native-android-csharp/DemoAndroid/Activities/WakeOnLanActivity.cs

using System.Net;
using System.Net.Sockets;
using Android.Content;
using Android.Hardware.Biometrics;
using Android.OS;
using Java.Lang;

namespace DemoAndroid;

[Activity(Label = "@string/app_name", MainLauncher = true, Theme = "@style/Default")]
public class WakeOnLanActivity : Activity
{
    private const int BiometricStrong = 0x0000000f;
    private const int BiometricWeak = 0x000000ff;
    private const int DeviceCredential = 0x00008000;

    private bool _authenticated = false;
    
    protected override void OnCreate(Bundle? savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        // Create your application here
        SetContentView(Resource.Layout.activity_wakeonlan);

        var button = FindViewById<Button>(Resource.Id.btnWakeOnLan);
        button.Click += (sender, args) =>
        {
            OnlyAuthenticated(WakePc);
        };
    }

    private void OnlyAuthenticated(Action action)
    {
        if (!_authenticated)
        {
            var callback = new BiometricCallback(this, () =>
            {
                _authenticated = true;
                action();
            });

            var prompt = new BiometricPrompt.Builder(this)
                .SetNegativeButton("Annuleren", MainExecutor, callback)
                .SetTitle("Bewijs jezelf met je vingerafdruk")
                .SetSubtitle("We willen zeker weten dat je niet Colinde bent")
                .SetAllowedAuthenticators(BiometricStrong)
                .Build();

            var cancellation = new CancellationSignal();

            prompt.Authenticate(cancellation, MainExecutor, callback);
            return;
        }

        action();
    }

    private void WakePc()
    {
        byte[] macAddress = [0x2c, 0x4d, 0x54, 0x4d, 0x10, 0x0f];
        var magicPacket = new byte[102];

        // Fill first 6 bytes with 0xFF
        for (var i = 0; i < 6; i++)
        {
            magicPacket[i] = 0xFF;
        }

        // Repeat MAC address 16 times
        for (var i = 1; i <= 16; i++)
        {
            for (var j = 0; j < 6; j++)
            {
                magicPacket[i * 6 + j] = macAddress[j];
            }
        }

        using var client = new UdpClient();
        client.EnableBroadcast = true;
        var endpoint = new IPEndPoint(IPAddress.Broadcast, 9); // 255.255.255.255 port 9

        for (var i = 0; i < 5; i++)
        {
            // Try 5 times
            client.Send(magicPacket, magicPacket.Length, endpoint);
        }
        
        Toast.MakeText(this, "Computer started!", ToastLength.Long)!.Show();
    }
}

/// <summary>
/// Biometric authentication callbacks
/// https://developer.android.com/identity/sign-in/biometric-auth#java
/// </summary>
class BiometricCallback : BiometricPrompt.AuthenticationCallback, IDialogInterfaceOnClickListener
{
    private Context _context;
    private readonly Action _onSuccess;

    public BiometricCallback(Context context, Action onSuccess)
    {
        _context = context;
        _onSuccess = onSuccess;
    }
    
    public override void OnAuthenticationError(BiometricErrorCode errorCode, ICharSequence? errString)
    {
        Toast.MakeText(_context, $"Authenticatie mislukt: {errorCode}, {errString?.ToString()}", ToastLength.Long)!.Show();
    }

    public override void OnAuthenticationSucceeded(BiometricPrompt.AuthenticationResult? result)
    {
        Toast.MakeText(_context, "Authentication succeeded!", ToastLength.Short)!.Show();
        _onSuccess();
    }

    public override void OnAuthenticationFailed()
    {
        Toast.MakeText(_context, "Authenticatie mislukt!", ToastLength.Long)!.Show();
    }

    public void OnClick(IDialogInterface? dialog, int which)
    {
        Toast.MakeText(_context, "Geannuleerd!", ToastLength.Long)!.Show();
    }
}