C#, NUnit: How to deal with testing of exceptions and deferred execution

Question

Say we have a method that looks like this:

public IEnumerable<Dog> GrowAll(this IEnumerable<Puppy> puppies)
{
    if(subjects == null)
        throw new ArgumentNullException("subjects");

    foreach(var puppy in puppies)
        yield return puppy.Grow();
}

If I test that by doing this:

Puppy[] puppies = null;
Assert.Throws<ArgumentNullException>(() => puppies.GrowAll());

The test will fail saying that it

Expected: <System.ArgumentNullException>
But was: null

I can fix that by changing the test to

Puppy[] puppies = null;
Assert.Throws<ArgumentNullException>(() => puppies.GrowAll().ToArray());

Is this just how you would usually do it? Or is there a better way to write the test? Or maybe a better way to write the method itself?

---

Tried to do the same with the built-in Select method, and it failed even without a ToArray or anything like that, so apparently there is something you can do about it... I just don't know what :p

Answer 1

The test is fine - your code isn't. You should make the code throw the exception as soon as it's called, by splitting the method in half:

public IEnumerable<Dog> GrowAll(this IEnumerable<Puppy> puppies)
{
    if(subjects == null)
        throw new ArgumentNullException("subjects");

    return GrowAllImpl(puppies);
}

private IEnumerable<Dog> GrowAllImpl(this IEnumerable<Puppy> puppies)
{
    foreach(var puppy in puppies)
        yield return puppy.Grow();
}