Wednesday, November 26, 2014

Slides from "OWIN and Composable Web Apps" at Campus Days Cph

Here are the OWIN slides from the talk I did at Campus Days Cph today. Enjoy :)

Friday, November 7, 2014

A First Look at C# 6 - Nameof operator

This is the fourth post about the C# 6 features I used when moving Nancy.Linker over to C# 6. The first part was about primary constructors. The second part was about auto properties. The third was about expression bodied members. This one in about the new nameof operator.

The nameof operator

The new nameof operator is a unary operator that takes, as its argument, a type, an identifier or a method and returns the name of the argument. That is, if you pass in the type Uri, you get back the string "System.Uri", if you pass in a variable you get back the name of the variable as a string.

In Nancy.Linker there was one opportunity for using nameof, which I coincidentally think represents a usage of nameof that will become widespread.
In one the private methods in Nancy.Linker there is a null check followed by a possible ArgumentException:

Notice how I am repeating the name of the res variable in a string in order to tell the ArgumentException which argument is in error. With the nameof operator I avoid this duplication and potential source of inconsistency as follows:

Note that in case I rename res and forget to update the usage inside nameof I get a compile error.

Do I Like This?

Yes. It lets me have fewer magic strings.

Wednesday, November 5, 2014

A First Look at Using C# 6 - Expression Bodied Members

This is the third post about the C# 6 features I used when moving Nancy.Linker over to C# 6. The first part was about primary constructors. The second part was about auto properties. This one is about the new expression bodied members feature.

Expression Bodied Methods

I like to break down my code down to short methods. Quite short. So I often have one-liner methods. In Nancy.Linker I had this for instance:

With C# 6 I can make this even shorter because methods that consist of just one expression, can be implemented as just that - one expression .... with a fat arrow in front of it. Like so:

Expression Bodied Properties

I didn't have an opportunity to use any expression bodies properties in Nancy.Linker, but I want to mention them anyway.

Just as single expression methods can be changed to be a fat arrow followed by the expression so can getter only properties that have just one expression in the getter. Note what I said there: Getter only properties can be expression bodied. Properties with setters cannot ... but ... think about it ... what would it mean to set a property consisting of only an expression? There is nothing there to assign to.

Seeing that expression bodied properties are getter only, it stands to reason that you don't have to state the 'get'.
As an example an expression bodied property looks like this:

Do I Like These?

Yes :-)
I like them because they cut down on boilerplate.

Monday, November 3, 2014

The 'Unit' in a Unit Test

How much should a unit test cover? What is the 'unit' which it should test? This seems to be a recurring discussion among developers. And rightly so - I am not going to claim to have the ultimate answer to those questions, but I will address one misconception that I hear repeated time and again, namely that the 'unit' of a unit test is a class in the production code. In this post I will show why this leads to bad testing practices and how thinking of the 'unit' as a unit of functionality leads to a better place.


To illustrate my point I'll play around with the following Employee class from an imagined resource allocation system. The system handles work assignments and can only assign them to employees that
  • do not have overlapping assignments
  • have the necessary skills
  • cost less per hour than the assignment pays
Instances of Employee have a number of work assignments each of which start at a certain date and likewise ends at a certain date. Employees also have a skill set and a minimum hourly cost. Employee instances use these properties to implement the business rules around work assignments, like so:

The implementation of Employee shown here is simplistic. Real business rules would no doubt be more involved. Nevertheless this class serves to illustrate the point.
If we follow the notion that the 'unit' of a unit test is a production class it is reasonable to assume that there is a test class corresponding to Employee and that it could look something like this:

Refactoring Production Code

Even though the employee class is simplistic it still has several reasons to change:
  • Changes to the business rules around matching employee skill set and assignment requirements
  • Changes to the business rules for employee availability, for instance introduction of part time assignments
  • Changes to the business rules around employee cost ans assignment rate
In other words the Employee class violates the Single Repsonsibility Principle.
As a first refactoring lets introduce a SkillSet class capable of comparing a set of skills held by an employee and a set skill needed by a task:

which would change the Employe class to this (hint: The changes are in line 12 and 19):

At this point the existing tests need just one follow one change in its constructor:

After that the tests will pass again and - importantly - they will still cover all our code including the SkillSet class.

Refactoring the Test Code?

The Employee_should class now tests not only the Employee class but also the SkillSet class. If we assume that the 'unit' of a unit test is a production class Employee_should is no longer a unit test. Following this line of thought leads towards two changes to the test code:
  • Mock out the SkillSet in the Employee_should. Typically this involves first introducing an interface on top of SkillSet and then using a mocked version of that in the Employee_should
  • Create a new test class for the SkillSet - Skillset_should

I'm not going to write that code, but I'm sure you can imagine it.

Test Code Smell

If we went ahead and did the refactorings to our test code it would be becoming quite smelly:
  • We would have lost all testing of the collaboration between Employee and SkillSet. This is pretty serious since that is in fact where part of the business rules are upheld. These parts of the rules would effectively become untested.
  • The Employee_should test class would become more complicated by the extra setup of the mocked SkillSet - this might not seem too bad at this point, but quickly becomes unwieldy when dealing with more complicated scenarios
  • The Skillset_should test would not add much - it almost just checks that the framework method IsSubsetOf works.

If we, on the other hand, follow the line of thought that the 'unit' of a unit test is a unit of behavior, we would just keep the unit tests as the are after the introduction of the SkillSet class. This is better because the tests are not hampering the freedom to refactor the production code.


I find it much more helpful to think about the 'unit' in unit test as a unit of behavior. Whether that unit of behavior is implemented in one or more production classes is an implementation detail of the production code.