Unit Testing – Best Practices for Assert Messages in Unit Tests

I don't think it's necessarily a bad thing, but I do think we should strive towards only having single asserts in our tests. This means you write a lot more tests and our tests would end up testing only one thing at a time.

Having said that, I would say maybe half of my tests actually only have one assert. I think it only becomes a code (test?) smell when you have about five or more asserts in your test.

How do you solve multiple asserts?

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;
  }

}