Last weekend, I wrote a more or less casual post on LinkedIn containing the ‘rules’ (it’s more of a list of terms and conditions, really) I set for myself when it comes to using AI. That post received some interesting comments that made me think and refine my thoughts on when (not) to use AI to support me in my work. Thank you to all of you who commented for doing so, and for showing me that there still is value in being active on LinkedIn in between all the AI-generated ‘content’. I really appreciate it. Now, AI and LLMs like ChatGPT or Claude can be very useful, that is, when used prudently. I think it is very important to be conscious and cautious when it comes to using AI, though, which is why I wrote that post. I wrote it mostly for myself, to structure my thoughts around AI, but also because I think it is important that others are at least conscious of what they’re doing and working with. That doesn’t mean you have to adhere to or even agree with my views and the way I use these tools, by the way. Different strokes for different folks. Because of the ephemeral nature of these LinkedIn posts, and the importance of the topic to me, I want to repeat the ‘rules’ (again, more of a T&C list) I wrote down here. This is the original, unchanged list from the post I wrote on February 15: I only use it to support me in completing tasks I understand. I need to be able to scrutinize the output the AI system produces and see if it is both sound and fit for the purpose I want to use it for. I never use it to explain to me something I don’t know yet or don’t understand enough. I have seen and read about too many hallucinations to trust them to teach me what I don’t understand. Instead, I use books, articles, and other content from authors and sources I do trust if I’m looking to learn something new. I never EVER use it for creative work. I don’t use AI-generated images anywhere, and all of my blogs, LinkedIn posts, comments, course material and other written text are 100% my own, warts and all. My views, my ideas, my voice. Interestingly, most of the comments were written in reaction to the first two bullet points at the time I wrote this blog post. I don’t know exactly why this is the case, it might be because the people who read it agree (which I doubt seeing the tsunami of AI-generated content that’s around these days), or maybe because there’s a bit of stigma around admitting to use AI for content generation. I don’t know. What I do know is that it is an important principle to me. I wrote about the reasons for that in an earlier blog post, so I won’t repeat myself here. Like so many terms and conditions, the list I wrote down in this post will probably evolve over time, but what will not change is me remaining very careful around where I use and where I don’t use AI to help me in my work. Especially now that the speed with which new developments in the AI space are presented to us and the claims around what it can and will do only get bigger, I think it is wise to remain cautious and look at these developments with a critical and very much human view.

When I build and release new features or bug fixes for RestAssured.Net, I rely heavily on the acceptance tests that I wrote over time. Next to serving as living documentation for the library, I run these tests both locally and on every push to GitHub to see if I didn’t accidentally break something, for different versions of .NET. But how reliable are these tests really? Can I trust them to pass and fail when they should? Did I cover all the things that are important? I speak, write and teach about the importance of testing your tests on a regular basis, so it makes sense to start walking the talk and get more insight into the quality of the RestAssured.Net test suite. One approach to learning more about the quality of your tests is through a technique called mutation testing. I speak about and demo testing your tests and using mutation testing to do so on a regular basis (you can watch a recent talk here), but until now, I’ve pretty much exclusively used PITest for Java. As RestAssured.Net is a C# library, I can’t use PITest, but I’d heard many good things about Stryker.NET, so this would be a perfect opportunity to finally use it. Adding Stryker.NET to the RestAssured.Net project The first step was to add Stryker.Net to the RestAssured.Net project. Stryker.NET is a dotnet tool, so installing it is straightforward: run dotnet new tool-manifest to create a new, project-specific tool manifest (this was the first local dotnet tool for this project) and then dotnet tool install dotnet-stryker to add Stryker.NET as a dotnet tool to the project. Running mutation tests for the first time Running mutation tests with Stryker.NET is just as straightforward: dotnet stryker --project RestAssured.Net.csproj from the tests project folder is all it takes. Because both my test suite (about 200 tests) and the project itself are relatively small code bases, and because my test suite runs quickly, running mutation tests for my entire project works for me. It still took around five minutes for the process to complete. If you have a larger code base, and longer-running test suites, you’ll see that mutation testing will take much, much longer. In that case, it’s probably best to start on a subset of your code base and a subset of your test suite. After five minutes and change, the results are in: Stryker.NET created 538 mutants from my application code base. Of these: 390 were killed, that is, at least one test failed because of this mutation, 117 survived, that is, the change did not make any of the tests fail, and 31 resulted in a timeout, which I’ll need to investigate further, but I suspect it has something to do with HTTP timeouts (RestAssured.Net is an HTTP API testing library, and all acceptance tests perform actual HTTP requests) This leads to an overall mutation testing score of 59.97%. Is that good? Is that bad? In all honesty, I don’t know, and I don’t care. Just like with code coverage, I am not a fan of setting fixed targets for this type of metric, as these will typically lead to writing tests for the sake of improving a score rather than for actual improvement of the code. What I am much more interested in is the information that Stryker.NET produced during the mutation testing process. Opening the HTML report I was surprised to see that out of the box, Stryker.NET produces a very good-looking and incredibly helpful HTML report. It provides both a high-level overview of the results: as well as in-depth detail for every mutant that was killed or that survived. It offers a breakdown of the results per namespace and per class, and it is the starting point for further drilling down into results for individual mutants. Let’s have a look and see if the report provides some useful, actionable information for us to improve the RestAssured.Net test suite. Missing coverage Like many other mutation testing tools, Stryker.NET provides code coverage information along with mutation coverage information. That is, if there is code in the application code base that was mutated, but that is not covered by any of the tests, Stryker.NET will inform you about it. Here’s an example: Stryker.NET changed the message of an exception thrown when RestAssured.Net is asked to deserialize a response body that is either null or empty. Apparently, there is no test in the test suite that covers this path in the code. As this particular code path deals with exception handling, it’s probably a good idea to add a test for it: [Test] public void EmptyResponseBodyThrowsTheExpectedException() { var de = Assert.Throws<DeserializationException>(() => { Location responseLocation = (Location)Given() .When() .Get($"{MOCK_SERVER_BASE_URL}/empty-response-body") .DeserializeTo(typeof(Location)); }); Assert.That(de?.Message, Is.EqualTo("Response content is null or empty.")); } I added the corresponding test in this commit. Removed code blocks Another type of mutant that Stryker.NET generates is the removal of a code block. Going by the mutation testing report, it seems like there are a few of these mutants that are not detected by any of the tests. Here’s an example: The return statement for the Put() method body, which is used to perform an HTTP PUT operation, is replaced with an empty method body, but this is not picked up by any of the tests. The same applies to the methods for HTTP PATCH, DELETE, HEAD and OPTIONS. Looking at the tests that cover the different HTTP verbs, this makes sense. While I do call each of these HTTP methods in a test, I don’t assert on the result for the aforementioned HTTP verbs. I am basically relying on the fact that no exception is thrown when I call Put() when I say ‘it works’. Let’s change that by at least asserting on a property of the response that is returned when these HTTP verbs are used: [Test] public void HttpPutCanBeUsed() { Given() .When() .Put($"{MOCK_SERVER_BASE_URL}/http-put") .Then() .StatusCode(200); } These assertions were added to the RestAssured.Net test suite in this commit. Improving testability The next signal I received from this initial mutation testing run is an interesting one. It tells me that even though I have acceptance tests that add cookies to the request and that only pass when the request contains the cookies I set, I’m not properly covering some logic that I added: To understand what is going on here, it is useful to know that a Cookie in C# offers a constructor that creates a Cookie specifying only a name and a value, but that a cookie has to have a domain value set. To enforce that, I added the logic you see in the screenshot. However, Stryker.NET tells me I’m not properly testing this logic, because changing its implementation doesn’t cause any tests to