Server.cs

using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Threading.Tasks;

namespace PlayGround
{
    class Program
    {
        private static async Task ClientAsync(Socket socket)
        {
            using (socket)
            {
                await using var ns = new NetworkStream(socket);
                await using var sw = new StreamWriter(ns);
                using var sr = new StreamReader(ns);

                while (true)
                {
                    var line = await sr.ReadLineAsync();
                    if (!string.IsNullOrEmpty(line)) continue;
                    await sw.WriteLineAsync("HTTP/1.1 200 OK\r\n\r\nHello world!");
                    await sw.FlushAsync();
                    break;
                }
            }
        }

        private static async Task ServerAsync()
        {
            var listener = new TcpListener(IPAddress.Any, 3000);
            listener.AllowNatTraversal(true);
            listener.Start();

            while (true)
            {
                var client = await listener.AcceptSocketAsync();
                var concurrentTask = ClientAsync(client);
            }
        }

        public static void Main()
        {
            ServerAsync().Wait();
        }
    }
}