public SomeButton_Click(object sender, RoutedEventArgs e)
 {
    var dialog = new DB_conn_win();
    if (dialog.ShowDialog() == true)
    {
      this.DB_query = dialog.Query;
    }
 }
    void Window_Loaded(object sender, RoutedEventArgs e)
    {
        DB_conn_win dialog = new DB_conn_win();

        dialog.Owner = this;
        dialog.ShowDialog();
        if (dialog.DialogResult != null && dialog.DialogResult.Value)
        {
            DB_Query = dialog.query;
        }
    }