How do I unsubscribe an anonymous delegate from an event?

I am unit testing (in NUnit) a method that fires an event, and I am using an
anonymous delegate to subscribe the test method to the event:

           // Create order
           Order testOrder = new Order();

           // Create anonymous delegate to subscribe to DeliveryNeeded
event
           testOrder.DeliveryNeeded +=
               delegate(object sender, DeliveryNeededEventArgs e)
               {
                   Assert.AreEqual(2, e.OrderItems.Count);
               };

The anonymous delegate works fine. At the end of the test method, I want to
unsubscribe from the event. How do I do that with an anonymous delegate?

The obvious possibility:

           // Unsubscribe from the DeliveryNeeded event
           testOrder.DeliveryNeeded -= delegate(object sender,
DeliveryNeededEventArgs e) { };

But that doesn't appear to work--the event still has a subscriber after that
code executes.

Thanks in advance!

David Veeneman
Foresight Systems