Question

我最近在C＃中编写了一个快速而肮脏的概念验证代理服务器，作为使Java Web应用程序与驻留在另一台服务器上的传统VB6应用程序进行通信的努力的一部分。这太荒谬了：

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：正文的长度+消息正文。

我有一个单独的ProxyClient类，表示与客户端的连接。它处理代理与单个客户端之间的所有交互。

我想知道的是，它们是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，您需要注意管理读取缓冲区，以免意外丢失字节，并且需要跟踪处理当前消息的距离。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，并在几个实例变量的帮助下管理缓冲区的状态和当前的消息处理状态。

我传递给BeginRead的回调函数的代码如下，以及上下文的相关实例变量。代码似乎工作正常“原样”，但我想知道它是否可以重构一点以使其更清晰（或者它可能已经是？）。

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东西提取到一个单独的MessageBuffer类中，并让我的读回调在它们到达时向它添加新的字节。然后MessageBuffer会担心当前BufferState之类的内容，并在收到完整消息时触发事件，然后ProxyClient可以进一步传播到主代理服务器代码，其中请求可以处理。

Answer 1

我必须克服类似的问题。这是我的解决方案（修改后适合您自己的示例）。

我们围绕Stream创建了一个包装器（NetworkStream的超类，它是TcpClient或其他任何类型的超类。它监视读取。读取某些数据时，会对其进行缓冲。当我们收到一个长度指示符（4个字节）时，我们检查是否有一个完整的消息（4个字节+消息体长度）。当我们这样做时，我们使用消息体引发MessageReceived事件，并从缓冲区中删除该消息。此技术自动处理碎片消息和每个数据包的多个消息情况。

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
}

Answer 2

我认为你使用过的设计很好，大概是我会做的和做过同样的事情。我不认为你通过重构其他类/结构来获得更多收益，而且从我看到你实际上通过这样做会使解决方案变得更加复杂。

我唯一的评论是两个是否读取第一个总是杂乱无章的长度而第二个总是身体足够强大。我总是对这样的方法保持警惕，好像它们由于不可预知的情况（例如另一端发送错误的长度）而以某种方式失去同步，因此很难恢复。相反，我会使用大缓冲区进行单次读取，以便始终从网络获取所有可用数据，然后检查缓冲区以提取完整的消息。这样一来，如果出现问题，当前缓冲区可以被丢弃以使事情恢复到干净状态，只有当前消息丢失而不是停止整个服务。

实际上，如果你的消息体很大并且两个单独的接收到达并且下一个消息在线上发送它的长度与前一个体的后半部分同时发生问题。如果发生这种情况，您的消息长度将最终附加到上一条消息的正文中，并且您已经处于上一段中所述的情况。

Answer 3

您可以使用yield return自动生成用于异步回调的状态机。杰弗里里希特通过他的AsyncEnumerator课程推广这项技巧，我已经玩弄了here这个想法。

Answer 4

你做这件事的方式没有错。但是对于我来说，我喜欢将数据的接收与处理它分开，这就是你似乎在想到的MessageBuffer类。我已经详细讨论了here。

C＃中是否存在众所周知的异步网络代码模式？

4 个答案: