Wednesday, May 28, 2014

Code coverage -- meaningless number or the Holy Grail?

Code coverage is a term which is often brought up when discussing TDD. Code coverage is the percentage of the lines of code that are executed by unit tests.



Many teams have a specific goal for the code coverage. For example, a typical goal is to have at least 80% code coverage. Others claim that code coverage is a meaningless number because you can have 100% code coverage with nonsense tests that have no value. So which coverage should one aim for?

Coverage is not a measure of quality

You can have 100% test coverage, and yet the tests may be worthless. A single test which simply executes every line in the production code without asserting on anything will yield a 100% coverage, but it won't detect any defects. Clearly, coverage can't be used to measure test quality. So what is it good for?

Coverage is an alarm bell

Although a 100% coverage does not provide much information, the opposite case does. If you have a test coverage of 20%, it means that almost none of the production code is tested. Sound the alarm and do something!

Hence, code coverage should be used as an indication that something is wrong and not as a measure of test quality. 

So which coverage should we aim for?

Personally, I rarely measure code coverage. If you do proper TDD (write test first and never touch production code before you have a failing test), your coverage will naturally be close to 100% because you already test for all the desired behavior. Some test runners decorate the source code as you type to indicate whether it's covered by tests, so untested code will be screaming at you.

If a team wants to run periodical code coverage analysis in order to enable the lack-of-tests alarm bell, I would aim for close to 100% on testable code and 0% on untestable code. Testable and untestable code should ideally be separated into different modules of the application, but that's another story which deserves a separate blog post at some point in the future...

Monday, May 19, 2014

Test-driven development of plug-ins

How can we do test-driven development in a plug-in environment where our production code lives inside of an application? This may be the case if we are developing plug-ins for applications like Microsoft Office, Adobe Photoshop or Schlumberger Petrel. There are two challenges with this:
  1. NUnit (or any other test runner of choice) may not be able to start the host application in order to execute the plug-ins
  2. Starting the host application may take a while. If it takes 30 seconds to start the application, it's hard to get into the efficient TDD cycle that I describe in the blog post "An efficient TDD workflow".

Abstraction and isolation

Abstraction, inversion of control and isolation are common strategies when we develop code which is dependent on the environment. The idea is to make abstractions to the environment and thereby omitting it when executing the tests. It's not always possible, though. Sometimes our plug-in interacts heavily with and is dependent upon the behavior of the environment.

So what do we do then? If we can't isolate them, join them!

Unit test runner as a plug-in

Both challenges above can be solved by creating your own test runner as a plug-in inside of the host application! Instead of letting NUnit start the host application (slow and perhaps not even possible), the host application is running the tests.

So instead of doing a slow TDD cycle like this:


We want to move the host application startup out of the cycle like this:


So how can we do this? Create a test runner inside the host application... as a plug-in!

Let's have a look on one specific case: a test runner as a plug-in in Petrel. Petrel faces challenge #2 mentioned above. It can run in a "unit testing mode" where NUnit tests can start Petrel and run the Petrel-dependent production code, but startup takes 30-60 seconds.

If you are using NUnit, this is quite simple. NUnit provides multiple layers of test runners, depending on how much you want to customize its behavior. A very rudimentary implementation which allows the user to select a test assembly, list tests and execute them could look like this:

public partial class TestRunnerControl : UserControl
  {
    private readonly Dictionary<string, List<string>> _assembliesAndTests = new Dictionary<string, List<string>>();

    private string _pluginFolder;

    public TestRunnerControl()
    {
      InitializeComponent();

      this.runButton.Image = PlayerImages.PlayForward;

      FindAndListTestAssemblies();
    }

    private void FindAndListTestAssemblies()
    {
      var pluginPath = Assembly.GetExecutingAssembly().Location;
      _pluginFolder = Path.GetDirectoryName(pluginPath);

