Unit Testing - Is it bad form to have unit test calling other unit tests

Question

I have a unit test called TestMakeAValidCall(). It tests my phone app making a valid call.

I am about to write another test called TestShowCallMessage() that needs to have a valid call made for the test. Is it bad form to just call TestMakeAValidCall() in that test?

For reference this is my TestMakeAValidCall() test.

    [TestMethod]
    public void TestMakeAValidCall()
    {
       //Arrange
        phone.InCall = false;
        phone.CurrentNumber = "";
        // Stub the call to the database
        data.Expect(x => x.GetWhiteListData()).
            Return(FillTestObjects.GetSingleEntryWhiteList());
        // Get some bogus data
        string phoneNumber = FillTestObjects.GetSingleEntryWhiteList().
            First().PhoneNumber;
        // Stub th call to MakeCall() so that it looks as if a call was made.
        phone.Expect(x => x.MakeCall(phoneNumber)).
            WhenCalled(invocation =>
                       {
                           phone.CurrentNumber = phoneNumber;
                           phone.InCall = true;
                       });

       //Act
        // Select the phone number
        deviceControlForm.SelectedNumber = phoneNumber;
        // Press the call button to make a call.
        deviceMediator.CallButtonPressed();

       //Assert
        Assert.IsTrue(phone.InCall);
        Assert.IsTrue(phone.CurrentNumber == phoneNumber);
    }

Answer 1

Refactor the setup to another method and call that method from both tests. Tests should not call other tests.

Answer 2

I think its a bad idea. You want your unit test to test one thing and one thing only. Instead of creating a call through your other test, mock out a call and pass it in as an argument.

Answer 3

IMHO, you should do one of the following:

• Create a method that returns a valid call, and use it separately for both tests (not one calling the other)
• Mock the valid call for the ShowCallMessageTest

Answer 4

A unit test should test one unit/function of your code by definition. Having it call other unit tests makes it test more than one unit. I break it up in to individual tests.

Answer 5

Yes - unit tests should be separate and should aim to test only one thing (or at least a small number of closely-related things). As an aside, the calls to data.Expect and phone.Expect in your test method are creating expectations rather than stub calls, which can make your tests brittle if you refactor...

Answer 6

To offer a counter point:

I strongly believe that well designed unit test should depend on one another!

Of course, that makes sense only if the testing framework is aware of these dependencies such that it can stop running dependent test when a dependency fails. Even better, such a framework can pass the fixture from test to test, such that can build upon a growing and extending fixture instead of rebuilding it from scratch for each single test. Of course, caching is done to take care no side-effects are introduced when more than one test depends from the same example.

We implemented this idea in the JExample extension for JUnit. There is no C# port yet, though there are ports for Ruby and Smalltalk and ... the most recent release of PHPUnit picked up both our ideas: dependencies and fixture reuse.

PS: folks are also using it for Groovy.