Говорилка

using System;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;

namespace jopa
{
    public static class Program
    {
        public static void Main(string[] args)
        {
            CheckUsage(args);
            StartClient(args[1]);
            StartServer(args[0]);
        }

        private static void CheckUsage(string[] args)
        {
            if (args.Length < 2 || !args.All(a => int.TryParse(a, out _)))
            {
                Console.WriteLine($"Usage: {Assembly.GetEntryAssembly().GetName().Name} <your port> <his port>");
                Environment.Exit(-1);
            }
        }

        private static void StartClient(string port)
        {
            new Thread(async () =>
            {
                while (true)
                {
                    using var client = new HttpClient();
                    var res = Console.ReadLine();
                    try
                    {
                        await client.PostAsync($"http://localhost:{port}", new StringContent(res))
                            .ConfigureAwait(false);
                    }
                    catch
                    {
                        Console.WriteLine($"Nobody listening on port {port}");
                    }
                }
            }).Start();
        }

        private static void StartServer(string port)
        {
            static async Task<string> ReadAll(Stream st)
            {
                using var reader = new StreamReader(st);
                return await reader.ReadToEndAsync();
            }
            new WebHostBuilder()
                .UseSetting(WebHostDefaults.SuppressStatusMessagesKey, "True")
                .UseUrls($"http://0.0.0.0:{port}")
                .UseKestrel()
                .Configure(ab =>
                    ab.Run(async context => Console.WriteLine(await ReadAll(context.Request.Body))))
                .Build()
                .Run();
        }
    }
}