Verifying code and testing with Pex

Pex, Automated White box testing for .NET

Earlier this week, Katrien posted an update on the list of Belgian TechDays 2009 speakers. This post featured a summary on all sessions, of which one was titled “Pex – Automated White Box Testing for .NET”. Here’s the abstract:

“Pex is an automated white box testing tool for .NET. Pex systematically tries to cover every reachable branch in a program by monitoring execution traces, and using a constraint solver to produce new test cases with different behavior. Pex can be applied to any existing .NET assembly without any pre-existing test suite. Pex will try to find counterexamples for all assertion statements in the code. Pex can be guided by hand-written parameterized unit tests, which are API usage scenarios with assertions. The result of the analysis is a test suite which can be persisted as unit tests in source code. The generated unit tests integrate with Visual Studio Team Test as well as other test frameworks. By construction, Pex produces small unit test suites with high code and assertion coverage, and reported failures always come with a test case that reproduces the issue. At Microsoft, this technique has proven highly effective in testing even an extremely well-tested component.”

After reading the second sentence in this abstract, I was thinking: “SWEET! Let’s try!”. So here goes…

Getting started

First of all, download the academic release of Pex at After installing this, Visual Studio 2008 (or 2010 if you are mr. or mrs. Cool), some context menus should be added. We will explore these later on in this post.

What we will do next is analyzing a piece of code in a fictive library of string extension methods. The following method is intended to mimic VB6’s Left method.


/// <summary>
/// Return leftmost characters from string for a certain length
/// </summary>
/// <param name="current">Current string</param>
/// <param name="length">Length to take</param>
/// <returns>Leftmost characters from string</returns>
public static string Left(this string current, int length)
    if (length < 0)
        throw new ArgumentOutOfRangeException("length", "Length should be >= 0");

    return current.Substring(0, length);


Great coding! I even throw an ArgumentOutOfRangeException if I receive a faulty length parameter.

Pexify this!

Analyzing this with Pex can be done in 2 manners: by running Pex Explorations, which will open a new add-in in Visual Studio and show me some results, or by generating a unit test for this method. Since I know this is good code, unit tests are not needed. I’ll pick the first option: right-click the above method and pick “Run Pex Explorations”.

Run Pex Explorations

A new add-in window opens in Visual Studio, showing me the output of calling my method with 4 different parameter combinations:

Pex Exploration Results

Frustrated, I scream: “WHAT?!? I did write good code! Pex schmex!” According to Pex, I didn’t. And actually, it is right. Pex explored all code execution paths in my Left method, of which two paths are not returning the correct results. For example, calling Substring(0, 2) on an empty string will throw an uncaught ArgumentOutOfRangeException. Luckily, Pex is also there to help.

When I right-click the first failing exploration, I can choose from some menu options. For example, I could assign this as a task to someone in Team Foundation Server.

Pex Exploration Options In this case, I’ll just pick “Add precondition”. This will actually show me a window of code which might help avoiding this uncaught exception.

Preview and Apply updates

Nice! It actually avoids the uncaught exception and provides the user of my code with a new ArgumentException thrown at the right location and with the right reason. After doing this for both failing explorations, my code looks like this:


/// <summary>
/// Return leftmost characters from string for a certain length
/// </summary>
/// <param name="current">Current string</param>
/// <param name="length">Length to take</param>
/// <returns>Leftmost characters from string</returns>
public static string Left(this string current, int length)
    // <pex>
    if (current == (string)null)
        throw new ArgumentNullException("current");
    if (length < 0 || current.Length < length)
        throw new ArgumentException("length < 0 || current.Length < length");
    // </pex>

    return current.Substring(0, length);


Great! This should work for any input now, returning a clear exception message when someone does provide faulty parameters.

Note that I could also run these explorations as a unit test. If someone introduces a new error, Pex will let me know.

More information

More information on Pex can be found on

kick it on

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Leave a Comment


8 responses

  1. Avatar for Sven
    Sven January 7th, 2009

    Nice post - keep it up ;)

  2. Avatar for maartenba
    maartenba January 7th, 2009

    I sure will!

  3. Avatar for rowan
    rowan January 7th, 2009

    looks like a handy tool, thanks for posting.

  4. Avatar for Dave Van den Eynde
    Dave Van den Eynde January 15th, 2009

    Does anyone else feel this violates the whole purpose of TDD?

  5. Avatar for maartenba
    maartenba January 15th, 2009

    I think this just adds an extra layer of verification on your code. In the example I gave, TDD would perhaps not provide me with a test that checks for a ceratin Exception but instead only tests the happy path.

  6. Avatar for Matt Penner
    Matt Penner January 17th, 2009

    I'm definitely a TDD yourself type guy. I don't like generated tests, but this looks like a great helper. As Maarten said this gives you an extra layer. I would definitely write the tests yourself, but run this to see if you missed anything. Self created tests can only test what you think to test.

    However, as Maarten so cleverly displayed, you still need to look at the entire procedure as a whole. Maarten, after you incorporated Pex's (can I say that?) additions your initial exception block at the end will never be used. This can be removed. ReSharper probably would have caught this (wow, tools are replacing all our intelligence huh? j/k).

    So, refactor your code after running Pex.

  7. Avatar for maartenba
    maartenba January 17th, 2009

    Have removed that check, indeed looked over it. Had no Resharper at hand...

  8. Avatar for Gary Evans
    Gary Evans January 28th, 2009

    One of the comments above was worried that this violates the whole idea of TDD - I've found it the exact opposite!
    Using Pex, you can structure your tests to be more general statements, and get the benefit of TDD but catch unexpected bugs.

    Check out my blog post on writing theories using Pex: http://taumuon-jabuka.blogs...

    Cheers, Gary