private void saleslogBttn_Click(object sender, EventArgs e)
 {
     if (saleslogform == null)
     {
         saleslogform             = new saleslogFrm();
         saleslogform.FormClosed += saleslogform_FormClosed;
     }
     saleslogform.Show(this);
     Hide();
 }
 void saleslogform_FormClosed(object sender, FormClosedEventArgs e)
 {
     saleslogform = null;
     Show();
 }