AVAudioSessionRouteChangeReasonKey Where is this hidden in Xamarin?

I cannot find the wrapper for "AVAudioSessionRouteChangeReasonKey" when trying to add an observer for this code: http://stackoverflow.com/a/21437382/548975

NSDictionary *interuptionDict = notification.userInfo;
NSInteger routeChangeReason = [[interuptionDict valueForKey:AVAudioSessionRouteChangeReasonKey] integerValue];

Thank you