Label valute sum?

Question

if (ukupnolabel.Text.Length > 0 && label3.Text.Length > 0)
{
  label4.Text = Convert.ToDouble(Convert.ToDouble(ukupnolabel.Text) * Convert.ToDouble(label3.Text) / 100).ToString("#,0.00");

}

 

Where to put this code, intialiye component not work, slection chaged datagrid dont work, form load dont work?

 

I miss somesthin??