--- title: "Testing Legacy Code with Typemock Isolator Deep Dive" url: "https://courses.osherove.com/courses/the-art-of-unit-testing-and-tdd-master-class-in-net/testing-legacy-code-with-typemock-isolator-deep-dive.md" canonical: "https://courses.osherove.com/courses/the-art-of-unit-testing-and-tdd-master-class-in-net/testing-legacy-code-with-typemock-isolator-deep-dive/" tags: [] author: "Roy Osherove" description: "Lesson 11 of 14 in The Art of Unit Testing & TDD Master Class in .NET (Legacy Code), 20:08. Free video by Roy Osherove." usage: "Licensed under CC BY 4.0 — reuse, quote and adapt with attribution (Roy Osherove, link to the original page)." license: "https://creativecommons.org/licenses/by/4.0/" --- # Testing Legacy Code with Typemock Isolator Deep Dive 1. [Courses](https://courses.osherove.com/#courses) 2. [The Art of Unit Testing & TDD Master Class in .NET](https://courses.osherove.com/courses/the-art-of-unit-testing-and-tdd-master-class-in-net/) 3. Legacy Code Lesson 11 of 14 Duration: 20:08 [Watch on YouTube](https://www.youtube.com/watch?v=vst-gq7-iqs) Roy Tests BlogEngine.NET with Isolator ## In this lesson Roy Osherove tests the same blog engine code with [Typemock Isolator](https://osherove.com/blog/2009/7/9/minimizing-unit-test-fragility-8-features-in-typemock-isolat.html "Minimizing unit Test Fragility – 8 features in Typemock Isolator to help (osherove.com)"), explaining why Visual Studio must be restarted for a [profiler-based framework](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-6#ch06lev1sec1 "Constrained and unconstrained frameworks (The Art of Unit Testing, 2nd ed., ch. 6)") and what the Isolated attribute does. He fakes the static methods of the trackback class to verify that Send was called, then follows each failure, which he calls [wishful invocation](https://osherove.com/blog/2010/3/19/code-excavations-wishful-invocations-perimeters-and-domain-s.html "Code excavations, wishful invocations, perimeters and domain specific unit test frameworks (osherove.com)"), faking a non-public static method, the blog settings singleton and all instances of a remote file class. The test passes without any debugging, and he asks the audience to get to the same point. ## Transcript 2,283 words [\[00:26\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=26s) So now let's look at the blog engine, but this time we're going to try to do it with [Typemock Isolator](https://osherove.com/blog/2006/9/27/using-typemock-vs-design-for-testability.html "Using TypeMock vs. Design for testability (osherove.com)"). So I'm going to show some of the APIs we can use and some of the thought process. We're not going to solve it fully. We're just going to start working with it. So our problem is first of all to to start using Typemock Isolator, I installed it and restarted Visual Studio. It's important to restart Visual Studio because because it's a profilerbased framework. The only way a profiler works is if you have an environment variable in the process where the tests run. [\[01:03\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=63s) And what isolator and [Moles](https://osherove.com/blog/2010/5/25/justmock-and-moles-a-short-overview-for-tdd-alpha-geeks.html "JustMock and Moles – A short overview for TDD alpha geeks (osherove.com)") and JustMock do they install a plugin into Visual Studio when loaded simply loads an environment variable. And so all the tests in Visual Studio run with that with the Visual Studio environment variables. And so they run with profiler enabled. If I didn't do this then my test will just fail because the profiler doesn't work. And then I added two references. Basically I went and said right click add reference. [\[01:36\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=96s) I looked for T for Typemock and I need the C# APIs and the core DL. So just this these two. If you're looking for them, they're in the Typemock Isolator 62 range act and assert and Typemock dl. Next thing I do is I created a in the [unit test](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-1#sigil_toc_id_17 "Defining unit testing, step by step (The Art of Unit Testing, 3rd ed., ch. 1)") project I added the class called manager tests [\[02:06\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=126s) And I added the isolated attribute. The isolate attribute makes sure that if that if I set static behavior in different locations in different tests then they get cleaned up between each test run. If we forget to do this our test can start behaving in a weird way. I could have instead do done just isolated at the test level and then I would have to remember to do it for each test. If I do it at the class level, it works for all the tests in the class. [\[02:43\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=163s) So this is the beginning of the test that I that I that I wrote. If I just want to call manager send I'm I'm still still going to call send. So we'll have this dependency and everything. [\[03:15\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=195s) If I was to call manager.send, I need to create a [fake](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-5#ch05lev1sec2 "Dynamically creating a fake object (The Art of Unit Testing, 2nd ed., ch. 5)") publishable bull item. So I can just call fake item here and also send in a URI. So we're just going to send in some URI here. [\[03:47\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=227s) Equals. And now I'm going to use isolator to fake it. Isolate fake an instance of I publishable. That's it. Now, if I just do this, what is my end? What should be my [end result](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-1#sigil_toc_id_19 "Exit point types (The Art of Unit Testing, 3rd ed., ch. 1)") here? What I'd like to do is I'd like to be able to ch make sure that trackback.sam got called because that's one end result. Another end result would be to track that pingback. [\[04:19\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=259s) So let's start with this. I want to track that trackback. So I should start tracking it by saying isolate fake static methods of type trackback. And the reason I'm not saying just a specific method I could have done this isolate fake or sorry isolate when called trackback send [\[04:52\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=292s) With anything ignore since it has to return something we can just say return [recursive fake](https://osherove.com/blog/2008/10/4/isolator-feature-focus-recursive-fakes.html "Isolator feature focus: Recursive Fakes (osherove.com)"). It's the same as saying ignore call. Just return whatever it needs to return. The reason I'm not doing this and remember I'm sending in here null because by default isolator doesn't care about parameters. If I wanted it to care about the parameter I would have to say with exact arguments [\[05:24\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=324s) And then it would care. Right now it doesn't care about the parameters. So it always returns a recursive fake. In this case it returns always a default value. I could have done this, but I might as well fake all the static methods of trackback if I don't care about it really because it's a different class and it's a different location. I might as well fake all of them because I don't care about any of these static. So, I'm just going to do that. [\[05:57\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=357s) Now, if I wanted to verify it got called, I can say isolate.verify And then I can choose for example I can verify that something non-public got called or if it's public I can say it was called for now we can say with any arguments trackback. And if it didn't get called my test will fail. So if I just run this, [\[06:46\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=406s) Okay. Now just to prove that we can get this to to be called. Even if I in my test I call it directly just to prove a point can say trackback. send with anything or just a fake item. Who cares? So, trackback message, sorry. New track back message with the fake item and the new URI [\[07:17\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=437s) With something. Did I miss anything? Too many braces. H >> the constructor has three parameters. >> Oh yeah, it has two URIs, right? Yeah. And again a new URI. So I'm just going to do this for now. [\[07:50\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=470s) Ctrl + D to duplicate. And hopefully now we will end up with something. Or you know what? just say this. Well, that's it because it doesn't matter for our demo purposes. I'm going to repeat the test. I pressed alt R and you can see here one passed. So, it actually got called. I remove this. [\[08:23\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=503s) I alt R again and the test fails. Okay. So now that we can track it, we can start doing interesting things. If we actually run this, our test fails because it didn't get called. But now we can go deep and find out why it's not called. What should happen? If you go into manager send, [\[08:54\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=534s) We'll see that somewhere we have a get URLs from content. Okay. Now, right now it's protected virtual because I I I I refactored it. What if I turn it back into static? Private static. Get yours from content. And just repeat the test for now. Build succeeded. Still failed. Okay, if I wanted to control get URLs from content so that I get back some URL because it's non-public, [\[09:31\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=571s) I can either turn it into public or I can use one of the these APIs. Isolate when non-public dot when called. Now the target object is there is no instance because it's a static method. So it's a type you can just say type of manager [\[10:03\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=603s) With the method name of get get urls from content and I can also tell it what to return when called will return. And now I can return a new list of URI URIs. So if it's expecting get URLs from content which is an innumerable URI [\[10:34\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=634s) We can say list and create a variable of list of URI equals new list of URI and initialize it with a couple of them or maybe just I http [\[11:11\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=671s) So we have this new list. We don't even need this for now. This list is going to be returned. I just make it even more readable. I think right now get else from content will return list. Now let's see what happens. So now I get a a weird stuff. System null reference exception object reference not sent to an instance on of an object. If I look at the stack trace I can see blog engine core get current instance. [\[11:41\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=701s) If I go back double click here blog settings.get instance and then somewhere here I have if blog settings instance enable trackback send. So I've come across another dependency and this is a very common pattern of work in [legacy code](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-12 "Working with legacy code (The Art of Unit Testing, 3rd ed., ch. 12)") that we've talked about which I call the [wishful invocation](https://osherove.com/blog/2010/3/19/code-excavations-wishful-invocations-perimeters-and-domain-s.html "Code excavations, wishful invocations, perimeters and domain specific unit test frameworks (osherove.com)"). So again basically what I'm doing is I'm trying to write my test here [\[12:13\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=733s) And then I invoke something and I wish that it works. If it does it's great. If it doesn't, then I look up at the track race trace. I see what I missed and I continue faking away until I can actually get through the path that I'm working on. So if I've realized that this is a dependency, if I didn't have isolator, I would have to [extract and override](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-3#ch03lev1sec5 "Variations on refactoring techniques (The Art of Unit Testing, 2nd ed., ch. 3)") this or something. But here now what I can do, I have several choices. I can just say [\[12:46\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=766s) Isolate when called blah blah blah ignore call or return recursive fake or I can just say will return true because I wanted to enable trackback send and this will take care of everything. If I didn't care about the result, I could have just faked all the static methods on the type of blog settings. [\[13:18\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=798s) That would make things even better for me because then I wouldn't care about each specific method. But right now, what you can see is that I can fake a chain of things. Blog settings has an instance and that will be fake by default. So if I run this again, no debugging yet. I look at the output and now I have another exception. The remote name could not be resolved. [\[13:49\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=829s) Something.com get response. So it's actually going out to the web and trying to download something because it's going through a remote file. So if I go here, it's saying remote file get file as string and then get trackback URL from page. So these two lines are actually a dependency as well. These are actually downloading the the content of the file. So how do I take over instances of things that are mued up? [\[14:22\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=862s) One way to do it would be to just use the isolator future syntax. So we can say we could create a fake instance of remote file. Or so the first way would be to say isolate fake instance of remote file file. Now remote file currently is internal. [\[14:54\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=894s) I'm going to make it public for demo purposes. But in real life, what you would probably do is you would use the [InternalsVisibleTo attribute](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-3#ch03lev1sec6 "Overcoming the encapsulation problem (The Art of Unit Testing, 2nd ed., ch. 3)") in the in the production code so that your test will see it as if it was public. So isolate fake instance of remote file. Let's get back the instance fake remote file. And now I can say isolate fake swap [\[15:26\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=926s) All instances of remote file with the fake file. I could have done instead isolate swap all instances on remote file with a recursive fake and then I wouldn't need the file at all. So this is like a shortcut. If I don't care about the specific behavior of the fake object, [\[15:58\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=958s) Then I can just say, "Look, fake an object for me and replace all the future objects with a fake one that just does all the defaults. If I care about how that fake would behave, then I would fake it on my own and I would tell it to replace it with my own." Okay, so what happens if I just fake it with with a recursive fake? I run the test again, go through the output, I get a verify exception was expected but was not called. Now I'm not getting an exception. [\[16:30\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=990s) So I'm not I don't know what the dependency is or not, but I might be just not going through the path as as as well as I thought. So now might be at the time to either debug or if I look at this get trackback URL from page it might be that it's getting a page content which is empty from that fake file. So what I would want to do maybe is to say get track URL from page. I can either go there and [\[17:04\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1024s) Make it match a specific regular expression as the fake content. And let's see what the regular expression is. It looks for trackback.ping blah blah blah blah blah blah blah. And what if it's too complicated? What if creating the content that is so complicated for this is actually has become so complicated it's a dependency. Well, in that case I can just fake the validation of the regular expression. [\[17:38\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1058s) So instead I'm just going to fake this get track URL from page is going to return some kind of non-null trackback URL. Okay. So this is going to I'm going to make this return non trackback URL. And this was initially static as well. So I'm going to make it static back again. Private static. So again I'm going to go here and say let's duplicate this and say when called on manager [\[18:09\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1089s) What was the name of it? Get track your from page. We'll return new what was it? What's supposed to happen? Supposed to get a track URL. What's the type URI? Okay, it's going to use that URI. So, I'm just going to send in a new URI [\[18:44\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1124s) URI that is keep forgetting the