If you've ever wanted to restrict a texbox control to accept only numbers and not even allow the user to type characters this is how to do it. Note that this code also allows the user to type non alpha characters like backspace.
private void txtNumber_KeyDown(object sender, KeyEventArgs e)
{
if ((e.KeyValue > 0 && e.KeyValue < 31) || (e.KeyValue > 47 && e.KeyValue < 58))