There are three general types of scenarios that are relevant to the discussion of negative tests in Test Case Designer.

Treat “impossible-to-test-for” scenarios differently than negative tests.

Impossible-to-test-for scenarios involve combinations of test inputs that will NEVER appear together in the real world. A good example would be using an Apple computer’s Operating System and trying to launch Internet Explorer. It cannot be done under typical circumstances. There is no point in trying to test for it, because Internet Explorer has not been available on Mac computers for years. The appropriate way to handle these scenarios is to prevent them from appearing in your tests using the Invalid Pair or Bound Pair feature.

Negative Tests are different. You want to include certain scenarios to confirm, for example, that certain types of users cannot perform certain actions. If you had different types of users for an airplane reservation system, for example, you might want to confirm that no role other than a Super-Admin User would be able to modify the ticket price. It is important to test for these kinds of scenarios.

It might be confusing to understand how to address negative tests in the context of generating Test Case Designer tests. Still, the effort you make to clearly understand your negative testing options will be well worth it because this topic comes up frequently in most projects.

Advice for Beginners: Keep negative tests separate from TCD tests

The easiest way to handle negative tests is to keep them separate from your TCD-generated tests. Many teams using our tool find it easiest to generate their positive tests. Then, outside of Test Case Designer, they will document negative tests like they currently do.

There is nothing wrong with this approach; it has the advantage of being “clean” and easy to explain. Depending upon your team's tools and processes and perhaps the quantity and nature of the particular negative tests you have in mind, it might be an appropriate solution for you.

If you decide to use this approach, consider using the “Notes” feature within TCD – some teams document their ideas for negative tests to ensure they don’t get lost.

Advice for Intermediate / Advanced Users: Distinguish between types 2 and 3

TCD sets of tests include scenarios that generally have the same number of steps. It is important that every test actually gets executed from start to finish. Why? Because of interactive coverage measurement. TCD tests ensure that you will achieve coverage of all of the interactions you have selected (2-way interactions, 3-way, etc.) If some of your tests stop part-way through, you will not achieve your desired coverage goal. The following example demonstrates this important point.

Imagine that you have a test that says:

  1. Fly from: India
  2. Fly to: France
  3. Departure Date: Tomorrow
  4. Class of Travel: First
  5. Number of Passengers: 500 <>
  6. Meal Preference: Vegetarian
  7. Type of Discount Available: XYZ
  8. Payment Type: CC, Cash, FF Miles
  9. Print ticket: Online, Email, Snail Mail

What is the problem with this kind of scenario that would risk “Killing the Script” in the 5th step? Steps 6 through 9 would never get executed. The Test Case Designer coverage algorithm has specifically selected the values that appear in each of the 9 steps to achieve your coverage objective. But if this test were to get killed after step 5, the following interactions that should be tested would never actually get executed:

  • From India & Vegetarian
  • To France & FF Miles
  • Depart Tomorrow& XYZ Discount type
  • First Class Travel & Snail Mail
  • etc.

Advice for Intermediate and Advanced Users: Consider including negative tests that allow execution of every step

Sometimes it is simple to avoid the problem described above. You can change the way you’re describing the Value that causes an error message to make it possible to achieve both of these objectives:

  • Trigger the error message to complete your negative test, and
  • Enable the tester to fix the problem and proceed to the end of the test.

In the flight reservation example above, here’s how we could change the description of the Value we enter into DesignWis to accomplish both of those goals:

  • Instead of entering “500” as the number of passengers, enter the following: “500, then confirm that the correct error message appears, then enter a valid number of passengers.”

Problem solved.

Beware of negative test ideas that would kill your test scripts before the final step. Document those outside of the Test Case Designer.

The example above assumes that a tester could trigger an error message with an invalid entry, fix the problem that caused it, and continue to execute the entire test script. Sometimes, a tester might be unable to fix the problem or continue executing steps. The following kind of test for an ATM is an example of a “script-killing” test:

“Confirm that when an ATM user enters their PIN incorrectly 5 times, then the machine should physically destroy the user’s card.”

If you had a set of tests that involved multiple steps and were supposed to happen after the user entered their PIN code, such as withdrawing a certain amount of money from a certain type of account, you would run into the problem described above. Important combinations of test inputs would not be tested. In this example, there is nothing that a tester would be able to do after the card was physically destroyed; it would not work to say, “now fix the problem and continue executing the test.” It would be impossible to resurrect the card once it had been physically destroyed.

If you have a script-killing test that you want to run, do not include it in your TCD scenarios. Instead, create a test for that outside of the tool.

Final caveat: TCD Automate allows you to create the scripts without any variables, so you could create the flow-killing scenarios there if you use BDD and want to keep all scripts in one place.
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