C# .NET 套接字 tcp begin/endreceive 客户端：endreceive 读取所有字节，但在 beginreceive 中，缓冲区大小自行更改

Question

so i have a server and client using the class Socket (on tcp).

I've been struggling with this problem for a few weeks, I've searched a lot but I don't understand how that problem is possible. I've been watching debugger a lot, watching values of my variables but can't see where the problem is.

For example, the client sends 100 bytes, and the server receives them, proceeds them and send back 150 bytes. But, on client side, the buffer changes size each time the client receives a message (i don't know why, maybe because of beginreceive).

So, when the server sends these 150 bytes, the client beginreceive, but I see that the buffer size is 100 (same size as the previous message the client has sent).

And the client does "endreceive", and read 150 bytes (the right amount of bytes sent by server).

When i want to get the string, i use an encoding function, the buffer containing message, and the number of bytes received.

So, i end up having a buffer of size of 100 bytes (wrong size) and the number of bytes read is 150 bytes (right amount).

I've modified my code many times, tried to define "new byte[bufferSize]", but the problem stays the same.

Sometimes, the client even receive message that the server has never sent! That's weird. The client seems sometimes to receive bytes from the "server buffer" (i haven't found how to check it)

       public void InitializeReceive(Socket soc)
        {
             //bufferSize is fixed to 256
            MyBuffer = new byte[bufferSize];

            soc.BeginReceive(MyBuffer , 0, MyBuffer .Length,
                SocketFlags.None, new AsyncCallback(Receiving), soc);

        }
        public void Receiving(IAsyncResult iar)
        {

            Socket tmp = (Socket)iar.AsyncState;

            int received= tmp.EndReceive(iar);

            if (received> 0)
            {
                 //problem is here. MyBuffer has a size of 100 bytes, received is 150 bytes
                string msg = Encoding.Unicode.GetString(MyBuffer, 0, received);



                //next message
                InitializeReceive(tmp);
            }

        }

i've tried checking heap stack, stuff like this too, but i don't really know how to diagnostic it.

The first messages between client and server are okay. But it becomes like this after a few exchanges, and the buffer that i fixed isn't full at all. The size of first messages were around 36 bytes, over 100 bytes, and so on. It doesn't seem to be a problem of size here

I don't even understand how it is possible. I hope someone has an explanation thank you !

EDIT :

So, as asked, I tried to make a minimal reproducible code.

I've tested it myself and yes, the problem still stays. I get the exception "System.ArgumentOutOfRangeException " in the client side.

Server code :

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
//
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using System.IO;
using System.Threading;

namespace BugServeurSocket1
{
    public partial class Form1 : Form
    {
        private Socket MonServeur, MonClient;


        int bufferSize = 256;
        private byte[] MonBuffer;

        int portServeur = 8000;

        string tAdversaire = "1";

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            MonBuffer = new byte[bufferSize];

            #region mise en place serveur
  
            IPAddress IPServeur = CorrectAddress(Dns.GetHostName());

            MonServeur = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

            MonServeur.Bind(new IPEndPoint(IPServeur, portServeur));

            MonServeur.Listen(10);

            MonServeur.BeginAccept(new AsyncCallback(SurDemandeConnexion), MonServeur);
            #endregion
        }

        //réseau
        #region fct perso
        #region verif adr ipv4
        private IPAddress CorrectAddress(string nPC)
        {
            IPAddress ipReponse = null;

            if (nPC.Length > 0)
            {
                IPAddress[] IPserveur = Dns.GetHostEntry(nPC).AddressList;

                for (int i = 0; i < IPserveur.Length; i++)
                {
                    Ping pingServeur = new Ping();
                    //envoyer ping
                    PingReply pingResponse = pingServeur.Send(IPserveur[i]);

                    if (pingResponse.Status == IPStatus.Success)

                        if (IPserveur[i].AddressFamily == AddressFamily.InterNetwork)
                        {

                            ipReponse = IPserveur[i];
                            break;
                        }
                }//for
            }

            return ipReponse;
        }
        #endregion
        #region reception connexion client
        public void SurDemandeConnexion(IAsyncResult iar)
        {
            Socket tmp = (Socket)iar.AsyncState;

            MonServeur.BeginAccept(new AsyncCallback(SurDemandeConnexion), MonServeur);//accepter nouvelles connexions

            MonClient = tmp.EndAccept(iar);

            //MonBuffer = new byte[256];//test
            MonBuffer = Encoding.Unicode.GetBytes("Connexion acceptée");
            MonClient.Send(MonBuffer, 0, MonBuffer.Length, SocketFlags.None);

            //ver7
            InitializeReceive(MonClient);
        }
        #endregion
        public void InitializeReceive(Socket soc)
        {
            MonBuffer = new byte[256];//empty buffer//test

            soc.BeginReceive(MonBuffer, 0, MonBuffer.Length,
                SocketFlags.None, new AsyncCallback(ReceiveMsg), soc);
        }
        public void ReceiveMsg(IAsyncResult iar)
        {
            Socket tmp = (Socket)iar.AsyncState;

            int received = tmp.EndReceive(iar);

            if (received > 0)
            {
                string msg = Encoding.Unicode.GetString(MonBuffer, 0, received);

                string[] msgSplit = msg.Split(':');

