Mathias Brandewinder on .NET, F#, VSTO and Excel development, and quantitative analysis / machine learning.
by Mathias 15. October 2009 18:03

I recently finished reading the Art of Unit Testing, by Roy Osherove (Manning); here are a few thoughts on the book.

Who to give it too

This is an excellent book for the C# developer with solid foundations in object-oriented design, who has already some exposure to writing unit tests. If you have worked on a decent-scale project, and found yourself thinking “hmmm, I am sure there is a smarter way to write these tests”, you should definitely get that book. Note that while it will be extremely useful to the test-driven development practitioner, this is NOT a book on TDD.More...

by Mathias 6. October 2009 06:09

Silicon Valley Code Camp version 4.0 took place this week-end, and was a big success, judging by the numbers and the happy faces. Congratulations to Peter Kellner and the team for a tremendous organization!

Personally, I wanted to give a big thank-you to the people who attended my session on Test-Driven Development – and for bearing with my voice, which was pretty shaky. I got sick this week and wasn’t sure until Saturday evening if I could do it, because on Thursday my voice was totally gone. I think I had more herbal tea with honey this week than in my entire life, but you guys made it all worth it: I had a great time giving my presentation, and you guys rocked!

As I said during the session, the theory behind TDD is pretty succinct, so there isn’t much in the slides themselves worth posting. Instead, I thought I would list a few pointers:

NUnit: you can find it here. I recommend checking out the Quick Start page, which covers most of what you need to start writing unit tests. I have written a post on data-driven tests here.

While we are talking about tools, I haven’t presented it during the session, but I really like TestDriven.Net. There is a free community version for your personal use. It’s a Visual Studio add-on which allows you to run and debug your tests from Visual Studio.

Even though it’s a Java book, and this session was for .NET developers, I really recommend Kent Beck’s book Test-Driven Development by Example. It’s very easy to read, and will get you started on the right foot. It’s also very well written – one of my favorite books!

The other book I recommend is the Art of Unit Testing, by Roy Osherove. I just finished it, and I wish I had it with me a few years ago, when I began writing tests seriously :) The book is technically about unit testing and not TDD, and it is a .NET book. I highly recommend it, it is chock-full of good advice, and covers way more than just testing.

That’s it! If you are interested in either the slides or code, let me know, and I’ll gladly post them, too. In the meanwhile, thanks again for coming, and… happy testing!

by Mathias 18. September 2009 06:12

ghostmantisI found a bug in my code the other day. It happens to everybody - apparently I am not the only one to write bugs – but the bug itself surprised me. In my experience, once you know a piece of code is buggy, it’s usually not too difficult to figure out what the origin of the problem might be (fixing it might). This bug surprised me, because I knew exactly the 10 lines of code where it was taking place, and yet I had no idea what was going on – I just couldn’t see the bug, even though it was floating in plain sight (hint: the pun is intended).

Here is the context. The code reads a double and converts it into a year and a quarter, based on the following convention: the input is of the form yyyy.q, for instance, 2010.2 represents the second quarter of 2010. Anything after the 2nd decimal is ignored, 2010.0 is “rounded up” to 1st quarter, and 2010.5 and above rounded down to 4th quarter.

Here is my original code:

public class DateConverter
    public static int ExtractYear(double dateAsDouble)
        int year = (int)dateAsDouble;
        return year;

    public static int ExtractQuarter(double dateAsDouble)
        int year = ExtractYear(dateAsDouble);
        int quarter = (int)(10 * (Math.Round(dateAsDouble, 1) - (double)year));
        if (quarter < 1)
            quarter = 1;
        if (quarter > 4)
            quarter = 4;
        return quarter;

Can you spot the bug?


by Mathias 7. September 2009 13:14

It’s that time of the year again: Silicon Valley Code Camp is coming up on October 3rd and 4th, at Foothill College. If you live in the Bay Area and like to talk code, this is an event you don’t want to miss. The previous editions rocked, and this year looks like it’s going to rock even harder, with well over 100 sessions and close to 1,000 registered! Oh, and did I mention it’s free?

This year again, I will give an introduction to Test-Driven Development for .NET developers. It’s a topic which is dear to my heart; in his book on TDD, Kent Beck says that it “is a way of managing fear during programming”, and I have to say that my life as a developer got significantly more peaceful after reading it. I can’t guarantee that you will feel the same, but I’ll do my best to share the goods!

The session is targeted for beginners. My goal is to get you quick-started so that you are ready to use it when you leave the room. I will write some code live, to show the methodology in action, using only tools you can get for free. This year, I think I will focus mostly on NUnit and keep it to a minimum on the tools provided in Visual Studio, unless there is strong popular demand; hopefully this will give me enough time to squeeze in a few minutes on mocks. I also just ordered Roy Osherove’s “The Art of Unit Testing”, which looks very promising, and may push me to modify my plan a bit.

I hope to see you there – and if you have questions or suggestions, let me know in the comments section!

by Mathias 24. July 2009 15:54

I really like the addition of [TestCase] in NUnit 2.5. A significant part of the code I write is math or finance oriented, and I find Data-Driven tests more convenient that “classic” unit tests to validate numeric procedures.

However, I got a bit frustrated today, because of the lack of tolerance mechanism in data-driven tests. Tolerance allows you to specify a margin of error (delta) on your test, and is supported in classic asserts:

public void ClassicToleranceAssert()
    double numerator = 10d;
    double denominator = 3d;
    Assert.AreEqual(3.33d, numerator / denominator, 0.01);
    Assert.That(3.33d, Is.EqualTo(numerator / denominator).Within(0.01));

You can specify how close the result should be from the expected test result, here +/- 0.01.

I came into some rounding problems with data driven tests today, and hoped I would be able to resolve them with the same mechanism. Here is roughly the situation:

[TestCase(10d, 2d, Result = 5d)]
[TestCase(10d, 3d, Result = 3.33d)]
public double Divide(double numerator, double denominator)
    return numerator / denominator;

Not surprisingly, the second test case fails – and when I looked for a similar tolerance mechanism, I found zilch.

The best solution I got was to do something like this:

[TestCase(10d, 2d, Result = 5d)]
[TestCase(10d, 3d, Result = 3.33d)]
public double Divide(double numerator, double denominator)
    return Math.Round(numerator / denominator, 2);

Of course, this works – but this is clumsy. I was really hoping that TestCase would support the same functionality as Assert, with a built-in delta tolerance. It seems particularly relevant: rounding error issues are typical in numerical procedures, a field where data-driven tests are especially adapted.

Maybe the feature exists, but is undocumented. If you know how to do this, sharing your wisdom will earn you a large serving of gratitude, and if it the feature doesn’t exist yet… maybe in NUnit 2.5.1?


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