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I'm trying to establish a bluetoothconnection to another phoner. Later on, it'll be a board with a HC5 module, but for debugging I'm just using a phone.

The problem is, that connect fails and throws an IO Exception: " read failed, socket might closed or timeout, read ret: -1"

A quick google-search shows a lot having this problem. The only way I can see it solved, is by using a method not public in the API 

Method m = mmDevice.getClass().getMethod("createRfcommSocket", new Class[] {int.class});
mmSocket = (BluetoothSocket) m.invoke(mmDevice, 1);

The problem just is, that createRfcommSocket has been removed, and the result from getMethod will be null.

My code is from the example: https://github.com/xamarin/monodroid-samples/tree/master/BluetoothChat which for connecting is:

        public ConnectThread(BluetoothDevice device, BluetoothChatService service)
        {
            UUID MY_UUID = UUID.FromString("00001101-0000-1000-8000-00805f9b34fb");
            mmDevice = device;
            _service = service;
            BluetoothSocket tmp = null;

            // Get a BluetoothSocket for a connection with the
            // given BluetoothDevice
            try
            {
                if ((int)Android.OS.Build.VERSION.SdkInt >= 10) // Gingerbread 2.3.3 2.3.4
                    tmp = device.CreateInsecureRfcommSocketToServiceRecord(MY_UUID);
                else
                    tmp = device.CreateRfcommSocketToServiceRecord(MY_UUID);


            }
            catch (Java.IO.IOException e)
            {
                Log.Error(TAG, "create() failed", e);
            }
            mmSocket = tmp;
        }

Since the 'original hack' doesn't work, and I haven't found any other solution, I'm hoping somebody here knows how to fix this.

Best regards!

Benjamin Larsen
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1 Answers1

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This is the issue I and others in my company have come across when working with Bluetooth devices from Android phones. It is well described on SO here: Service Discovery Failed Exception Using Bluetooth On Android

The method you describe as being removed is, in fact, still operational. And it is the one we have successfully used. We attempt connection using the 

tmp = device.CreateRfcommSocketToServiceRecord(MY_UUID);

method you show above. Use a try...catch block around tmp.Connect() (your code does not show the Connect call) In the catch block "re-create" the BluetoothSocket using the createRfcommSocket method. I used a small method to do this:

       private BluetoothSocket CreateRfcommSocket(BluetoothDevice bTdevice)
    { // This is an "undocumented" call that is needed to (mostly) avoid a Bluetooth Connection error
      // introduced in Android v4.2 and higher. It is used as a "fallback" connection.
      // Full paths version of code!
      //Java.Lang.Reflect.Method mi = device.Class.GetMethod("createRfcommSocket", new Java.Lang.Class[] { Java.Lang.Integer.Type });
      //_bluetoothSocket = (BluetoothSocket)mi.Invoke(device, 1);

        // Compact version of above
        var mi = bTdevice.Class.GetMethod("createRfcommSocket", Integer.Type);
        return (BluetoothSocket)mi.Invoke(bTdevice, 1);
    }

This is the method indicated here: Xamarin Forum Post

I have tested this on Android 4.4.2 and Android 8.0 and it works on these systems. Another person in the company has tested the Java equivalent code on Android: 4.2.2, 4.4.2, 7.0 and 8.0 and it works in every case.

JamesT
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