Unit Testing – Does Code Smell in Unit Tests Matter

Expectations should be different but if you're testing with a common set of expectations across several unit tests you should use variables and objects that are shared.

This is also why you have the setUp and tearDown methods for in xUnit frameworks.

Some frameworks take this one step further letting you set up with nesting such as Jasmine (BDD/TDD for Javascript).

I think this is what the author is getting at.

In my code sample, I have a timing window that takes an output quantity plus a start and stop time. The intent is to draw an output window over a 24 hour timespan. A wrinkle is added when start time is greater than stop time because that's a timing window that spans midnight.

You can write unit tests that fully exercise the object without exposing the private variables. Those private bools and timespans are the collaborators he's referring to when exposing the internals for unit testing. According to the book, exposing those internals would NOT be a code smell since they are collaborators.

Exposing the double output would be a code smell since it's not a collaborator - it's an element explicitly hidden by the class itself that has conditional logic within GetOutput to determine what should be returned.

Digging into the bools / timespans would make the unit tests more comprehensive. He says this is good.
Digging into the double output would require additional logic in your unit test that mirrored what GetOutput was doing. This would be the code smell he's referring to.

public class TimeWindow
{
  private bool isConst;
  private bool spansMidnight;
  private TimeSpan start1;
  private TimeSpan stop1;
  private TimeSpan start2;
  private TimeSpan stop2;
  private double output;

  public TimeWindow( double out, TimeSpan start, TimeSpan stop)
  {
    output = out;

    if( start == stop )
      isConst = true;
    else if( start > stop )
    {
      spansMidnight = true;
      start1 = midnight;
      stop1 = stop;
      start2 = start;
      stop2 = midnight;
    }
    else 
    {
      start1 = start;
      stop1 = stop;
    }
  }

  public double GetOutput( TimeSpan time )
  {
    // some logic here on what / how to return
    ...
    return output;
  }

}