DisplayInformation.InitializeOrientation.cs

private void InitializeOrientation()
{
  _didChangeStatusBarOrientationObserver = NSNotificationCenter
    .DefaultCenter
    .AddObserver(
      UIApplication.DidChangeStatusBarOrientationNotification, 
      n => {
        UpdateCurrentOrientation();
        OrientationChanged?.Invoke(this, CurrentOrientation);
      }
    );
  UpdateCurrentOrientation();
}

private void UpdateCurrentOrientation()
{
  var currentOrientationMask = UIApplication.SharedApplication
     .StatusBarOrientation;
  switch (currentOrientationMask)
  {
    case UIInterfaceOrientation.LandscapeLeft:
      CurrentOrientation = DisplayOrientations.LandscapeFlipped;
      break;
    case UIInterfaceOrientation.LandscapeRight:
      CurrentOrientation = DisplayOrientations.Landscape;
      break;
    case UIInterfaceOrientation.Portrait:
      CurrentOrientation = DisplayOrientations.Portrait;
      break;
    case UIInterfaceOrientation.PortraitUpsideDown:
      CurrentOrientation = DisplayOrientations.PortraitFlipped;
      break;
  }
  NativeOrientation = CurrentOrientation;
}