First off, don't use Parse on user input: use TryParse instead:
double d1, d2;
if (!double.TryParse(textBox2.Text, out d1))
{
MessageBox.Show(string.Format("{0} is not a number", textBox2.Text));
return;
}
if (!double.TryParse(textBox3.Text, out d2))
{
MessageBox.Show(string.Format("{0} is not a number", textBox3.Text));
return;
}
textBox1.Text = (d1 * d2 / 100).ToString("#.00");
That way, your app won't crash if your user misskeys...