Mathias Brandewinder on .NET, F#, VSTO and Excel development, and quantitative analysis / machine learning.
by Mathias 4. December 2011 15:47

I am in the middle of “Working Effectively with Legacy Code”, and found it every bit as great as it was said to be. In the book, Feathers introduces the concept of Seams and Enabling Points:

a Seam is a place where you can alter behavior in your program without editing it in that place

every seam has an enabling point, a place where you can make the decision to use one behavior or another.

The idea - as I understand it - is that an enabling point is a hook for testability, a place where you can replace the behavior of a piece of code with your own controlled behavior, and validate that the results are as expected.

The reason I am bringing this up is that I have been writing lots of F# lately, and it made me realize that a functional style provides lots of enabling points, and can be much easier to test than object-oriented code.

Here is a simplified, but representative, example of the problem I was looking at: I needed to pick a random item in a list. In C#, a method along these lines would do the job:

public T PickFrom(IList<T> list)
   var random = new Random();
   return list[random.Next(list.Count())];

However, this code is utterly untestable; it’s also probably a terrible idea to instantiate a new Random every time this is called, so we modify it this way:

public T PickFrom(IList<T> list, Random random)
   return list[random.Next(list.Count())];

This is much better: now we have a decent Enabling Point, because the list of arguments of the method contains everything that is used inside the method. However, this is still untestable, but for a different reason: by definition, Random.Next() will return different values every time PickFrom is called, and expecting a repeatable result from PickFrom is a bit of a desperate enterprise.


by Mathias 13. November 2011 11:03

We were doing some pair-programming with Petar recently, Test-Driven Development style, and started talking about how figuring out where to begin with the tests is often the hardest part. Petar noticed that when writing a test, I was typically starting at the end, first writing an Assert, and then coding my way backwards in the test – and that it helped getting things started.

I hadn’t realized I was doing it, and suspected it was coming from Kent Beck’s “Test-Driven Development, by Example”. Sure enough, the Patterns section of the book lists the following:

Assert First. When should you write the asserts? Try writing them first.

So why would this be a good idea?

I think the reason it works well, is that it helps focus the effort on one single goal at a time, and requires clarifying what that goal is. Starting with the Assert forces you to imagine one single fact that should be true once you have implemented the feature, and to think about how you are going to verify that the feature is indeed working.

Once the Assert is in place, you can now write the story backwards: what is the method that was called to get the result being checked, and  where does it belong? What classes and setup is required to make that method call? And, now that the story is written, what is it really saying, and what should the test method name be?

In other words, begin with the Assert, figure out the Act part, Arrange the actors, and (re)name the test method.

I think what trips some people is that while a good test will look like a little story, progressing from a beginning to a logical end, the process leading to it follows a completely opposite direction. Kent Beck points the Self-Similarity in the entire process: write stories which describe what the application will do once done, write tests which describe what the feature does once the code is implemented, and write asserts which will pass once the test is complete. Always start with the end in mind, and do exactly what it takes to achieve your goal.


by Mathias 25. September 2011 05:31


It’s this time of the year again: on Saturday & Sunday October 8 + 9, Silicon Valley CodeCamp is taking place at Foothill College in Los Altos Hill. There are currently over 200 sessions listed, and 2000 people signed up already. I am expecting lots of fun - again.

I’ll be giving 3 talks this year:

  • Sat, 11:45 - Beginning TDD for C# Developers.
  • Sun, 1:15: For Those About to Mock.
  • Sun, 2:45: An excursion in F#

Hope to see you there, and also that I will have some energy left to attend some of the other talks!

More information about my talks here.

by Mathias 13. February 2011 13:46

I have been a fan of Test-Driven Development for a long time; it has helped me write better code and keep my sanity more than once. However, until now, I haven’t really looked into Behavior-Driven Development, even though I have often heard it described as a natural next step from TDD. A recent piece in MSDN Magazine re-ignited my interest, and helped me figure out one point I had misunderstood, namely how BDD and TDD fit together, so I started looking into existing frameworks.

Most of them follow a similar approach: write in a plain-text file a description of the feature in Gherkin, a “feature description” language that is human readable, let the framework generate test stubs which map to the story, and progressively fill the stubs as the feature gets implemented.

I am probably (too) used to writing tests as code, but something about the idea of starting from text files just doesn’t feel right to me. I understand the appeal of Gherkin as a platform-independent specification language, and of letting the product owner write specifications – but the thought of having to maintain two sets of files (the features and the actual tests) worries me. I may warm up to it in due time, but in the meanwhile I came across StoryQ, a framework which felt much easier to adopt for me.

StoryQ is a tiny dll, which permits to write stories as tests in C#, using a fluent interface, with all the comfort and safety of strong typing and intellisense; Gherkin stories can be produced as an output of the tests, and a separate utility allows you to create code templates from Gherkin.

