问我按下按钮后，电话里没有显示通知

EN

namespace PresentationNoti
{
    [Activity(Label = "@string/app_name", Theme = "@style/AppTheme", MainLauncher = true)]
    public class MainActivity : AppCompatActivity
    {
        Button btn1, btn2;
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.activity_main);
            btn1 = (Button)FindViewById(Resource.Id.btn1);
            btn2 = (Button)FindViewById(Resource.Id.btn2);
            btn1.Click += Btn1_Click;
            btn2.Click += Btn2_Click;
        }

        private void Btn2_Click(object sender, System.EventArgs e)
        {
            NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this);
            mBuilder.SetSmallIcon(Resource.Drawable.birthday);
            mBuilder.SetContentTitle("It's your birthday");
            mBuilder.SetContentText("Happy B-day Birthday Boy/Girl");
            Intent resultIntent = new Intent(this, typeof(MainActivity));
            TaskStackBuilder stackBuilder = TaskStackBuilder.Create(this);
            stackBuilder.AddParentStack(Java.Lang.Class.FromType(typeof(MainActivity)));
            // Adds the Intent that starts the Activity to the top of the stack
            stackBuilder.AddNextIntent(resultIntent);
            PendingIntent resultPendingIntent = stackBuilder.GetPendingIntent(0, (int)PendingIntentFlags.UpdateCurrent);
            mBuilder.SetContentIntent(resultPendingIntent);
            NotificationManager mNotificationManager = (NotificationManager)GetSystemService(Context.NotificationService);
            mNotificationManager.Notify(0, mBuilder.Build());
        }

        private void Btn1_Click(object sender, System.EventArgs e)
        {
            NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this);
            mBuilder.SetSmallIcon(Resource.Drawable.birthday);
            mBuilder.SetContentTitle("It's your birthday");
            mBuilder.SetContentText("Happy B-day Birthday Boy/Girl");
            NotificationManager mNotificationManager = (NotificationManager)GetSystemService(Context.NotificationService);
            mNotificationManager.Notify(0, mBuilder.Build());
        }
    }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#90aacc">
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="basic notification"
        android:textColor="#000000"
        android:layout_marginTop="15dp"
        android:layout_gravity="center"
        android:textSize="25sp"
        android:id="@+id/btn1"
    />
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="advanced notification"
        android:textColor="#000000"
        android:layout_marginTop="15dp"
        android:layout_gravity="center"
        android:textSize="25sp"
        android:id="@+id/btn2"
    />
</LinearLayout>