问为什么NotificationManager notificationManager = (NotificationManager)GetSystemService(LocalNotificationService)；显示错误？

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NotificationManager notificationManager =(NotificationManager)GetSystemService(LocalNotificationService);

public class LocalNotificationService : ILocalNotificationService       
{       
    int _notificationIconId { get; set; }       
    readonly DateTime _jan1st1970 = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);       
    internal string _randomNumber;
    public const string URGENT_CHANNEL = "main";
    public void LocalNotification(string title, string body, int id, DateTime notifyTime){       

        //long repeateDay = 1000 * 60 * 60 * 24;       
        long repeateForMinute = 86400000; // In milliseconds      
        long totalMilliSeconds = (long)(notifyTime.ToUniversalTime() - _jan1st1970).TotalMilliseconds;       
        if (totalMilliSeconds < JavaSystem.CurrentTimeMillis()){       
            totalMilliSeconds = totalMilliSeconds + repeateForMinute;       
        }



        string chanName = "mainChanel";
        var importance = NotificationImportance.High;
        NotificationChannel chan = new NotificationChannel(URGENT_CHANNEL, chanName, importance);
        chan.EnableVibration(true);
        chan.LockscreenVisibility = NotificationVisibility.Public;
        NotificationManager notificationManager = (NotificationManager)GetSystemService(LocalNotificationService); // shows error: GetSystemService is not allowed at this context
        notificationManager.CreateNotificationChannel(chan);
        var intent = CreateIntent(id);       
        var localNotification = new LocalNotification();       
        localNotification.Title = title;       
        localNotification.Body = body;       
        localNotification.Id = id;       
        localNotification.NotifyTime = notifyTime;       

        if (_notificationIconId != 0){       
            localNotification.IconId = _notificationIconId;       
        }       
        else{       
            localNotification.IconId = Resource.Drawable.ic_mr_button_connecting_30_light;       
        }       

        var serializedNotification = SerializeNotification(localNotification);       
        intent.PutExtra(ScheduledAlarmHandler.LocalNotificationKey, serializedNotification);       

        Random generator = new Random();         
        _randomNumber = generator.Next(100000, 999999).ToString("D6");        

        var pendingIntent = PendingIntent.GetBroadcast(Application.Context, Convert.ToInt32(_randomNumber), intent, PendingIntentFlags.Immutable);       
        var alarmManager = GetAlarmManager();       
        alarmManager.SetRepeating(AlarmType.RtcWakeup, totalMilliSeconds, repeateForMinute, pendingIntent);       
    }










    public void Cancel(int id){       

        var intent = CreateIntent(id);       
        var pendingIntent = PendingIntent.GetBroadcast(Application.Context, Convert.ToInt32(_randomNumber), intent, PendingIntentFlags.Immutable);       
        var alarmManager = GetAlarmManager();       
        alarmManager.Cancel(pendingIntent);       
        var notificationManager = NotificationManagerCompat.From(Application.Context);       
        notificationManager.CancelAll();       
        notificationManager.Cancel(id);       
    }       

    public static Intent GetLauncherActivity(){       

        var packageName = Application.Context.PackageName;       
        return Application.Context.PackageManager.GetLaunchIntentForPackage(packageName);       
    }       


    private Intent CreateIntent(int id){       

        return new Intent(Application.Context, typeof(ScheduledAlarmHandler))       
            .SetAction("LocalNotifierIntent" + id);       
    }       

    private AlarmManager GetAlarmManager(){       

        var alarmManager = Application.Context.GetSystemService(Context.AlarmService) as AlarmManager;       
        return alarmManager;       
    }       

    private string SerializeNotification(LocalNotification notification){       

        var xmlSerializer = new XmlSerializer(notification.GetType());       

        using (var stringWriter = new StringWriter()){       
            xmlSerializer.Serialize(stringWriter, notification);       
            return stringWriter.ToString();       
        }       
    }       
}       

[BroadcastReceiver(Enabled = true, Label = "Local Notifications Broadcast Receiver")]       
public class ScheduledAlarmHandler : BroadcastReceiver{       

    public const string LocalNotificationKey = "LocalNotification";       

    public override void OnReceive(Context context, Intent intent){       
        var extra = intent.GetStringExtra(LocalNotificationKey);       
        var notification = DeserializeNotification(extra);       
        //Generating notification       
        var builder = new NotificationCompat.Builder(Application.Context)       
            .SetContentTitle(notification.Title) 
            .SetChannelId("main")
            .SetContentText(notification.Body)       
            .SetSmallIcon(notification.IconId)       
            .SetSound(RingtoneManager.GetDefaultUri(RingtoneType.Alarm))       
            .SetAutoCancel(true);       

        var resultIntent = LocalNotificationService.GetLauncherActivity();       
        resultIntent.SetFlags(ActivityFlags.NewTask | ActivityFlags.ClearTask);       
        var stackBuilder = Android.Support.V4.App.TaskStackBuilder.Create(Application.Context);       
        stackBuilder.AddNextIntent(resultIntent);       

        Random random = new Random();       
        int randomNumber = random.Next(9999 - 1000) + 1000;        

        var resultPendingIntent =       
            stackBuilder.GetPendingIntent(randomNumber, (int)PendingIntentFlags.Immutable);       
        builder.SetContentIntent(resultPendingIntent);       
        // Sending notification       
        var notificationManager = NotificationManagerCompat.From(Application.Context);       
        notificationManager.Notify(randomNumber, builder.Build());       
    }       

    private LocalNotification DeserializeNotification(string notificationString){       

        var xmlSerializer = new XmlSerializer(typeof(LocalNotification));       
        using (var stringReader = new StringReader(notificationString))       
        {       
            var notification = (LocalNotification)xmlSerializer.Deserialize(stringReader);       
            return notification;       
        }       
    }



  /*  private void createNotificationChannel()
    {
        var channelName = "main";
        var channelDescription = "The main channel";

        // set the vibration patterm for the channel
        long[] vibrationPattern = { 100, 200, 300, 400, 500, 400, 300, 200, 400 };

        // Creating an Audio Attribute
        var alarmAttributes = new AudioAttributes.Builder()
                               .SetContentType(AudioContentType.Sonification)
                               .SetUsage(AudioUsageKind.Alarm)
                               .Build();

        // Create the uri for the alarm file
        // Note: Make sure you add the mp3 file in your Android project under Resources/raw/soundFile.mp3 and set the build as Android Resource
        var alarmUri = Android.Net.Uri.Parse("MyApp.Android/Resources/raw/alarm.mp3");


        // create chan1  which is the urgent notifications channel
        var chan1 = new NotificationChannel(channelName, channelName, NotificationImportance.High)
        {
            Description = channelDescription
        };


        // set the channel properties
        chan1.EnableLights(true);
        chan1.LightColor = Color.Red;
        chan1.EnableVibration(true);
        chan1.SetVibrationPattern(vibrationPattern);
        chan1.SetSound(alarmUri, alarmAttributes);          // this is where the sound is set for the channel
        chan1.SetBypassDnd(true);
        chan1.LockscreenVisibility = NotificationVisibility.Public;

        // finally create the channel
        var manager = (NotificationManager)GetSystemService(NotificationService);

    }
    */
}