      AppDomain tempDomain = AppDomain.CreateDomain("tmpDomain", null, new AppDomainSetup { ApplicationBase = _pluginFolder });
      tempDomain.DoCallBack(LoadAssemblies);
      AppDomain.Unload(tempDomain);

      foreach (var testDll in _assembliesAndTests.Keys)
      {
        this.testAssemblyComboBox.Items.Add(testDll);
      }
    }

    private void LoadAssemblies()
    {
      foreach (
        var dllPath
          in Directory.GetFiles(_pluginFolder)
            .Where(f => f.EndsWith(".dll", true, CultureInfo.InvariantCulture) && f.Contains("PetrelTest")))
      {
        try
        {
          Assembly assembly = Assembly.LoadFrom(dllPath);

          var dllFilename = Path.GetFileName(dllPath);

          try
          {
            var typesInAssembly = assembly.GetTypes();

            foreach (var type in typesInAssembly)
            {
              var attributes = type.GetCustomAttributes(true);

              if (attributes.Any(a => a is TestFixtureAttribute))
              {
                if (!_assembliesAndTests.ContainsKey(dllFilename))
                {
                  _assembliesAndTests[dllFilename] = new List<string>();
                }

                _assembliesAndTests[dllFilename].Add(type.FullName);
              }
            }

            Ms.MessageLog("*** Found types in " + assembly.FullName);
          }
          catch (Exception e)
          {
            Ms.MessageLog("--- Could not find types in " + assembly.FullName);
          }
        }
        catch (Exception e)
        {
          Ms.MessageLog("--Could not load  " + dllPath);
        }
      }
    }  

    private void TestAssemblySelected(object sender, EventArgs e)
    {
      this.testClassComboBox.Items.Clear();

      var testAssembly = this.testAssemblyComboBox.SelectedItem as string;
      if (!string.IsNullOrEmpty(testAssembly))
      {
        foreach (var testClass in _assembliesAndTests[testAssembly])
        {
          this.testClassComboBox.Items.Add(testClass);
        }
      }
    }

    private void runButton_Click(object sender, EventArgs e)
    {     
      if (!CoreExtensions.Host.Initialized)
      {
        CoreExtensions.Host.InitializeService();
      }

      var results = RunTests();

      ReportResults(results);
    }

    private void ReportResults(TestResult results)
    {
      var resultsToBeUnrolled = new List<TestResult>();
      resultsToBeUnrolled.Add(results);

      var resultList = new List<TestResult>();
      while (resultsToBeUnrolled.Any())
      {
        var unrollableResult = resultsToBeUnrolled.First();
        resultsToBeUnrolled.Remove(unrollableResult);

        if (unrollableResult.Results == null)
        {
          resultList.Add(unrollableResult);
        }
        else
        {
          foreach (TestResult childResult in unrollableResult.Results)
          {
            resultsToBeUnrolled.Add(childResult);
          }
        }
      }

      int successCount = resultList.Count(r => r.IsSuccess);
      int failureCount = resultList.Count(r => r.IsFailure);
      int errorCount = resultList.Count(r => r.IsError);

      string successString = string.Format("{0} tests passed. ", successCount);
      string failureString = string.Format("{0} Tests failed. ", failureCount);
      string errorString = string.Format("{0} Tests had error(s). ", errorCount);

      string summary = successString + failureString + errorString;

      this.resultSummaryTextBox.Text = summary;

      this.resultSummaryTextBox.Select(0, summary.Length);
      this.resultSummaryTextBox.SelectionColor = Color.FromArgb(80, 80, 80);

      if (successCount > 0)
      {
        this.resultSummaryTextBox.Select(0, successString.Length);
        this.resultSummaryTextBox.SelectionColor = Color.DarkGreen;
      }

      if (failureCount > 0)
      {
        this.resultSummaryTextBox.Select(successString.Length, failureString.Length);
        this.resultSummaryTextBox.SelectionColor = Color.Red;
      }

      if (errorCount > 0)
      {
        this.resultSummaryTextBox.Select(successString.Length + failureString.Length, errorString.Length);
        this.resultSummaryTextBox.SelectionColor = Color.Red;
      }

      this.resultSummaryTextBox.Select(0, summary.Length);
      this.resultSummaryTextBox.SelectionAlignment = HorizontalAlignment.Center;

      this.resultSummaryTextBox.Select(0, 0);

