Psychic Debugging, Part Two

Eric Lippert — Thu, 06 Sep 2007 18:31:00 GMT

A number of readers have the mysterious fifth sense which gives them the ability to deduce that the GetBars method from yesterday's post contains a yield return and is therefore an iterator. Remember, as the standard states (in section 10.14.4):

[...] execution of the code in the iterator block occurs when the enumerator object's MoveNext method is invoked.

Since the test program does not invoke MoveNext, the check for null is never executed, and therefore the exception is never thrown.

Since most of the interesting new sequence operators available in C# 3.0 are implemented with iterators it probably will be increasingly important for developers to understand a bit more about how iterators work behind the scenes. I may do some blog posts on that over the next little while.

Psychic Debugging, Part One

Eric Lippert — Wed, 05 Sep 2007 20:30:00 GMT

Here is a compiler bug report I got the other day. The user is trying to write a unit test for a method which takes a Foo and returns a collection of Bars. The test is supposed to confirm that GetBars throws an exception if the argument is null. The test was failing with “got no exception”. The user was wondering if somehow the compiler had optimized away the call.

    static void Test() {
        bool gotException = false;
        try {
            IEnumerable<Bar> bars = Foo.GetBars(null);
        }
        catch (ArgumentNullException ex) {
            Console.WriteLine("SUCCESS: Got expected exception");
            gotException = true;
        }
        catch (Exception ex) {
            Console.WriteLine("FAILURE: Got unexpected exception");
            gotException = true;
        }
        finally {
            if (!gotException)
                Console.WriteLine("FAILURE: Got no exception");
        }
    }

Several people (including myself and Cyrus) psychically debugged this one independently. The user did not include the source code of GetBars, which is what necessitated the use of our psychic powers. Because I am a nice guy, I will give you the beginning:

    static public IEnumerable<Bar> GetBars(Foo foo) {
        if (foo == null)
            throw new ArgumentNullException("foo");

Our psychic powers correctly told us that the behaviour of the test program is correct and expected. What do your psychic powers tell you about the rest of the implementation of GetBars? What is going on here?

Fabulous Adventures In Coding : Psychic Debugging

Psychic Debugging, Part Two

Psychic Debugging, Part One