The form seen in the designer is not actually an instance of the Form1 class
you're designing but a simple form with the relavent controls added to it.
Custom controls however are instances of the real thing so you could create
a custom control that covered the form entirely and do design-time detection
on that. You may also be able to do design time detection on a derived form
that uses visual inheritance. In any case, the DesignMode property is never
valid in the constructor.