I created a biometric authentication service that starts an activity and registers a callback to a static EventHandler for the result.
The handler:
public class BiometricHandler : IBiometricHandler
{
private TaskCompletionSource<byte[]> taskCompletionSource;
public Task<byte[]> StartBiometricAuth()
{
Intent intent = new Intent(Android.App.Application.Context, typeof(BiometricActivity));
intent.AddFlags(ActivityFlags.NewTask);
Android.App.Application.Context.StartActivity(intent);
taskCompletionSource = new TaskCompletionSource<byte[]>();
BiometricActivity.BiometricEventHandler += BiometricCompleted;
return taskCompletionSource.Task;
}
private void BiometricCompleted(object sender, BiometricEventArgs e)
{
taskCompletionSource.SetResult(e.Success ? e.Payload : new byte[] { });
BiometricActivity.BiometricEventHandler -= BiometricCompleted;
}
}
And the activity (not the actual code obviously):
public class BiometricActivity : Activity
{
public static event EventHandler<BiometricEventArgs> BiometricEventHandler;
private readonly int BIOMETRIC_REQUEST = 1;
protected override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
Intent intent = new Intent(Application.Context, typeof(BiometricAuth));
StartActivityForResult(intent, BIOMETRIC_REQUEST);
}
protected override void OnActivityResult(int requestCode, [GeneratedEnum] Result resultCode, Intent data)
{
BiometricEventHandler?.Invoke(
this,
new BiometricEventArgs(
true, new byte[] {1, 2, 3}
);
Finish();
}
}
The above code throws a NullReferenceException:
0xFFFFFFFFFFFFFFFF in System.Diagnostics.Debugger.Mono_UnhandledException_internal
0x1 in System.Diagnostics.Debugger.Mono_UnhandledException at /Users/builder/jenkins/workspace/archive-mono/2020-02/android/release/mcs/class/corlib/System.Diagnostics/Debugger.cs:125,4
0x20 in Android.Runtime.DynamicMethodNameCounter.57
0x6 in Xamarin.Forms.Platform.Android.PlatformConfigurationExtensions.OnThisPlatform<Xamarin.Forms.Application> at D:\a\_work\1\s\Xamarin.Forms.Platform.Android\PlatformConfigurationExtensions.cs:8,4
0xC in Xamarin.Forms.Platform.Android.AppCompat.FragmentContainer.OnResume at D:\a\_work\1\s\Xamarin.Forms.Platform.Android\AppCompat\FragmentContainer.cs:126,4
0x8 in AndroidX.Fragment.App.Fragment.n_OnResume at C:\a\_work\1\s\generated\androidx.fragment.fragment\obj\Release\monoandroid12.0\generated\src\AndroidX.Fragment.App.Fragment.cs:2570,4
0x11 in Android.Runtime.DynamicMethodNameCounter.57
And the console:
**System.NullReferenceException:** 'Object reference not set to an instance of an object.'
I narrowed it down to the Finish() method.
There is no exception when it is not called.
I tried calling Finish() from the handler using BroadcastReceiver, same result.
Why does Finish() throw this exception?