                //pseudo
                string msgKey = msgSplit[0];

                #region recherche adversaire
                if (msgKey == "JEU_CHERCHER_ADVERSAIRE")
                {
                        MonBuffer = Encoding.Unicode.GetBytes("TESTJEU_CHERCHER_ADVERSAIRE:" + tAdversaire + ":"
+ tAdversaire + "=adversaire_RECHERCHE;joueur_RECHERCHANT");

                       MonClient.Send(MonBuffer, 0, MonBuffer.Length, SocketFlags.None);
                }
                #endregion

                //next msg
                InitializeReceive(tmp);
            }
        }
        #endregion
    }
}

Client code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.IO; 
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Threading;
//
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;

namespace BugClientSocket1
{
    public partial class Form1 : Form
    {
        private Socket MonClient;

        //buffer
        int bufferSize = 256;
        private byte[] MonBuffer;

        int portServeur = 8000;

        string pseudoID = "1:";
        string tAdversaire = "1";

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            tPseudo.Text = pseudoID;

            MonBuffer = new byte[bufferSize];
        }

        private IPAddress CorrectAddress(string nPC)
        {
            IPAddress ipReponse = null;

            try
            {
                if (nPC.Length > 0)
                {
                    IPAddress[] IPserveur = Dns.GetHostEntry(nPC).AddressList; 
                    for (int i = 0; i < IPserveur.Length; i++)
                    {
                        Ping pingServeur = new Ping();
                        PingReply pingResponse = pingServeur.Send(IPserveur[i]);

                        if (pingResponse.Status == IPStatus.Success)
                            if (IPserveur[i].AddressFamily == AddressFamily.InterNetwork)
                            {
                                ipReponse = IPserveur[i];
                                break;
                            }
                    }//for
                }

                return ipReponse;
            }
            catch (SocketException e)
            {
                return null;
            }
        }

        private void BConnecter_Click(object sender, EventArgs e)
        {
            MonClient = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

            MonClient.Blocking = false;


            //tserveur.txt -> replace with your PC name
            IPAddress IPserveur = CorrectAddress(tServeur.Text);

            if (IPserveur != null)
                MonClient.BeginConnect(new IPEndPoint(IPserveur, portServeur), new AsyncCallback(OnConnection), MonClient);

        }
        #region Se connecter au serveur
        public void OnConnection(IAsyncResult iar)
        {
            Socket tmp = (Socket)iar.AsyncState;

            if (tmp.Connected)
            {
                byte[] msg = Encoding.Unicode.GetBytes("PSEUDO:" + tPseudo.Text);

                //send pseudo
                MonClient.Send(msg, 0, msg.Length, SocketFlags.None);

                //next msg
                InitializeReceive(tmp);

            }
            else
            {
                MessageBox.Show("Serveur inaccessible");
            }

        }
        #endregion
        #region Debut reception msg + suite
        //cfr beginaccept-> beginreceive ici
        public void InitializeReceive(Socket soc)
        {
            MonBuffer = new byte[bufferSize];

            soc.BeginReceive(MonBuffer, 0, MonBuffer.Length,
                SocketFlags.None, new AsyncCallback(ReceiveMsg), soc);

        }
        public void ReceiveMsg(IAsyncResult iar)
        {
            Socket tmp = (Socket)iar.AsyncState;

            int received = tmp.EndReceive(iar);

            if (received > 0)
            {
                string msg = Encoding.Unicode.GetString(MonBuffer, 0, received);

                //next msg
                InitializeReceive(tmp);
            }

        }
        #endregion

        private void BChercher_Click(object sender, EventArgs e)
        {
            if (MonClient != null && tPseudoAdversaire.Text != "")
            {
                //MonBuffer = new byte[bufferSize];
                MonBuffer = Encoding.Unicode.GetBytes("JEU_CHERCHER_ADVERSAIRE:" + tAdversaire + ":"
                    + pseudoID + "=adversaire_RECHERCHE;joueur_RECHERCHANT");

                //send pseudo
                MonClient.Send(MonBuffer, 0, MonBuffer.Length, SocketFlags.None);

            }

        }
    }
}

I did let text on purpose (such as JEU_CHERCHER...), to get the exact same problem.

My application is a winform in visual studio 2019

To reproduce the bug, here the steps:

1. Put breakpoints in method "beginreceive" and "end receive", and variable "received"; to see the number of byte and string. Also check size of MonBuffer when arriving to these break points
2. clic on "connexion" button to connect to server
3. Server receive "pseudo" and accept client
4. Server send message saying "ok you are connected"
5. Clic on "chercher" (search) to send a new message to server
6. Server got message, proceed it, then answer it with a new message
7. Client receive message, and here the bug : buffer size is smaller than number of received byte
8. Exception occurs

If you don't understand something (because i didn't translate all in english), please tell me. Thank you

Answer 1

To sum it up, the solution is:

• Avoid global variable as buffer when possible. But here, I need global variable as buffer for receiving data
• Never use the same buffer for receive and send message (bytes)
• Use local variable to send at least, and here, I use the global variable buffer to receive