A minimalist local echo server.

TcpEchoServer.cs

    class EchoTcpServer
    {
        readonly int _port;

        public EchoTcpServer( int port )
        {
            _port = port;
        }

        public async Task<Exception?> Run( CancellationToken cancel )
        {
            TcpListener listener = new TcpListener( IPAddress.Loopback, _port );
            listener.Start();
            try
            {
                try
                {
                    using( TcpClient client = await listener.AcceptTcpClientAsync() )
                    {
                        NetworkStream stream = client.GetStream();
                        // Yes... this works like a charm! :)
                        await stream.CopyToAsync( stream, cancel );
                    }
                }
                finally
                {
                    listener.Stop();
                }
            }
            catch( OperationCanceledException )
            {
            }
            catch( Exception ex )
            {
                return ex;
            }
            return null;
        }