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Ignore Irrelevant Return Values

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This is an installment in my Zero-Friction TDD series.

Sometimes, you don't care about the return value from a particular operation. The simplest example is if you want to check that creating a new instance of a specific type will throw an exception if supplied with wrong parameter values:

 [ExpectedException(typeof(ArgumentNullException))]

 [TestMethod]

 public void CreateWithNullTextWillThrow()

 {

     // Fixture setup

     int anonymousNumber = 8;

     string nullText = null;

     // Exercise system

     new Plop(anonymousNumber, nullText);

     // Verify outcome (expected exception)

     // Teardown

 }

In this example, even though the new operation is supposed to return a value, we don't care about it, since we only want to test that the correct exception type is being thrown. To save myself from having to stop and think about a variable name (even though in this case, I'd just have used sut), I prefer to not declare and assign the variable at all.

In general, I believe that APIs should follow the principle of Command-Query Separation, but sometimes it makes sense to break this rule. When you have an operation that both return a value and have side-effects, testing the side-effect via state-based testing doesn't require the return value.

Imaging that you have a method with this signature:

 public object CommandQuery(int number)

Testing the side-effect may look like this:

 [TestMethod]

 public void CommandQueryWillUpdateNumberProperty()

 {

     // Fixture setup

     int expectedNumber = 89;

     int anonymousNumber = 11;

     string anonymousText = "Anonymous text";

     Plop sut = new Plop(anonymousNumber, anonymousText);

     // Exercise system

     sut.CommandQuery(expectedNumber);

     // Verify outcome

     Assert.AreEqual<int>(expectedNumber, sut.Number, "CommandQuery");

     // Teardown

 }

Notice that I've ignored the return value of the CommandQuery method.

Save yourself the time and effort it takes to declare and assign variables if you don't use them anyway.

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