      // Set grid results
      this.resultsGridView.Rows.Clear();

      int firstErrorIdx = -1;

      foreach (var result in resultList)
      {
        var testName = result.Name;
        var image = result.IsSuccess ? GeneralActionImages.Ok : StatusImages.Error;

        int idx = this.resultsGridView.Rows.Add(testName, image);

        if (firstErrorIdx == -1 && !result.IsSuccess)
        {
          firstErrorIdx = idx;
        }
      }

      if (firstErrorIdx != -1)
      {
        this.resultsGridView.FirstDisplayedScrollingRowIndex = firstErrorIdx;
      }
    }

    private TestResult RunTests()
    {
      var testAssembly = this.testAssemblyComboBox.SelectedItem as string;
      var testClass = this.testClassComboBox.SelectedItem as string;

      TestPackage testPackage = new TestPackage(Path.Combine(_pluginFolder, testAssembly));
      TestExecutionContext.CurrentContext.TestPackage = testPackage;

      TestSuiteBuilder builder = new TestSuiteBuilder();
      TestSuite suite = builder.Build(testPackage);

      var testFixtures = FindTestFixtures(suite);
      var desiredTest = testFixtures.First(f => f.TestName.FullName == testClass);
      var testFilter = new NameFilter(desiredTest.TestName);
      TestResult result = suite.Run(new NullListener(), testFilter);

      return result;
    }

    private IEnumerable<TestFixture> FindTestFixtures(Test test)
    {
      var testFixtures = new List<TestFixture>();

      foreach (Test child in test.Tests)
      {
        if (child is TestFixture)
        {
          testFixtures.Add(child as TestFixture);
        }
        else
        {
          testFixtures.AddRange(FindTestFixtures(child));
        }
      }

      return testFixtures;
    }
  }

Note that this code sample is not complete, but it shows how to find and run the tests. Use the test runner control in a plug-in inside the host application, and it will look like this:


Whenever a test is failing, Visual Studio will break at the offending NUnit Assert statement:


So how does this allow for an efficient code-test cycle? By using Visual Studio's Edit & Continue! Whenever you want to edit the production code or the test code, press pause in Visual Studio, edit as needed, and run the tests again. Hence, you can write code and (re)run tests at will without restarting the host application.

It's not as efficient as writing proper unit tests that execute in milliseconds, but it's far better than waiting for the host application on every test execution.

Wednesday, May 14, 2014

An efficient TDD workflow

How do you do your everyday TDD work? There are many ways to skin a cat, but I like to do it in a cyclic fashion:


I start by writing a very simple failing test. I then implement enough of the production code to just make this test pass. I then add more details to the test which make the test fail again. I implement more of the production code so that it passes again. Rinse and repeat until your tests are covering all the use cases.

Let's say that I want to write a Matrix class with an inversion method. I'd start with a super simple test case where I assert that the inverse of the identity matrix I is an identity matrix. I'd then add more and more cases:

  1. Create test which asserts that the inverse of the unity matrix I is a unity matrix. Production code returns an empty matrix, so it will fail
  2. Fix the production code so that it always returns I. The test will pass.
  3. Add new test case which asserts that the inverse of a different 2x2 matrix is correct. The test will fail
  4. Fix the production code so that it calculates the inverse of any 2x2 matrix. The test will pass
  5. Add new test case with a 3x3 matrix. The test will fail
  6. Fix the production code so that it calculates the inverse of any size matrix. The test will pass.
  7. Add new test case which asserts that calculating the inverse of a singular matrix throws an ArithmetricException. The test will fail because it tries to divide by zero
  8. Fix the production code so that it handles singular matrices correctly. The test will pass.
...and the show goes on until I am satisfied. Note that I'm not writing the entire test and then the entire production code method. Both the test and the production code evolve in parallel -- but I always have a failing test before doing anything with the production code.

Furthermore, I like to have the production code and the test code side-by-side like this so that I don't need to switch back and forth between the files:


If you are using a good test runner like NCrunch, you don't even need to run the tests manually. They are run automatically as you type, and you will have instant feedback when the test fails or passes!