• May 18, 2011
• in geek
• 2 min read

When lazy evaluation attacks

I just had a lovely object lesson in lazy evaluation of Iterators. I wanted to have method that would return an enumerator over an encapsulated set after doing some sanity checking:

public IEnumerable<Subscription> Filter(Func<Subscription, bool> filter) {
    if(filter == null) {
        throw new ArgumentNullException("filter","cannot execute with a null filter");
    }
    foreach(var subInfo in _subscriptions.ToArray()) {
        Subscription sub;
        try {
            var subDoc = XDocFactory.LoadFrom(subInfo.Path, MimeType.TEXT_XML);
            sub = new Subscription(subDoc );
            if(filter(sub) {
              continue;
            }
        } catch(Exception e) {
            _log.Warn(string.Format("unable to retrieve subscription for path '{0}'", subInfo.Path), e);
            continue;
        }
        yield return sub;
    }
}

I was testing registering a subscription in the repository with this code:

IEnumerable<Subscription> query;
try {
  query = _repository.Filter(handler);
} catch(ArgumentException e) {
  return;
}
foreach(var sub in query) {
   ...
}

And the test would throw a ArgumentNullException because handler was null. What? But, but i clearly had a try/catch around it! Well, here's where clever bit me. By using yield, the method had turned into an enumerator instead of a method call that returned an enumerable. That means that the method body would get squirreled away into an enumerator closure that would not get executed until the first MoveNext(). And that in turn meant that my sanity check on handler didn't happen at Filter() but at the first iteration of the foreach.

Instead of doing "return an Iterator for subscriptions", I needed to do "check the arguments" and then "return an Iterators for subscriptions" as a separate action. This can be accomplished by factoring the yield into a method called by Filter() instead of being in Filter() itself:

public IEnumerable<Subscription> Filter(Func<Subscription, bool> filter) {
    if(filter == null) {
        throw new ArgumentException("cannot execute with a null filter");
    }
    return BuildSubscriptionEnumerator(Func<Subscription, bool> filter);
}

public IEnumerable<Subscription> BuildSubscriptionEnumerator(Func<Subscription, bool> filter) {
    foreach(var subInfo in _subscriptions.ToArray()) {
        Subscription sub;
        try {
            var subDoc = XDocFactory.LoadFrom(subInfo.Path, MimeType.TEXT_XML);
            sub = new Subscription(subDoc );
            if(filter(sub) {
              continue;
            }
        } catch(Exception e) {
            _log.Warn(string.Format("unable to retrieve subscription for path '{0}'", subInfo.Path), e);
            continue;
        }
        yield return sub;
    }
}

Now the sanity check happens at Filter() call time, while the enumeration of subscription still only occurs as its being iterated over, allowing for additional filtering and Skip/Take additions without having to traverse the entire possible set.