fail. Now, I might be able to test this specific logic with a few added acceptance tests, but it really is only a small piece of logic, and I should be able to test that logic in isolation, right? Well, not with the code written in the way it currently is… So, time to extract that piece of logic into a class of its own, which will improve both the modularity of the code and allow me to test it in isolation. First, let’s extract the logic into a CookieUtils class: internal class CookieUtils { internal Cookie SetDomainFor(Cookie cookie, string hostname) { if (string.IsNullOrEmpty(cookie.Domain)) { cookie.Domain = hostname; } return cookie; } } I deliberately made this class internal as I don’t want it to be directly accessible to RestAssured.Net users. However, as I do need to access it in the tests, I have to add this little snippet to the RestAssured.Net.csproj file: <ItemGroup> <InternalsVisibleTo Include="$(MSBuildProjectName).Tests" /> </ItemGroup> Now, I can add unit tests that should cover both paths in the SetDomainFor() logic: [Test] public void CookieDomainIsSetToDefaultValueWhenNotSpecified() { Cookie cookie = new Cookie("cookie_name", "cookie_value"); CookieUtils cookieUtils = new CookieUtils(); cookie = cookieUtils.SetDomainFor(cookie, "localhost"); Assert.That(cookie.Domain, Is.EqualTo("localhost")); } [Test] public void CookieDomainIsUnchangedWhenSpecifiedAlready() { Cookie cookie = new Cookie("cookie_name", "cookie_value", "/my_path", "strawberry.com"); CookieUtils cookieUtils = new CookieUtils(); cookie = cookieUtils.SetDomainFor(cookie, "localhost"); Assert.That(cookie.Domain, Is.EqualTo("strawberry.com")); } These changes were added to the RestAssured.Net source and test code in this commit. An interesting mutation So far, all the signals that appeared in the mutation testing report generated by Stryker.NET have been valuable, as in: they have pointed me at code that isn’t covered by any tests yet, to tests that could be improved, and they have led to code refactoring to improve testability. Using Stryker.NET (and mutation testing in general) does sometimes lead to some, well, interesting mutations, like this one: I’m checking that a certain string is either null or an empty string, and if either condition is true, RestAssured.Net throws an exception. Perfectly valid. However, Stryker.NET changes the logical OR to a logical AND (a common mutation), which makes it impossible for the condition to evaluate to true. Is that even a useful mutation to make? Well, to some extent, it is. Even if the code doesn’t make sense anymore after it has been mutated, it does tell you that your tests for this logical condition probably need some improvement. In this case, I don’t have to add more tests, as we discussed this exact statement earlier (remember that it had no test coverage at all). It did make me look at this statement once again, though, and I only then realized that I could simplify this code snippet to if (string.IsNullOrEmpty(responseBodyAsString)) { throw new DeserializationException("Response content is null or empty."); } Instead of a custom-built logical OR, I am now using a construct built into C#, which is arguably the safer choice. In general, if your mutation testing tool generates several (or even many) mutants for the same code statement or block, it might be a good idea to have another look at that code and see if it can be simplified. This was just a very small example, but I think this observation holds true in general. This change was added to the RestAssured.Net source and test code in this commit. Running mutation tests again and inspecting the results Now that several (supposed) improvements to the tests and the code have been made, let’s run the mutation tests another time to see if the changes improved our score. In short: 397 mutants were killed now, up from 390 (that’s good) 111 mutants survived, down from 117 (that’s also good) there were 32 timeouts, up from 31 (that needs some further investigation) Overall, the mutation testing score went up from 59,97% to 61,11%. This might not seem like much, but it is definitely a step in the right direction. The most important thing for me right now is that my tests for RestAssured.Net have improved, my code has improved and I learned a lot about mutation testing and Stryker.NET in the process. Am I going to run mutation tests every time I make a change? Probably not. There is quite a lot of information to go through, and that takes time, time that I don’t want to spend for every build. For that reason, I’m also not going to make these mutation tests part of the build and test pipeline for RestAssured.Net, at least not any time soon. This was nonetheless both a very valuable and a very enjoyable exercise, and I’ll definitely keep improving the tests and the code for RestAssured.Net using the suggestions that Stryker.NET presents.

