Casper said:
I have a custom web form control, that can contain other custom
controls.
Is it possible to add designer support for this, so I can drag a
custom control onto another control?
Yes and no.
Your outer control could derive from Panel, for instance, in which case you
could drag other controls onto it in the same way as a Panel control works.
But it would be _exactly_ like a Panel control. If I remember correctly,
there is a trade-off between this "drag" behavior and the ability to have
your outer control do its own design-time rendering. I believe that the
designer will ignore your outer control designer's GetDesignTimeHtml method
and simply render the inner controls.
Alternatively, your outer control could support templates. You couldn't
directly drag controls onto it then, but you could enter template editing
mode and then drag controls into the template editing area.