C# 异步套接字服务器 - 回调中的 BeginReceive

Question

我有一个用 C# 编写的简单异步套接字服务器(几乎是 Microsoft 的 example )，但是这个示例的问题是它只接受来自客户端的一条消息，然后关闭。我希望此服务器在收到任何消息后保持事件状态。

这个问题之前有人问过here ，答案和评论解释说解决这个问题只是在 SendCallback 函数中调用 handler.beginReceive，但是要做到这一点需要传入一个 state 变量。这是我不确定用异步编程做的事情，因为我对它很陌生。

在下面的示例中，我如何将 state 对象从 Send 函数传送到 SendCallback 函数？

服务器代码:

// Asynchronous Server Socket Example
// http://msdn.microsoft.com/en-us/library/fx6588te.aspx

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;

// State object for reading client data asynchronously
public class StateObject
{
    // Client  socket.
    public Socket workSocket = null;
    // Size of receive buffer.
    public const int BufferSize = 1024;
    // Receive buffer.
    public byte[] buffer = new byte[BufferSize];
    // Received data string.
    public StringBuilder sb = new StringBuilder();
}

public class GetState
{

}

public class AsynchronousSocketListener
{
    // Thread signal.
    public static ManualResetEvent allDone = new ManualResetEvent(false);

    public AsynchronousSocketListener()
    {
    }

    public static void StartListening()
    {
        // Data buffer for incoming data.
        byte[] bytes = new Byte[1024];

        // Establish the local endpoint for the socket.
        // The DNS name of the computer
        // running the listener is "host.contoso.com".
        IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName());
        IPAddress ipAddress = ipHostInfo.AddressList[0];
        IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);

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

        // Bind the socket to the local endpoint and listen for incoming connections.
        try
        {
            listener.Bind(localEndPoint);
            listener.Listen(100);

            while (true)
            {
                // Set the event to nonsignaled state.
                allDone.Reset();

                // Start an asynchronous socket to listen for connections.
                Console.WriteLine("Waiting for a connection...");
                listener.BeginAccept(new AsyncCallback(AcceptCallback), listener);

                // Wait until a connection is made before continuing.
                allDone.WaitOne();
            }

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

        Console.WriteLine("\nPress ENTER to continue...");
        Console.Read();

    }

    public static void AcceptCallback(IAsyncResult ar)
    {
        // Signal the main thread to continue.
        allDone.Set();

        // Get the socket that handles the client request.
        Socket listener = (Socket)ar.AsyncState;
        Socket handler = listener.EndAccept(ar);

        // Create the state object.
        StateObject state = new StateObject();
        state.workSocket = handler;
        handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
    }

    public static void ReadCallback(IAsyncResult ar)
    {
        String content = String.Empty;

        // Retrieve the state object and the handler socket
        // from the asynchronous state object.
        StateObject state = (StateObject)ar.AsyncState;
        Socket handler = state.workSocket;

        // Read data from the client socket. 
        int bytesRead = handler.EndReceive(ar);

        if (bytesRead > 0)
        {
            // There  might be more data, so store the data received so far.
            state.sb.Append(Encoding.ASCII.GetString(
                state.buffer, 0, bytesRead));

            // Check for end-of-file tag. If it is not there, read 
            // more data.
            content = state.sb.ToString();
            if (content != null)
            {
                // All the data has been read from the 
                // client. Display it on the console.
                Console.WriteLine("Read {0} bytes from socket. \n Data : {1}",
                    content.Length, content);
                // Echo the data back to the client.
                Send(handler, content, state);
            }
            else
            {
                // Not all data received. Get more.
                handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
            }
        }
    }

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

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

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

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

            /// ******* WIP ******** //
            //handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
            //handler.Shutdown(SocketShutdown.Both);
            //handler.Close();

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


    public static int Main(String[] args)
    {
        StartListening();
        return 0;
    }
}

Answer 1

我会这样做:

public class AsynchronousSocketServer
{
    private Socket listener;
    private byte[] buffer = new byte[8192]; // Buffer to store data from clients.

    public void StartListening()
    {
        listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        listener.Bind(new IPEndPoint(localIPAddress, listeningPort));
        listener.Listen(20);
        listener.BeginAccept(OnSocketAccepted, null);
    }

    private void OnSocketAccepted(IAsyncResult result)
    {
        // This is the client socket, where you send/receive data from after accepting. Keep it in a List<Socket> collection if you need to.
        Socket client = listener.EndAccept(result);

        // Pass in the client socket as the state object, so you can access it in the callback.
        client.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, OnDataReceived, client); // Start receiving data from this client.
        listener.BeginAccept(OnSocketAccepted, null); // Start a new async accept operation to accept incoming connections from other clients.
    }

    private void OnDataReceived(IAsyncResult result)
    {
        // This is the client that sent you data. AsyncState is exactly what you passed into the state parameter in BeginReceive
        Socket client = result.AsyncState as Socket;
        int received = client.EndReceive(result);

        // Handle received data in buffer, send reply to client etc...

        // Start a new async receive on the client to receive more data.
        client.BeginReceive(buffer, 0, buffer.Length, SocketFlags.None, OnDataReceived, client);
    }
}

如您所见，它有很多递归回调。基本思想是您调用 BeginAccept 开始接受第一个连接，然后一旦您的回调触发您接受与 EndAccept 的连接并开始一个新的 BeginAccept 开始接受更多连接。当您开始与客户端通信时，同样的逻辑适用于 BeginReceive 和 EndReceive。使用此逻辑，服务器将持续接受传入连接，客户端也将能够持续向服务器发送数据。

在您的示例中，您从 AsyncState 获取了监听器，但您没有再次调用 BeginAccept 来接受更多传入连接，这可能解释了为什么您的服务器只接受 1 个连接并关闭。

对于 BeginSend 中的 state 参数，您实际上可以将 BeginReceive 放在 Send() 之后> 省去麻烦的方法:

Send(handler, content, state);
handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);