C# UDP 服务器异步多客户端 |客户端断开连接时的 SocketException

Question

我一直在用 C# 编写一个套接字服务器程序（我的灵感来自这个post），我的问题是当客户端断开异常时“现有连接被远程主机强行关闭”在调用EndReceiveFrom()并返回0时出现，ref clientEP变为客户端常闭。我不明白为什么我的 DoReceiveFrom() 函数在没有可读的情况下被调用。我可能错过了什么。怎么了？

问题出现在那里：

int dataLen = this.serverSocket.EndReceiveFrom(iar, ref clientEP);

完整源代码：

class UDPServer
{
    private Socket serverSocket = null;
    private List<EndPoint> clientList = new List<EndPoint>();
    private List<Tuple<EndPoint, byte[]>> dataList = new List<Tuple<EndPoint, byte[]>>();
    private byte[] byteData = new byte[1024];
    private int port = 4242;

    public List<Tuple<EndPoint, byte[]>> DataList
    {
        private set { this.dataList = value; }
        get { return (this.dataList); }
    }

    public UDPServer(int port)
    {
        this.port = port;
    }

    public void Start()
    {
        this.serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        this.serverSocket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        this.serverSocket.Bind(new IPEndPoint(IPAddress.Any, this.port));
        EndPoint newClientEP = new IPEndPoint(IPAddress.Any, 0);
        this.serverSocket.BeginReceiveFrom(this.byteData, 0, this.byteData.Length, SocketFlags.None, ref newClientEP, DoReceiveFrom, newClientEP);
    }

    private void DoReceiveFrom(IAsyncResult iar)
    {
        try
        {
            EndPoint clientEP = new IPEndPoint(IPAddress.Any, 0);
            int dataLen = this.serverSocket.EndReceiveFrom(iar, ref clientEP);
            byte[] data = new byte[dataLen];
            Array.Copy(this.byteData, data, dataLen);

            if (!this.clientList.Any(client => client.Equals(clientEP)))
                this.clientList.Add(clientEP);

            EndPoint newClientEP = new IPEndPoint(IPAddress.Any, 0);
            this.serverSocket.BeginReceiveFrom(this.byteData, 0, this.byteData.Length, SocketFlags.None, ref newClientEP, DoReceiveFrom, newClientEP);

            DataList.Add(Tuple.Create(clientEP, data));
        }
        catch (ObjectDisposedException)
        {
        }
    }

    public void SendTo(byte[] data, EndPoint clientEP)
    {
        try
        {
            this.serverSocket.SendTo(data, clientEP);
        }
        catch (System.Net.Sockets.SocketException)
        {
            this.clientList.Remove(clientEP);
        }
    }

    public void SendToAll(byte[] data)
    {
        foreach (var client in this.clientList)
        {
            this.SendTo(data, client);
        }
    }

    public void Stop()
    {
        this.serverSocket.Close();
        this.serverSocket = null;

        this.dataList.Clear();
        this.clientList.Clear();
    }
}

例外：

An existing connection was forcibly closed by the remote host

---

更新： 我尝试在另一台电脑上运行我的客户端（netcat）并且不再出现异常，即使在 SendTo() 时，在我的 clientList 中删除我的客户端也是有问题的. 我还是不明白发生了什么。

Answer 1

一切都应该如此。

这是所有异步方法的工作方式：您调用 BeginDo() 并将 AsyncCallback 委托的实现传递给它（在您的例如 DoReceiveFrom）。之后您的实现立即开始执行 - BeginDo() 不是阻塞调用。

在您的实现中，您必须调用 EndDo()，这将 阻塞，直到发生以下两种情况之一：您调用 BeginDo( )，实际上做某事，或者它抛出异常。就像客户端断开连接时的情况一样。

Async 方法上的source。

你需要做的是让整个事情发挥作用

1. 确保正确处理客户端断开连接的异常
2. 无论 EndReceiveFrom 以何种方式结束，请确保调用 BeginReceiveFrom。最好在调用 EndReceiveFrom 之后立即调用 BeginReceiveFrom。这是必需的，因为当您的服务器在这些调用之间时，它实际上并没有监听套接字。

我会在 EndReceiveFrom 周围放置另一个 try-catch。

更新：

private void DoReceiveFrom(IAsyncResult iar)
{
    try
    {
        EndPoint clientEP = new IPEndPoint(IPAddress.Any, 0);
        int dataLen = 0;
        byte[] data = null;
        try
        {
            dataLen = this.serverSocket.EndReceiveFrom(iar, ref clientEP);
            data = new byte[dataLen];
            Array.Copy(this.byteData, data, dataLen);
        }
        catch(Exception e)
        {
        }
        finally
        {
            EndPoint newClientEP = new IPEndPoint(IPAddress.Any, 0);
            this.serverSocket.BeginReceiveFrom(this.byteData, 0, this.byteData.Length, SocketFlags.None, ref newClientEP, DoReceiveFrom, newClientEP);
        }

        if (!this.clientList.Any(client => client.Equals(clientEP)))
            this.clientList.Add(clientEP);

        DataList.Add(Tuple.Create(clientEP, data));
    }
    catch (ObjectDisposedException)
    {
    }
}