how do i get TcpListener to accept multiple connections and work with each one individually?

Question

I have an SMTP listener that works well but is only able to receive one connection. My C# code is below and I am running it as a service. My goal is to have it runnign on a server and parsing multiple smtp messages sent to it.

currently it parses the first message and stops working. how can I get it to accept the 2nd, 3rd, 4th... SMTP message and process it like it does the first?

here is my code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Net;
using System.IO;  

namespace SMTP_Listener
{
    class Program
    {
        static void Main(string[] args)
        {


            TcpListener listener = new TcpListener(IPAddress.Any , 8000);
            TcpClient client;
            NetworkStream ns;

            listener.Start();

            Console.WriteLine("Awaiting connection...");
            client = listener.AcceptTcpClient();
            Console.WriteLine("Connection accepted!");

            ns = client.GetStream();

            using (StreamWriter writer = new StreamWriter(ns))
            {
                writer.WriteLine("220 localhost SMTP server ready.");
                writer.Flush();

                using (StreamReader reader = new StreamReader(ns))
                {
                    string response = reader.ReadLine();

                    if (!response.StartsWith("HELO") && !response.StartsWith("EHLO"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    string remote = response.Replace("HELO", string.Empty).Replace("EHLO", string.Empty).Trim();

                    writer.WriteLine("250 localhost Hello " + remote);
                    writer.Flush();

                    response = reader.ReadLine();

                    if (!response.StartsWith("MAIL FROM:"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    remote = response.Replace("RCPT TO:", string.Empty).Trim();
                    writer.WriteLine("250 " + remote + " I like that guy too!");
                    writer.Flush();

                    response = reader.ReadLine();

                    if (!response.StartsWith("RCPT TO:"))
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    remote = response.Replace("MAIL FROM:", string.Empty).Trim();
                    writer.WriteLine("250 " + remote + " I like that guy!");
                    writer.Flush();

                    response = reader.ReadLine();

                    if (response.Trim() != "DATA")
                    {
                        writer.WriteLine("500 UNKNOWN COMMAND");
                        writer.Flush();
                        ns.Close();
                        return;
                    }

                    writer.WriteLine("354 Enter message. When finished, enter \".\" on a line by itself");
                    writer.Flush();

                    int counter = 0;
                    StringBuilder message = new StringBuilder();

                    while ((response = reader.ReadLine().Trim()) != ".")
                    {
                        message.AppendLine(response);
                        counter++;

                        if (counter == 1000000)
                        {
                            ns.Close();
                            return;  // Seriously? 1 million lines in a message?
                        }
                    }

                    writer.WriteLine("250 OK");
                    writer.Flush();
                    ns.Close();
                    // Insert "message" into DB
                    Console.WriteLine("Received message:");
                    Console.WriteLine(message.ToString());
                }
            }

            Console.ReadKey();
        }
    }
}

Answer 1

Late answer because this question did not had the answer that I was looking for. Here is what I was looking for:

using System.Net.Sockets;

class Program
{
    static void Main()
    {
        var server = new TcpListener(System.Net.IPAddress.Any, 80);
        server.Start();

        // Wait for connection...
        server.BeginAcceptTcpClient(OnClientConnecting, server);

        Console.ReadLine();        
    }

    static void OnClientConnecting(IAsyncResult ar)
    {
        try
        {
            Console.WriteLine("Client connecting...");

            if (ar.AsyncState is null)
                throw new Exception("AsyncState is null. Pass it as an argument to BeginAcceptSocket method");

            // Get the server. This was passed as an argument to BeginAcceptSocket method
            TcpListener s = (TcpListener)ar.AsyncState;

            // listen for more clients. Note its callback is this same method (recusive call)
            s.BeginAcceptTcpClient(OnClientConnecting, s);

            // Get the client that is connecting to this server
            using TcpClient client = s.EndAcceptTcpClient(ar);

            Console.WriteLine("Client connected succesfully");

            // read data sent to this server by client that just connected
            byte[] buffer = new byte[1024];
            var i = client.Client.Receive(buffer);
            Console.WriteLine($"Received {i} bytes from client");

            // reply back the same data that was received to the client
            var k = client.Client.Send(buffer, 0, i, SocketFlags.None);
            Console.WriteLine($"Sent {k} bytes to slient as reply");

            // close the tcp connection
            client.Close();
        }
        catch (Exception exception)
        {
            Console.WriteLine(exception);
        }
    }
}

Then when I make a request to http://localhost/foo on my browser I get this:

GET /test HTTP/1.1
Host: localhost
Connection: keep-alive
Cache-Control: max-age=0
sec-ch-ua: "Chromium";v="106", "Google Chrome";v="106", "Not;A=Brand";v="99"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
Upgrade-Insecure-Requests: 1
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/106.0.0.0 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Sec-Fetch-Site: none
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,es;q=0.8

Answer 2

As per your code, you are starting one listener and receiving and processing message and closing program.

You need to maintain a listener and TcpClient object can be passed to another function to process the message received. listener.Start();

        Console.WriteLine("Awaiting connection...");
        client = listener.AcceptTcpClient();
        Console.WriteLine("Connection accepted!");

Answer 3

I know this is old question but I am sure many will like this answer.

// 1
while (listening)
{
    TcpClient client = listener.AcceptTcpClient();
    // Start a thread to handle this client...
    new Thread(() => HandleClient(client)).Start();
}

// 2
while (listening)
{
    TcpClient client = listener.AcceptTcpClient();
    // Start a task to handle this client...
    Task.Run(() => HandleClient(client));
}

// 3
public async void StartListener() //non blocking listener
{
    listener = new TcpListener(ipAddress, port);
    listener.Start();
    while (listening)
    {
        TcpClient client = await listener.AcceptTcpClientAsync().ConfigureAwait(false);//non blocking waiting                    
        // We are already in the new task to handle this client...   
        HandleClient(client);
    }
}
//... in your code
StartListener();
//...
//use Thread.CurrentThread.ManagedThreadId to check task/thread id to make yourself sure

Answer 4

You can factor out most of your code into a separate thread:

static void Main(string[] args)
{
    TcpListener listener = new TcpListener(IPAddress.Any , 8000);
    TcpClient client;
    listener.Start();

    while (true) // Add your exit flag here
    {
        client = listener.AcceptTcpClient();
        ThreadPool.QueueUserWorkItem(ThreadProc, client);
    }
}
private static void ThreadProc(object obj)
{
    var client = (TcpClient)obj;
    // Do your work here
}

Answer 5

You almost certainly want to spin each connection into another thread. So you have the "accept" call in a loop:

while (listening)
{
    TcpClient client = listener.AcceptTcpClient();
    // Start a thread to handle this client...
    new Thread(() => HandleClient(client)).Start();
}

Obviously you'll want to adjust how you spawn threads (maybe use the thread pool, maybe TPL etc) and how you stop the listener gracefully.