Question

I have an interface method

  public void Execute(ICommand command); 

which needs to pass known subtypes of ICommand to an apropriate Handle(SpecificCommand command) method implementation and do some generic handling of unknown types. I am looking for a universal (i.e. not requiring a giant switch) method of doing so, something similar to

  Handle(command as command.GetType()); // this obviously does not compile 

I know I could register the handlers somehow, e.g. store them as delegates in a dictionary, but this still requires duplicating the handling logic (once in the specific Handle(...) method signature, once in the delegate reqistration). If I populate the dictionary by inspecting my class with reflection (looking for Handle(XXX command) methods), I’ll get a performance hit.

To summarize: how can I downcast an object (upcasted by the call to Execute(ICommand command)) to invoke a method requiring a concrete type without knowing which type it is at compile time.

Answer 1

The cast is emitted at compile-time, so you need to know the type at compile-time. The overloading is also determined at compile-time – so by the time you actually know the concrete type to use, it’s too late.

I don’t see that you’d actually be duplicating any logic by using delegates. Alternatively, if you do it with reflection, you can build delegates very easily using Delegate.CreateDelegate – you’ll only get a performance hit once, and after that it’ll be very fast. See my blog entry about Delegate.CreateDelegate for more information.

I think I’d decide to use a hand-built dictionary or one built with reflection based on how many methods I had and how often they change. You’ll probably find KeyedByTypeCollection useful for the dictionary.