private async void btn_add_Click(object sender, RoutedEventArgs e)
 {
     ContentDialog dialog = new AddEditContact(this, -1);
     await dialog.ShowAsync();            
 }
        private async void btn_edit_Click(object sender, RoutedEventArgs e)
        {
            if (allowAction)
            {
                int count = listview_contacts.Items.Count;
                int editId = -1;
                for (int i = 0; i < count; i++)
                {
                    Object item = listview_contacts.Items[i];
                    if (item is ListViewContactItem)
                    {
                        if (((ListViewContactItem)item).IsCheck)
                        {
                            editId = ((ListViewContactItem)item).Id;
                            break;
                        }

                    }
                }

                if (editId > 0)
                {
                    AddEditContact dialog = new AddEditContact(this, editId);
                    await dialog.ShowAsync();
                }
            }
        }