HTTP I don't know why must be a psychological thing. something.com. So now get track URL from page is going to return something that's not null. Repeat the test. Alt R. Let's see the output. And now it actually succeeded. And I didn't do even a single bit of debugging yet. [\[19:14\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1154s) Okay. But at some point in legacy code, I would get into the idea of debugging. Sometimes you do have to debug. Are there any questions so far? So I didn't touch the code except actually make it harder to test so that it would feel a bit more like legacy. Are there any questions so far? Okay. So, what I want you to do, I want you to install Typemock Isolator. And I want you to get to this stage [\[19:45\]](https://www.youtube.com/watch?v=vst-gq7-iqs&t=1185s) Where you can actually prove that trackback.send got called. Okay, ## Related - [Important tools for legacy code unit testing](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-10#ch10lev1sec4) · The Art of Unit Testing, 2nd ed., ch. 10 The 2nd edition's section on important tools for legacy code unit testing, including profiler-based isolation tools like Isolator. - [Video: Typemock Basics - Lesson 1 (Dependency breaking 101)](https://osherove.com/blog/2008/1/21/video-typemock-basics-lesson-1-dependency-breaking-101.html) · osherove.com Roy's short screencast on breaking dependencies in legacy code with Typemock, the same tool used in this deep dive. - [Easier legacy unit testing with TypeMock.NET](https://osherove.com/blog/2005/3/20/easier-legacy-unit-testing-with-typemocknet.html) · osherove.com Roy's early post on using TypeMock to test legacy code that can't easily be refactored. - [Isolator feature focus: Recursive Fakes](https://osherove.com/blog/2008/10/4/isolator-feature-focus-recursive-fakes.html) · osherove.com Explains Isolator's recursive fakes, which return fake objects from properties as the lesson's fakes do. ### Also linked in this lesson #### The Art of Unit Testing, 3rd ed. - [Defining unit testing, step by step](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-1#sigil_toc_id_17), ch. 1 - [Exit point types](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-1#sigil_toc_id_19), ch. 1 - [Working with legacy code](https://livebook.manning.com/book/the-art-of-unit-testing-third-edition/chapter-12), ch. 12 #### The Art of Unit Testing, 2nd ed. - [Constrained and unconstrained frameworks](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-6#ch06lev1sec1), ch. 6 - [Dynamically creating a fake object](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-5#ch05lev1sec2), ch. 5 - [Variations on refactoring techniques](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-3#ch03lev1sec5), ch. 3 - [Overcoming the encapsulation problem](https://livebook.manning.com/book/the-art-of-unit-testing-second-edition/chapter-3#ch03lev1sec6), ch. 3 #### osherove.com - [Minimizing unit Test Fragility – 8 features in Typemock Isolator to help](https://osherove.com/blog/2009/7/9/minimizing-unit-test-fragility-8-features-in-typemock-isolat.html) - [Code excavations, wishful invocations, perimeters and domain specific unit test frameworks](https://osherove.com/blog/2010/3/19/code-excavations-wishful-invocations-perimeters-and-domain-s.html) - [Using TypeMock vs. Design for testability](https://osherove.com/blog/2006/9/27/using-typemock-vs-design-for-testability.html) - [JustMock and Moles – A short overview for TDD alpha geeks](https://osherove.com/blog/2010/5/25/justmock-and-moles-a-short-overview-for-tdd-alpha-geeks.html) [Previous lesson Static Forwarding on a real legacy app](https://courses.osherove.com/courses/the-art-of-unit-testing-and-tdd-master-class-in-net/static-forwarding-on-a-real-legacy-app/) [Next lesson Code Excavations and Domain Specific Test Frameworks](https://courses.osherove.com/courses/the-art-of-unit-testing-and-tdd-master-class-in-net/code-excavations-and-domain-specific-test-frameworks/) --- ## About the author **Roy Osherove**: Senior Solutions Architect at AWS working on AI for production and AI-driven development; author of The Art of Unit Testing and Elastic Leadership, with about 20 years of consulting and training on unit testing, TDD, team leadership and continuous delivery. ### Sites - [RobotPaper](https://robotpaper.ai/): This blog: AI patterns and anti-patterns, AI and coding agents, and engineering leadership. - [osherove.com](https://osherove.com): Roy's personal site. Author of The Art of Unit Testing and Elastic Leadership; consulting and training on unit testing, TDD, team leadership and continuous delivery, plus about 20 years of blog posts. - [The Art of Unit Testing](https://www.artofunittesting.com): The Art of Unit Testing, 3rd Edition (Manning): readable, maintainable, trustworthy unit tests, plus TDD workshops in C#, JavaScript, Java and Kotlin. - [5whys.com](https://www.5whys.com): Roy's writing on software team leadership: self-organizing teams, surviving chaos, learning mode and the Elastic Leadership model. - [Pipeline Driven](https://pipelinedriven.org): Continuous delivery and DevOps practices: build patterns, pipeline-friendly testing, DevOps metrics and enterprise DevOps. - [Elastic Leadership](https://www.elasticleadership.com): Elastic Leadership, Roy's framework for software team leaders (survival, learning and self-organizing modes). ### Social - X: https://x.com/royosherove - LinkedIn: https://www.linkedin.com/in/osherove/ ### Contact - Contact form: https://forms.gle/jQSx5KoXvrVA1JCK7 ### License Licensed under CC BY 4.0 — reuse, quote and adapt with attribution (Roy Osherove, link to the original page). License: https://creativecommons.org/licenses/by/4.0/