C# 为什么“Flush”不强制字节流到网络流中？

Question

我有一个项目，我试图将序列化对象发送到服务器，然后等待“OK”或“ERROR”消息返回。

我似乎遇到了与以下海报类似的问题：TcpClient send/close problem

问题是我似乎能够发送原始对象的唯一方法是关闭连接，但是（当然）我迫不及待地想看看服务器是否成功处理了对象。

private void button4_Click(object sender, EventArgs e)
{
    RequestPacket req = new RequestPacket();

    /// ... Fill out request packet ...

    /// Connect to the SERVER to send the message...
    TcpClient Client = new TcpClient("localhost", 10287);
    using (NetworkStream ns = Client.GetStream())
    {
        XmlSerializer xml = new XmlSerializer(typeof(RequestPacket));
        xml.Serialize(ns, req);

        /// NOTE: This doesn't seem to do anything.... 
        ///       The server doesn't get the object I just serialized.
        ///       However, if I use ns.Close() it does... 
        ///          but then I can't get the response.
        ns.Flush();

        // Get the response. It should be "OK".
        ResponsePacket resp;

        XmlSerializer xml2 = new XmlSerializer(typeof(ResponsePacket));
        resp = (ResponsePacket)xml2.Deserialize(ns);


        /// ... EVALUATE RESPONSE ...
    }

    Client.Close()
}

更新：回应一位评论者，我认为客户不会有错。它只是在等待对象，并且对象永远不会出现，直到我关闭套接字....但是，如果我错了，我会很乐意公开吃乌鸦。 =) 这是客户端：

    static void Main(string[] args)
    {
        // Read the port from the command line, use 10287 for default
        CMD cmd = new CMD(args);
        int port = 10287;

        if (cmd.ContainsKey("p")) port = Convert.ToInt32(cmd["p"]);

        TcpListener l = new TcpListener(port);
        l.Start();

        while (true)
        {
            // Wait for a socket connection.
            TcpClient c = l.AcceptTcpClient();
            
            Thread T = new Thread(ProcessSocket);

            T.Start(c);
        }
    }


    static void ProcessSocket(object c)
    {
        TcpClient C = (TcpClient)c;

        try
        {
            RequestPacket rp;
            //// Handle the request here.
            using (NetworkStream ns = C.GetStream())
            {
                XmlSerializer xml = new XmlSerializer(typeof(RequestPacket));
                rp = (RequestPacket)xml.Deserialize(ns);
            }

            ProcessPacket(rp);
        }
        catch
        {
            // not much to do except ignore it and go on.
        }
    }

是的....就是这么简单。

Answer 1

您应该使用链接到您的网络流的 StreamWriter/Reader，.Flush 对 NetworkStream 没有任何作用，请参见此处：

http://www.c-sharpcorner.com/UploadFile/dottys/SocketProgDTRP11222005023030AM/SocketProgDTRP.aspx

Answer 2

呃哦，你可以责怪Nagle's algorithm。虽然它与 C# 无关，但它是 TCP/IP 堆栈的默认行为。使用SetSocketOption 方法启用NoDelay 套接字选项。但请注意，禁用 Nagle 算法会降低吞吐量。

我也不确定您在套接字上使用的那个流，因为我根本不是 C# 开发人员，但请尝试删除它的实例，以便它确实可以写入 :-)

Answer 3

显然，短版本是，当使用 XmlSerializer（或任何其他大 blob）将数据推送到 NetworkStream 时，它只会无限期地保持线路打开，等待写入更多信息。它只会在您关闭连接后刷新连接。这会造成这种方法非常适合发送但不适合接收的情况。或相反亦然。它成为一种单向通信，对于通过同一连接继续来回通信毫无用处。

我不得不解决一些表面上看起来如此优雅的问题，但回到我以前的 C 时代，我已经诉诸于先发送“字节数”数据包，然后才是实际数据包.这使我能够在另一端读取确切的字节数，因此我永远不会陷入阻塞模式。

为了简化我的生活，我创建了一个类，其中包含一些用于发送和接收的静态方法。这个类可以通过网络发送任何 XML-serializable 类，所以它可以做我需要它做的事情。

如果有人有更优雅的解决方案，我愿意倾听。

public class PacketTransit
{
    public static void SendPacket(TcpClient C, object Packet)
    {
        MemoryStream ms = new MemoryStream();
        XmlSerializer xml = new XmlSerializer(Packet.GetType());
        xml.Serialize(ms, Packet);
        ms.Position = 0;
        byte[] b = ms.GetBuffer();
        ms.Dispose();

        byte [] sizePacket = BitConverter.GetBytes(b.Length);
        // Send the 4-byte size packet first.
        C.Client.Send(sizePacket, sizePacket.Length, SocketFlags.None);
        C.Client.Send(b, b.Length, SocketFlags.None);
    }

    /// The string is the XML file that needs to be converted.
    public static string ReceivePacket(TcpClient C, Type PacketType)
    {
        byte [] FirstTen = new byte[1024];
        int size = 0;
        byte[] sizePacket = BitConverter.GetBytes(size);

        // Get the size packet
        int sp = C.Client.Receive(sizePacket, sizePacket.Length, SocketFlags.None);
        if (sp <= 0) return "";

        size = BitConverter.ToInt32(sizePacket, 0);

        // read until "size" is met
        StringBuilder sb = new StringBuilder();
        while (size > 0)
        {
            byte[] b = new byte[1024];
            int x = size;
            if (x > 1024) x = 1024;
            int r = C.Client.Receive(b, x, SocketFlags.None);
            size -= r;
            sb.Append(UTF8Encoding.UTF8.GetString(b));
        }

        return sb.ToString();
    }

    /// The XML data that needs to be converted back to the appropriate type.
    public static object Decode(string PacketData, Type PacketType)
    {
        MemoryStream ms = new MemoryStream(UTF8Encoding.UTF8.GetBytes(PacketData));
        XmlSerializer xml = new XmlSerializer(PacketType);
        object obj = xml.Deserialize(ms);
        ms.Dispose();

        return obj;
    }

    public static RequestPacket GetRequestPacket(TcpClient C)
    {
        string str = ReceivePacket(C, typeof(RequestPacket));

        if (str == "") return new RequestPacket();

        RequestPacket req = (RequestPacket) Decode(str, typeof(RequestPacket));

        return req;
    }

    public static ResponsePacket GetResponsePacket(TcpClient C)
    {
        string str = ReceivePacket(C, typeof(ResponsePacket));

        if (str == "") return new ResponsePacket();

        ResponsePacket res = (ResponsePacket)Decode(str, typeof(ResponsePacket));

        return res;
    }
}

要使用这个类，我只需要调用PacketTransit.SendPacket(myTcpClient, SomePacket) 来发送任何给定的 XML-Serializable 对象。然后我可以使用PacketTransit.GetResponsePacket 或PacketTransit.GetRequestPacket 在另一端接收它。

对我来说，这很有效，但它比最初预期的要多得多。

Answer 4

我相信这里真正的问题可能是 XmlDeserializer 在从流中读取 EOS 之前可能不会返回。您可能需要关闭发送流以进行输出以强制执行此操作。