Unable to open ESP32 socket from Xamarin.Forms app after upgrading to Android 9

Question

Opening the TCP socket which is listening on port 3000 on the ESP32 from the Xamarin.Forms app does not work anymore after updating the phone to Android 9. On Android 8 this worked but after updating the socket can not be opened.

The following exception is thrown by the app:

System.InvalidOperationException: The operation is not allowed on non-connected sockets.

I've already tried some methods for fixing this problem:

• Trying the app on an older phone (Android 6) which works
• Trying opening the socket on the ESP32 from a PC using netcat which works
• Trying opening a socket on a pc from the app which works
• Trying opening the socket on the ESP32 from a different TCP client app on the smartphone which works

All of the above work, but trying to open the TCP socket on the ESP32 from the xamarin forms app doesn't.

I have also tried

• enabeling cleartextTrafficPermitted which doesn't work
• adding the "INTERNET" permission to the android manifest which doesn't work

For the Xamarin.Forms app this code is used

var client = new TcpClient();
            var result = client.BeginConnect("192.168.1.1", 3000, null, null);

            var success = result.AsyncWaitHandle.WaitOne(TimeSpan.FromSeconds(1));

            try
            {
                NetworkStream nwStream = client.GetStream();
                byte[] bytesToSend = ASCIIEncoding.ASCII.GetBytes(message);

                nwStream.Write(bytesToSend, 0, bytesToSend.Length);
            }
            catch (Exception e)
            {
                Console.WriteLine(e);                
            }

            client.EndConnect(result);

On the listener side I use this for listening:

client_socket = accept(soc, (struct sockaddr *)&remote_addr, &socklen);

read(client_socket, recv_buf, sizeof(recv_buf));

It seems like Android can't open the socket on the ESP32, the question is why?

Answer 1

I had a socket connection issue too when upgrading to Android 9. I had a ionic-cordova and a Xamarin form app to try basic tcp socket connection on local wifi (hosted on a raspbery)

It was all working on previous android version, and from laptop but i was getting timeout in android pie.

Turns out, since my Raspberry had no internet connection, when trying to open a socket by IP, Android 9 was not using the wlan network but tried to used its 4G connection to Internet and failed. After I turned off 4G, it managed to connect to the Rasp.

my simple code sample for opening connection in Xamarin:

 private TcpClient m_client;

async public void connect(string address, int port)
        {
            if(m_client.Connected)
            {
                m_client.Close();
            }

            try
            {
                await m_client.ConnectAsync(address, port);
            }catch(Exception e)
            {
                Console.WriteLine($"error while connecting: {e.Message}");
            }
            Console.WriteLine($"is connected : {m_client.Connected}");

        }