Wow! solution 3 is greatly helped me. But there e is mistake in number_handler().
private void txtSerial_KeyPress(object sender, KeyPressEventArgs e)
{
number_handler(e);
}
private void txtID_KeyPress(object sender, KeyPressEventArgs e)
{
number_handler(e);
}
private void number_handler(KeyPressEventArgs e)
{
char var = e.KeyChar;
if (var == 8)
e.Handled = false;
else if (var < '0' || var > '9')
e.Handled = true;
}