Vidar’s Musings

When trying to introduce unit tests into an existing project, code dependencies is often a problem. During a teeth-grinding session grappling with this problem I came to think about pure functions.

Pure functions are functions that (according to Wikipedia’s definition):

• always evaluates the same result value given the same argument value(s). The function result value cannot depend on any hidden information or state that may change as program execution proceeds, nor can it depend on any external input from I/O devices, and
• evaluation of the result does not cause any semantically observable side effect or output, such as mutation of mutable objects or output to I/O devices.

Seems to me that pure functions would be a wet dream for unit testing. Imagine, you could pass in whatever the function needed to execute, and then check its return value. No mocking about with mock objects with side effect expectations needed!

But alas, we live in a world with non-pure functions. Even in functional programming, which does explicitly keep any state, you find them. In an object oriented language a primary idea is that objects should carry state. An object has a state. Period. However, if we can be inspired by the notion of pure functions, and make methods that have less dependencies, we will be much better off when it comes to unit testing.

I my experience, the ASP.NET programming model with ASPX code behind files is a model that promotes writing methods that are problematic to create unit tests for.

Take, for example, the following (theoretical, I admit) code behind snippet example:

protected void Page_Load(object sender, EventArgs e)

    {

        ShowOrHideEditButton(Page.User.Identity.Name);

    }

    void ShowOrHideEditButton(string currentUser)

    {

        if (Request.QueryString["user"].Equals(currentUser))

        {

            EditButton.Enabled = true;

        }

        else

        {

            EditButton.Enabled = false;

        }

    }

So, what is the problem here? Actually, there are two problems.

1. The ShowOrHideMethod is not only dependant on its input parameters, but also on another member variable. When looking at the method signature, you do not see this, and might be misleaded about what the outcome of the methods is dependent on.
2. The method does not return any value, but has a side-effect on its environment (setting the edit button visibility).

These two issues, makes it troublesome to test the method in a unit test. You have spend time to set up the methods context, and you have to measure the method’s side effects in order to make assesments about its result.

A much better approach would be something like this:

    protected void Page_Load(object sender, EventArgs e)

    {

        EditButton.Enabled = ShowOrHideEditButton(Page.User.Identity.Name, Request.QueryString["user"]);

    }

    bool ShowOrHideEditButton(string currentUserName, string profilePerson)

    {

        return (currentUserName.Equals(profilePerson));

    }

This way, we can easily test the ShowOrHideEditButton method in a unit test without having to use a great deal test code to set up the test context.

For increased testability, I would suggest the following for better method purity:

• A method should evaluate the same result, given the same argument value(s) and object state.
• A method execution result should be represented as, in order of preference: 1) a return value, 2) changed object state (mutation), or 3) a side-effect on an I/O device

In my current project, we see an increasing use of JavaScript in our web application. Web 2.0, ya know. Hence, we have a growing need for being able to create automated JavaScript tests. A colleague of mine, Aslak Hellesøy, suggested that we should have a look at script.aculo.us’s unit test framework.

Fair enough, the script.aculo.us unit test framework seemed to do the trick. We use both prototype and script.aculo.us in our project, so the script.aculo.us unit test framework seemed to fit in nicely. My next concern would be to make it as easy as possible for the developers to run the tests as well as integrating
it into our continuous integration builds (we use TFS). The test results should somehow be made available in the test run report. The steps needed to achieve this would be something like
this:

1. Write the JavaScript unit tests and place them in an HTML file
2. From a Visual Studio Team System test method, fire up a browser that loads the HTML file and runs the test
3. Fetch the result of the JavaScript unit tests and incorporate them into the test report

script.aculo.us provides the pieces for 1 and 2 above out of the box. It also supports POSTing the results back to a web server using Ajax. That could be an approach that would be a part of the solution for point 3 above. However, I felt that this approach is not an optimal solution. First of all because you would have to to
make the tests available thru a web server, the web server would need to receive the results, and store them somewhere. Then, the Visual Studio test would have to fetch the results and incorporate them. It all adds up to seem quite fragile. It is important that a test run has as few external dependencies as possible. Hence, I
wanted a separate approach. The browser should be able to load the JavaScript tests directly from the file system (file:///…). Then, after running the tests we should be able to traverse the DOM of
the HTML page directly to fetch the results.

As a result, we ended up with this approach:

• Fire up Internet Explorer from .NET via its COM interface (Internet Explorer Object, SHDocVw.dll). You can find code for this here.
• Load the file directly from the file system (file:///…) and execute the tests
• Traverse the HMTL DOM to fetch the test results (using Microsoft HTML Object Library, mshtml.dll)
• Report any error messages back using Assert.fail(”…message…”)