This blog post is another one in the ‘writing things down to structure my thinking on where I want my career to go’ series. I will get back to writing technical and automation blog posts soon, but I need to finish my contract testing course first. One of the things I like to do most in life is traveling and seeing new places. Well, seeing new places, mostly, as the novelty of waiting, flying and staying in hotel rooms has definitely worn off by now. I am in the privileged position (really, that is what it is: I’m privileged, and I fully realize that) that I get to scratch this travel itch professionally on a regular basis these days. Over the last few years, I have been invited to contribute to meetups and conferences abroad, and I also get to run in-house training sessions with companies outside the Netherlands a couple of times per year. Most of this traveling takes place within Europe, but for the last three years, I have been able to travel outside of Europe once every year (South Africa in 2022, Canada in 2023 and the United States in 2024), and needless to say I have enjoyed those opportunities very much. To give you an idea of the amount of traveling I do: for 2025, I now have four work-related trips abroad scheduled, and I am pretty sure at least a few more will be added to that before the year ends (it’s only just February…). That might not be much travel by some people’s standards, but for me, it is. And it seems the number of opportunities I get for traveling increase year over year, to the point where I have to say ‘no’ to several of these opportunities. Say no? Why? I thought you just said you loved to travel? Yes, that’s true. I do love to travel. But I also love spending time at home with my family, and that comes first. Always. Now, my sons are getting older, and being away from home for a few days doesn’t put as much pressure on them and on my wife as it did a few years ago. Still, I always need to find a balance between spending time with them and spending time at work. I am away from home for work not just when I’m abroad. I run evening training sessions with clients here in the Netherlands on a regular basis, too, as well as training sessions in my evenings for clients in different time zones, mainly US-based clients. And all that adds up. I try to only be away from home one night per week, but often, it’s two. When I travel abroad, it’s even more than that. Again, I’m not complaining. Not at all. It is an absolute privilege to get to travel for work and get paid to do that, but I cannot do that indefinitely, and that’s why I have made a decision: With a few exceptions (more on those below), I am going to say ‘no’ to conferences abroad from now on. This is a tough decision for me to make, but sometimes that’s exactly what you need to do. Tough, because I have very fond memories of all the conferences and meetups abroad I have contributed to. My first one, Romanian Testing Conference in 2017. My first keynote abroad, UKStar in 2019. My first one outside of Europe, Targeting Quality in 2023. They were all amazing, because of the travel and sightseeing (when time allowed), but also because of all the people I have met at these conferences. Yet, I can meet at least some of these people at conferences here in the Netherlands, too. Test Automation Days, the TestNet events, the Dutch Testing Day and TestMass all provide a great opportunity for me to catch up with my network. Sometimes, international conferences come to the Netherlands, too, like AutomationSTAR this year. And then there are plenty of smaller meetups here in the Netherlands (and Belgium) where I can meet and catch up with people as well. Plus, the money. I am not going to be a hypocrite and say that money doesn’t play into this. For the reasons mentioned above, I have a limited number of opportunities to travel every year, and I prefer to spend those on in-house training sessions with clients abroad, simply because the pay is much better. Even when a conference compensates flights and hotel (as they should) and offer a speaker or workshop facilitator fee (a nice bonus), it will be significantly less of a payday than when I run a training session with a client. That’s not the fault of those conferences, not at all, especially when they’re compensating their speakers fairly, but this is simply a matter of numbers and budgets. At the moment, I have one, maybe two contributions to conferences abroad coming up, and I gave them my word, so I’ll be there. That’s the SAST 30-year anniversary conference in October, plus one other conference that I’m talking to but haven’t received a ‘yes’ or ‘no’ from yet. Other than that, if conferences reach out to me, it’s likely to be a ‘no’ from now on, unless: the event pays a fee comparable to my rate for in-house training I can combine the event with paid in-house training (for example with a sponsor) it is a country or region I really, really want to visit, either for personal reasons or because I want to grow my professional network there I don’t see the first one happening soon, and the list of destinations for the third one is very short (Norway, Canada, New Zealand, that’s pretty much it), so unless we can arrange paid in-house training alongside the conference, the answer will be a ‘no’ from me. Will this reduce the number of travel opportunities for me? Maybe. Maybe not. Again, I see the number of requests I get for in-house training abroad growing, too, and if that dies down, it’ll be a sign for me that I’ll have to work harder to create those opportunities. For 2025, things are looking pretty good, with trips for training to Romania, North Macedonia and Denmark already scheduled, and several leads for more in the pipeline. And if the number of opportunities does go down, that’s fine, too. I’m happy to spend that time with family, working on other things, or riding my bike. And I’m sure there will be a few opportunities to speak at online meetups, events and webinars, too.

