 事件，并从缓冲区中删除消息。该技术自动处理碎片化消息和每个数据包包含多个消息的情况。

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
}