public override void OnStop()
        {
            base.OnStop();

            FilePickerActivity filePicker = Activity as FilePickerActivity;

            if (filePicker != null)
            {
                filePicker.FilterEvent -= NewFileFilter;
            }
        }
        public override void OnStart()
        {
            base.OnStart();

            _fileNameFilter = null;
            FilePickerActivity filePicker = Activity as FilePickerActivity;

            if (filePicker != null)
            {
                filePicker.FilterEvent += NewFileFilter;
            }
        }