public void CancelEdit()

// Subscribe to the <see cref="ArcGIS.Desktop.Editing.Events.RowDeletedEvent"/> for the first feature layer's table in the active map.  The event handler can be used to intercept and handle row deletion events, such as canceling a delete operation if certain conditions are met.
// subscribe to the RowDeletedEvent for the appropriate table
// in preparation for the event: public static Guid _currentRowDeletedGuid = Guid.Empty;
Table cancelDeleteTable = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault().GetTable();
RowDeletedEvent.Subscribe(rowChangedEventArgs =>
  {
    // RowEvent callbacks are always called on the QueuedTask so there is no need 
    // to wrap your code within a QueuedTask.Run lambda.
    var row = rowChangedEventArgs.Row;
    // check for re-entry 
    if (_currentRowDeletedGuid == rowChangedEventArgs.Guid)
      return;
    // cancel the delete if the feature is in Police District 5
    var fldIdx = row.FindField("POLICE_DISTRICT");
    if (fldIdx != -1)
    {
      var value = row[fldIdx].ToString();
      if (value == "5")
      {
        //cancel with dialog
        // Note - feature edits on Hosted and Standard Feature Services cannot be cancelled.
        rowChangedEventArgs.CancelEdit("Delete Event\nAre you sure", true);
        // or cancel without a dialog
        // args.CancelEdit();
      }
    }
    _currentRowDeletedGuid = rowChangedEventArgs.Guid;
  }, cancelDeleteTable);