Rather than talk about it, let’s see a quick code example. I have a regular NUnit TestFixture with one Test, which represents a Story I am interested in: when I pay the check at the restaurant, I need to add a tip to the check. There are 2 scenarios I am interested in: when I am happy, I’ll tip a nice 20%, but when I am not, there will be zero tip. This is how it could look like in StoryQ:

using NUnit.Framework;
using StoryQ;

public class CalculateTip
   public void CalculatingTheTip()
      new Story("Calculating the Tip")
      .InOrderTo("Pay the check")
      .IWant("Add tip to check")

      .WithScenario("Unhappy with service")
      .Given(CheckTotalIs, 100d)
      .When(IAmHappyWithService, false)
      .Then(TipShouldBe, 0d)

      .WithScenario("Happy with service")
      .Given(CheckTotalIs, 100d)
      .When(IAmHappyWithService, true)
      .Then(TipShouldBe, 20d)


   public double CheckTotal { get; set; }

   public bool IsHappy { get; set; }

   public void CheckTotalIs(double total)
      this.CheckTotal = total;

   public void IAmHappyWithService(bool isHappy)
      this.IsHappy = isHappy;

   public void TipShouldBe(double expectedTip)
      var tip = TipCalculator.Tip(CheckTotal, IsHappy);
      Assert.AreEqual(expectedTip, tip);

(The TipCalculator class is a simple class I implemented on the side).

This test can now be run just like any other NUnit test; when I ran this with ReSharper within Visual Studio, I immediately saw the output below. Pretty nice, I say.


What I liked so far

  • Painless transition for someone used to TDD. For someone like me, who is used to write unit tests within Visual Studio, this is completely straightforward. No new language to learn, a process pretty similar to what I am used to – a breeze.
  • Completely smooth integration with NUnit and ReSharper: no plugin to install, no tweaks, it just worked.
  • Fluent interface: the fluent interface provides guidance as you write the story, and hints at what steps are expected next.
  • Passing arguments: I like the API for expressing the Given/When/Then steps. Passing arguments feels very natural.
  • xml report: I have not played much with it yet, but there is an option to produce an xml file with the results of the tests, which should work well with a continuous integration server.

What I didn’t like that much

  • Execute: at some point I inadvertently deleted the .Execute() call at the end of the Story, and it took me a while to figure out why all my tests were passing, but no output was produced. More generally, I would have preferred something like Verify(), which seems clearer, but that’s nitpicking.
  • Multiple scenarios in one test: once I figured out that I could chain multiple scenarios in one story, I was a happy camper, but all the examples I saw on the project page have one story / one scenario per test method. It’s only when I used the WPF story converter that I realized I could do this.
  • Crash of the WPF converter: the converter is awesome – but the first time I ran it it crashed.

So where do I go from there? So far, I really enjoyed playing with StoryQ – enough that I want to give it a go on a real project. I expect that the path to getting comfortable with BDD will be similar to TDD: writing lots of tests, some of them fairly bad, until over time a certain feeling for what’s right or very wrong develops… In spite of my reservations, I am skeptical but curious (after all, I have been known to be wrong sometimes…), so I also plan to give SpecFlow a try.

by Mathias 12. December 2010 17:00

I recently began playing with ASP.NET MVC (never too late), and so far I really enjoy it. One aspect I really appreciate is its testability – I can write fairly straightforward unit tests to verify that the application behaves as I believe it should, as well as make sure I understand what is going on.

One point which got me stumped was how to test for authorization. A controller, or some of its methods, can be decorated with the attribute [Authorize], restricting users who can access the method by role or name. In the default ASP.NET MVC 2 template, when a user isn’t authorized to a specific area, he gets re-directed to the LogOn method on the AccountController.

So far so good. However, I ran into unexpected issues when I attempted to unit test that. Suppose there are two roles in our web application, Chicken and Pigs, and that we have a Controller that leads to a pigs-only area of the web site:

[Authorize(Roles = "Pig")]
public class PigsOnlyController : Controller
   public ActionResult Index()
      return View("Index");

My first thought was to mock the ControllerContext and do something along these lines:

public void OnlyPigsShouldAccessIndex()
   var context = new Mock<ControllerContext>();
   var userName = "PIG";
   context.SetupGet(p => p.HttpContext.User.Identity.Name).Returns(userName);
   context.SetupGet(p => p.HttpContext.Request.IsAuthenticated).Returns(true);
   context.Setup(p => p.HttpContext.User.IsInRole("Pig")).Returns(true);

   var controller = new PigsOnlyController();
   controller.ControllerContext = context.Object;

   // check what controller.Index() returns

However, while the web application itself behaved properly (Users in the Pig role get to the Index page, whereas Chicken get redirected to the Logon page), the test wasn’t doing what I expected: both Pigs and Chicken were happily reaching the Pigs-Only area.

As is often the case, I found out why on StackOverflow; the reason is that the re-direction is not the responsibility of the Controller. If the controller is properly decorated, the Index method won’t even be invoked, and where the call gets redirected to is handled in a different part.

So how do you unit test this behavior? In this case, we trust the framework to handle the redirection, so the only functionality we need to ascertain is that the Controller has the proper attribute.

Instead of validating the redirection, we can verify that the Controller class has an Authorize attribute, with the proper roles specified:

public void ControllerShouldAuthorizePigsButNotChicken()
   var information = typeof(PigsOnlyController);
   var attributes = information.GetCustomAttributes(typeof(AuthorizeAttribute), false);
   Assert.AreEqual(1, attributes.Length);

   var authorization = attributes[0] as AuthorizeAttribute;
   var authorizedRoles = authorization.Roles;

   var roles = authorizedRoles.Split(',');

I am not totally happy with the hard-coded strings “Pig” and “Chicken” in the test, but I don’t see a way around it; maybe it’s a sign that this test is more of an integration test than a unit test? If you know of a better way to test for that aspect, I am all ears!


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