Is there a reason why it’s a delegate and not an event? With an event, multiple sources could subscribe to it, but as a delegate it can only be set once:
You can subscribe multiple time to a delegate as well.
However, in any case I really discourage this. It leads to very brittle code. The only reason that field is there is for backwards compatibility. Instead I recommend that you only use the path callback on the seeker.StartPath method.