As a (sort of) follow-up post to my yearly review for 2024, in this post, I would like to go over the changes, bug fixes and new features that have been introduced in RestAssured .NET in 2024. This year, I released 7 new versions of the library, and while none of the versions included changes that were worthy of a blog post on its own, I thought it would be a good idea to wrap them all up in a single overview. Basically, this blog post is an extended version of the library’s CHANGELOG. I’ll go through the new versions chronologically, starting with the first release of 2024. Version 4.2.2 - released April 23 RestAssured .NET 4.2.2 fixes a bug that prevented JSON responses that are an array to be properly verified. In other words, if the JSON response body looks like this: [ { "id": 1, "text": "Do the dishes" }, { "id": 2, "text": "Clean out the trash" }, { "id": 3, "text": "Read the newspaper" } ] I would expect this test to pass: [Test] public void JsonArrayResponseBodyElementCanBeVerifiedUsingNHamcrestMatcher() { Given() .When() .Get("http://localhost:9876/json-array-response-body") .Then() .StatusCode(200) .Body("$[1].text", NHamcrest.Is.EqualTo("Clean out the trash")); } but prior to this version, it threw a Newtonsoft.Json.JsonReaderException. The solution? Adding a try-catch that first tries to parse the JSON response as a JObject (equal to existing behaviour), catch the JsonReaderException and try again, now parsing the JSON response into a JArray. That made the newly added test pass without failing any other tests. Another demonstration of the added value of having a decent set of tests. RestAssured .NET is slowly growing and becoming more complex, and having a test suite I can run locally, and that always runs when I push code to GitHub is an invaluable safety net for me. These tests run in a few seconds, yet they give me invaluable feedback on the effect of new features, bug fixes and code refactoring efforts. I haven’t heard back from the person submitting the original issue, but I assume that this fixed their issue. Version 4.3.0 - released August 16 I love learning about how people use RestAssured .NET, because invariably they will use it in ways I haven’t foreseen. I was unfamiliar with the concept of server-sent events (SSE) in APIs, for example, yet there are people looking to test these kinds of APIs using RestAssured .NET. It turned out that what this user was looking for was a way to set the HttpCompletionOption value on the System.Net.Http.HttpClient that is wrapped by RestAssured .NET. To enable this, I added a method to the DSL that looks like this: Given() .UseHttpCompletionOption(HttpCompletionOption.ResponseHeadersRead) I also added the option to specify the HttpCompletionOption to be used in a RequestSpecification as well as in the global config. A straightforward fix that solved the problem for this specific user. The only thing I don’t like here is that I don’t know of a way to test this locally. Do you? I would love to hear it. Version 4.3.1 - release August 22 Another user pointed out to me that trying to verify that the value of a JSON response body element is an empty array also threw an exception. So, if the JSON response body looks like this: { "success": true, "errors": [] } this test should pass, but instead it threw a Newtonsoft.Json.JsonSerializationException: [Test] public void JsonResponseBodyElementEmptyArrayValueCanBeVerifiedUsingNHamcrestMatcher() { Given() .When() .Get("http://localhost:9876/json-empty-array-response-body") .Then() .StatusCode(200) .Body("$.errors", NHamcrest.Is.OfLength(0)); } The fix? Adding some code that checks if the element returned when evaluating the JsonPath expression is a JArray or a JObject and using the right matching logic accordingly. I used my preferred procedure here: first, write a failing test that reproduces the issue then, make the test pass without breaking any other tests refactor the code, document and release Does this procedure sound familiar to you? Version 4.4.0 - released October 21 As you can probably tell from the semantic versioning, this version introduced a new feature to RestAssured .NET: the ability to use NTLM authentication when making an HTTP call. To enable this, I added a new method to the DSL: