result - so if the validation is deferred, the test will fail.</p>  <p>The final interesting test for the moment is also around deferred execution, using a helper class called ThrowingEnumerable. This is a sequence which blows up with an InvalidOperationException if you ever try to iterate over it. Essentially, we want to check two things:</p>  <ul>   <li>Just calling Where doesn&#39;t start iterating over the source sequence </li>    <li>When we call GetEnumerator() to get an iterator and then MoveNext() o