Question

All right, do not get angry now, I know there are several questions about this topic, but I still have some doubts.

I think I totally agree about not testing private functions, I find it reasonable, but how can I test public methods that set private variables?

Let’s say that the constructor set some private variables and I want to test that those variable are correctly set after the constructor is called. Is it a valid test? How should I check the value of the private variables without adding public getters?

I add a not real scenario example to try to be clearer:

public class ClassToTest
{
   private bool _isOn;

   public void SwitchOn() { _isOn = true; }
   public void SwitchOff(){ _isOn = false; }

   public void update()
   {
      if (_isOn)
        DoSomething();
   }

   private void DoSomething()
   {
     //this could also execute a function of an external dependency. But still the dependency could not have a public function to test if the behavior actually ran.
   }
}

how can I test that SwitchOn and SwitchOff work properly if I cannot test against the _isOn value? (this is an example, it implies that I will not write a public getter and the functions do not return a value because they do not need to)

Answer 1

Tests should use the Assemble/Activate/Assert pattern:

test switch on {
  x = new ClassToTest();
  x.SwitchOn();   //  <--  Assemble an on switch
  EmptyMailQueue();   // <-- Assemble nothing in the Q

  PossiblySendUserEmail();  // <-- Activate the feature

  assert(mailQueue.count() == 1);
  }

test switch on {
  x = new ClassToTest();
  x.SwitchOff();
  EmptyMailQueue();

  PossiblySendUserEmail();

  assert(mailQueue.count() == 0); // <-- Assert switch is off so no mails sent
  }

You assert the actual reason you have a switch. Testing the switch itself breaks the rule “Don’t TDD getters and setters”.