Given() .NtlmAuth() // This one uses default NTLM credentials for the current user .NtlmAuth("username", "password", "domain") // This one uses custom NTLM credentials As I had no idea how to write a proper test for this, even though I had tested it before releasing using Fiddler, I released a beta version first that the person submitting the issue could use to verify the solution. I’m happy to say that it worked for them and that the solution could be released properly. Again, if someone can think of a way to add a proper test for NTLM authentication to the test suite, I would love to hear it. All that the current tests do is run the code and see if no exception is thrown. Not a good test, but until I find a better way, it will have to do. Version 4.5.0 - released November 19 This version introduced not one, but two changes. First, since .NET 9 was officially released earlier that week (or maybe the week before, I forgot), I needed to release a RestAssured .NET version that targets .NET 9, so I did. Just like with .NET 8, I didn’t really have to change anything to the code other than adding net9.0 to the TargetFrameworks and add .NET 9 to the build pipeline for the library to make sure that every change is tested on .NET 9, too. Happy to say it all ‘just worked’. The other change took more effort: a user reported that they could not override the ResponseLogLevel set in a RequestSpecification at the individual test level. The reason? In the existing code, the response was logged directly after the HTTP call completed, so before any calls to Log() for the response. When Log() is called on the response, it was then logged again. I have no idea how I completely overlooked this until now, but I did. Rewriting the code to make this work took longer than I expected, but I managed in the end, through quite a bit of trial and error and lots of humand-centered testing (again, no idea how to write automated tests for this). The logging functionality of RestAssured .NET is something I intend to rewrite in the future, for a couple of reasons: It’s impossible to write automated tests for it (or at least I don’t know how to do this) Ideally, I want the logging to be more configurable and extensible to give users more flexibility than they have at the moment Version 4.5.1 - released November 20 As one does, I found an issue with the updated logging logic almost immediately after releasing 4.5.0 to the public: masking of sensitive headers and cookies didn’t work anymore when specified as part of a RequestSpecification. Lucky for me, this was a quick fix, but a bit embarrassing nonetheless. Had I had proper automated tests for the logging in place, I probably would have caught this before releasing 4.5.0…. Anyway, it’s fixed now, as far as I can tell. Version 4.6.0 - released December 9 The final RestAssured .NET release of 2024 added the capability to strip the ; charset=<some_charset> from the Content-Type header in a request. It turns out, some APIs explicitly expect this header to not contain the charset suffix, but the way I create a request, or rather, the way .NET creates a StringContent object, will add it by default. This issue was a great example of one of the main reasons why I started this project: there is so much I don’t know yet about HTTP, APIs, C#/.NET and other technologies, and working on these issues and improving RestAssured .NET gives me an opportunity to learn them. I make a habit of writing what I learned down in the issue on GitHub, so I can review it later, and so I can point others to these links and thoughts, too. So, if you’re looking for a way to strip the charset identifier from the Content-Type header in the request, you can now do that by passing an optional second boolean argument to Body() (defaults to false): Given() .Body(your_body_goes_here, stripCharset: true) That’s it! As you can see, lots of small changes, bug fixes and new features have been added to RestAssured .NET this year. Oh, and before I forget: with every release, I also made sure to update the dependencies I use to create and test RestAssured .NET to their latest versions. I consider that good housekeeping, and it’s all part of keeping a library up to date. I am looking forward to seeing the library evolve and improve further in 2025.