Is the callBack method called before the assignment or after here?

Question

Hi,

I have the code below which is basically calling a Domain Service in a SilverLight Application.

LoadOperation<tCity> loadOperation = _dataContext.Load(query,callBack, true);

Can you tell me which operation is done first?

Is the callBack method called before loadOperation variable is assigned or after it is assigned?

Thanks

Answer 1

Assuming it's meant to be an asynchronous operation, it could happen either way, in theory. The asynchronous operation should occur in another thread, and if that finishes before Load returns, the callback could be called before the assignment completes.

In practice, I'd expect the async call to take much longer than whatever housekeeping Load does at the end of the method - but I also wouldn't put that assumption into the code. Unless there's explicit synchronization to ensure that the assignment occurs before the callback, I don't think it's a good idea to rely on it.

Even if at the moment the assignment always happens first, consider:

• What happens if there's no network connection at the moment? The async call could fail very quickly.
• What happens if some caching is added client-side? The call could succeed very quickly.
• I don't know what kind of testing you're able to do against the RIA services, but sometimes you may want to be able to mock asynchronous calls by making them execute the callback on the same thread - which means the callback could happen in tests before the assignment. You could avoid this by forcing a genuinely asynchronous mock call, but handling threading in tests can get hairy; sometimes it's easiest just to make everything synchronous.

EDIT: I've been thinking about this more, and trying to work out the reasons behind my gut feeling that you shouldn't make this assumption, even though it's almost always going to be fine in reality.

Relying on the order of operations is against the spirit of asynchronicity.

You should (IMO) be setting something off, and be ready for it to come back at any time. That's how you should be thinking about it. Once you start down the slippery slope of "I'm sure I'll be able to just do a little bit of work before the response is returned" you end up in a world of uncertainty.

Answer 2

First, I would say write your callback without any assumptions. But aside from that I don't see how the callback could possibly occur before the assignment. The load operation would have to return immediately after the thread is spun.

Answer 3

There are 3 possible answers to this very specific RIA Services question:

1. It returns the assignment before the callback.
2. It may be possible for the callback to occur before the assignment.
3. You do not care.

Case 1: Based on a .Net Reflector investigation of the actual load method in question, it appears impossible for it to call the callback before the return occurs. (If anyone wants to argue that they are welcome to explain the intricacies of spinning up background threads).

Case 2: Proof that "the sky is falling" is possible would have to be shown in the reflected code. (If anyone wants to support this they are also welcome to explain the intricacies of spinning up background threads).

Case 3: In reality, the return value of a RIA Services load method is normally used to assign a lazy loading data source. It is not used by the callback. The callback is passed its own context, of the loaded data, as a parameter.

StackOverflow is all about practical code answers, so the only practical answer is option 3:

You do not care (as you do/should not use the assignment value from the callback).