c# - 套接字读取卡住

Question

我正在开发以学习为目的的小程序。

我的程序是简单的 Telnet 控制台。

我使用 visual studio 2010 Express 创建了非常酷的 UI，现在我正在尝试与远程服务器(Creston 控制处理器)建立通信。我从 this Microsoft article 复制并粘贴了类(class)当我执行类(class)时，程序卡住了。我不确定如何正确描述发生的情况，但在基本词中所有控件(包括关闭按钮)都停止工作。

这是我类(class)的代码:(我添加了一些调试行)；

public class StateObject
{
    // Client socket.
    public Socket workSocket = null;
    // Size of receive buffer.
    public const int BufferSize = 256;
    // Receive buffer.
    public byte[] buffer = new byte[BufferSize];
    // Received data string.
    public StringBuilder sb = new StringBuilder();
    public string TempString = string.Empty;
    public int TotalBytesRead = 0;
    public char[] charBuffer = new char[1000];
}
public class AsynchronousClient
{
    // The port number for the remote device.
    private const int port = 23;

    // ManualResetEvent instances signal completion.
    private static ManualResetEvent connectDone =
        new ManualResetEvent(false);
    private static ManualResetEvent sendDone =
        new ManualResetEvent(false);
    private static ManualResetEvent receiveDone =
        new ManualResetEvent(false);

    // The response from the remote device.
    private static String response = String.Empty;
    public AsynchronousClient()
    {

    }
    public void New()
    {
        StartClient();
    }
    private static void StartClient()
    {
        // Connect to a remote device.
        try
        {
            // Establish the remote endpoint for the socket.
            // The name of the 
            // remote device is "host.contoso.com".
            IPEndPoint remoteEP = new IPEndPoint(IPAddress.Parse("10.106.6.60"), port);

            // Create a TCP/IP socket.
            Socket client = new Socket(AddressFamily.InterNetwork,
                SocketType.Stream, ProtocolType.Tcp);

            // Connect to the remote endpoint.
            client.BeginConnect(remoteEP,
                new AsyncCallback(ConnectCallback), client);
            connectDone.WaitOne();
            Console.WriteLine("StartSend");
            // Send test data to the remote device.
            //Send(client, "hostname\n");
            //sendDone.WaitOne();
            Console.WriteLine("WaitForResponse");
            // Receive the response from the remote device.
            Receive(client);
            receiveDone.WaitOne();

            // Write the response to the console.
            Console.WriteLine("Read From Socket : {0}", response);
            Console.WriteLine("ReleaseSocket");
            // Release the socket.
            client.Shutdown(SocketShutdown.Both);
            client.Disconnect(true);

        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ConnectCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.
            Socket client = (Socket)ar.AsyncState;

            // Complete the connection.
            client.EndConnect(ar);

            Console.WriteLine("Socket connected to {0}",
                client.RemoteEndPoint.ToString());

            // Signal that the connection has been made.
            connectDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Receive(Socket client)
    {
        try
        {
            // Create the state object.
            Console.WriteLine("Receive");
            StateObject state = new StateObject();
            state.workSocket = client;

            // Begin receiving the data from the remote device.
            client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void ReceiveCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the state object and the client socket 
            // from the asynchronous state object.
            Console.WriteLine("Start Read");
            StateObject state = (StateObject)ar.AsyncState;
            Socket client = state.workSocket;

            // Read data from the remote device.
            int bytesRead = client.EndReceive(ar);
            Console.WriteLine("Bytes read: {0}", bytesRead);
            if (bytesRead > 0)
            {
                // There might be more data, so store the data received so far.
                string tsTempString = Encoding.ASCII.GetString(state.buffer, 0, bytesRead);
                state.TempString += tsTempString;
                Console.WriteLine("String {0}",  tsTempString);
                state.sb.Append(Encoding.ASCII.GetString(state.buffer, 0, bytesRead));

                // Get the rest of the data.
                client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
            }
            else
            {
                // All the data has arrived; put it in response.
                if (state.sb.Length > 1)
                {
                    response = state.sb.ToString();
                }
                // Signal that all bytes have been received.
                receiveDone.Set();
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

    private static void Send(Socket client, String data)
    {
        Console.WriteLine("Send");
        // Convert the string data to byte data using ASCII encoding.
        byte[] byteData = Encoding.ASCII.GetBytes(data);

        // Begin sending the data to the remote device.
        client.BeginSend(byteData, 0, byteData.Length, 0,
            new AsyncCallback(SendCallback), client);
    }

    private static void SendCallback(IAsyncResult ar)
    {
        try
        {
            Console.WriteLine("SendCallBack");
            // Retrieve the socket from the state object.
            Socket client = (Socket)ar.AsyncState;

            // Complete sending the data to the remote device.
            int bytesSent = client.EndSend(ar);
            Console.WriteLine("Sent {0} bytes to server.", bytesSent);

            // Signal that all bytes have been sent.
            sendDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }

}

当连接到服务器时，它会响应以下几行:

CP3 Console
Warning: Another console session is open

CP3>

当我运行程序时，我得到以下输出:

Socket connected to 10.106.6.60:23
StartSend
WaitForResponse
Receive
Start Read
Bytes read: 13
String CP3 Console

Start Read
Bytes read: 43
String Warning: Another console session is open 

Start Read
Bytes read: 6
String 
CP3>
The thread '<No Name>' (0x1d80) has exited with code 0 (0x0).
The thread '<No Name>' (0x24bc) has exited with code 0 (0x0).

我尝试了不同的读取流的方法，但都没有成功。程序仍然卡在同一个地方。

似乎在读取最后一个字节“>”后程序没有重新执行方法“ReceiveCallback”以退出循环。

我感觉它与调用“ReceiveCallback”的方式有关，但我无法弄清楚 client.beginReceive() 参数的实际作用。

Answer 1

进一步阐述 Roemer 的回答:EndReceive 只有在对话终止时才会返回 0。这通常意味着已收到带有 FIN 或 RST 标志的 TCP 数据包，最常见的原因是对等方(在您的情况下为服务器)关闭其端点或退出。 (还有各种其他情况；例如，防火墙或中间节点可能会生成 RST。)

(Roemer 写道“BeginReceive never returns zero...”，但显然是指 EndReceive，因为 BeginReceive 返回 IAsyncResult 对象。)

如果对方没有结束对话，EndReceive 将不会返回 0，您将永远不会进入回调的 else 分支调用 receiveDone.Set()。

从您的描述中并不能完全清楚服务器的行为方式，但它最后发送的 6 个字节显然是一个换行符，后跟“CP3>”提示符。那时它正在等待您的客户发送一些东西。它不会关闭连接。

TCP 是一种字节流服务——它不提供记录边界。所以套接字层无法知道服务器何时“发送完毕”，除非服务器关闭连接。您为客户提供三种选择:

1. 在您收到服务器的输出时对其进行解析并识别它何时等待输入(在本例中，这意味着识别“CP3>”提示)
2. 在任意时间过去且没有来自服务器的进一步数据后，确定服务器已完成
3. 别管它，只需在收到数据时从服务器输出数据即可

第三个选项是最简单的一个。建立连接后执行第一个 BeginReceive。然后，在您的回调方法中，如果对话仍处于打开状态且未发生错误，则处理您收到的数据并再次调用 BeginReceive。