OK, so I have been trying to look at information on testing, different testing libraries what not and so forth.
It seems to me that people always clearly define the difference of these test as one being high level and one being low level. And that a properly tested out application would contain both style tests as well as integration test, etc. etc.
But it seems like every article about the test types ends with something like "in practice it could be hard to see what the difference actually is". I find it weird that people seem so preachy that you must have both test to even reach any sort of full code coverage and at the same time not have any good info/examples on what each might look like.
I am asking because I am starting a new project, that promises to be much bigger and more involved than anything i have done in the past. I would like to keep a good workflow with my test and make sure I am not creating gaps in my testing as I go (in the past projects have been small, and any gaps I might have had don't seem to cause any major issues in production that weren't simple t0 correct)
I know that it seems a good Acceptance Test, naturally leads you to your unit test and once you get it, it is this magical thing that happens and your development life because just that much more blissful.
Anyway, does anyone know of any good discussions on getting started on a good testing workflow, one that talks about moving between your acceptance/integration tests to you unit test and what not.
example for .net would be great, but since most testing frameworks cucumber(gherkin)/rspec etc. are all meant to be fairly readable any example should be good.
"in practice it could be hard to see what the difference actually is":
I can only give general testing advice. Big software projects normally involve some kind of hierarchy.
In the developing phase developers do normally white box tests for small units (unit tests). Knowledge of internals can be used here to reduce the number of test cases (if two functions are orthogonal, you don't need to test all possible combinations). This is the lower end in the testing hierarchy.
In integration software components are put together and different persons (testers) test only the behaviour at the interfaces (black box testing). These persons don't have knowledge of internals, just the specs of the interfaces.
System integration combines even bigger systems.
and so on.
In order for testing to be effective, each level relies on fully pretested components tested in a lower level. Otherwise a higher testing level would have to include basic functionality of its components, but can only use black box testing (which has to assume the worst case, that is dependencies between features to test). So the number of test cases is increasing exponentially, making testing with full coverage impossible.
The hierarchical approach optimizes test efforts here.
Refer to the video on different types of testing. Different types of test techniques are compared with a table.
At work, we have a three-tier product. There is a client application which the users use and it queries data from a server which forwards those requests to a SQL database. We don't allow the client to have direct access to the SQL server.
The client product is what I'm wanting to unit test, but it has over 1.2 million lines of C# code and is a very old product. It was not designed with unit testing in mind and the lead developers for this product are generally opposed to unit testing mostly because of risk vs reward concerns, as well as how redesign would be required to reduce the amount of mocking that would need to be done. The redesign of these core, low-level client libraries and objects also has them concerned.
My philosophy is to certainly never neglect unit testing (because we'll always be too busy for it, and it'll always seem risky, and thus will never ever get done) and take an iterative approach to implementing unit tests.
I'm interested in hearing solutions to this situation. I'm sure many of you have encountered the situation of having to add unit testing into existing infrastructure. How could unit tests be added iteratively into the code base without hindering productivity and release cycles?
In situations like this (and we've in fact been undergoing the same process with an old webforms to MVC transition) is to simply start testing new code. Over time, eventually that old code will be rewritten or refactored.
Before 'new' code is considered valid, it must be unit tested and code reviewed. Over time, eventually you will find that more and more of your solution is now under test, and less and less old code is being called.
I found Michael Feathers' Working Effectively with Legacy Code useful when researching this topic.
My experiences:
Implement a way for system (end to end) tests
When you add new functionality write system tests and design the new functionality with unit tests
When you change existing functionality write system tests beforehand
Do NOT try to rewrite existing modules for testing them with unit tests
This way you get unit tests for new functionality and create a (althought wide) security net for old functionality. Over time more and more parts of your system get system tests and (in percent) you get more coverage with unit tests. Redesigning old code just to get unit test coverage is to costly.
I'm with your lead developers. It's 1.2 million lines of code which means which means that there is PLENTY of room for error when testing and redesigning. Its not designed for UT so it would probably take a non-trivial effort to rewrite the code so that it can be tested. I'm sure unit testing would be very time consuming. Plus, it's an old product which presumably means many of the bugs have already been found and fixed. If it ain't broke, dont fix it. Wouldn't you rather move on to the more interesting aspects of the project than testing and refactoring old code?
That said, if I absolutely thought it needed to happen, I would probably just write the tests for the pieces I touch, as I touch them. If I dont touch them, I dont test them.
Background: We have been handed over a very large codebase (1.4 million lines) that is primarily in C#. The application consists primarily of asp.net 2.0 style asmx web services accessing data in a SQL server 2008 database and also in various XML files. There are no existing automated tests in place. We have an automated nightly build in place (CC.NET).
We want to introduce some level of automated testing, but refactoring in granular level unit tests for this amount of code seems unlikely. Our first thought is to find a way to construct automated tests that simply call each web service with a given set of parameters to give us some level of code coverage. Seemed like the quickest way to get the highest amount of code coverage with some automated tests. Is this even called unit testing or would this be considered something else?
How would I go about isolating the data stores to get consistent test results? Would any test tools work better for this approach than others? xUnit? MS tests? NUnit?
Any suggestions to get us started in the right direction would be greatly appreciated. Thanks
My company has been doing something similar with our code base (C rather than C#) that totals about a million lines. The steps have gone something like this:
1) Write some automated tests like you describe that do system level tests.
2) Implement the rule that new code shall have a unit test.
3) When an area has a few bugs against it the process of fixing those bugs should include writing a basic unit test.
The point is that 3 shouldn't require a complete unit test (if it's easier people will do it). If you move from 0% test coverage to 40% coverage of a particular module then you've made great progress.
Though 6 months in you may only be up to 5% of the total code base that 5% is the code that is changing the most and where you are most likely to introduce bugs. The code I work on now is about 60% covered by integration tests and 15% (by line) covered by unit tests. That doesn't seem like a lot but it does provide significant value and our development effort has benefited from it.
edit: in response to one of the other comments the current set of integration tests we run take about 14 hours at the moment. We're now looking at running some in parallel to speed them up.
The tests that you describe sound like more of an end-to-end or integration test than a unit test, strictly speaking. But that's not necessarily a bad thing! In your situation, it may be productive to work down from the end-to-end tests towards the unit tests, rather than up as you would on a new codebase.
Write at least one simple test for each web service API, to ensure that you have some coverage of everything
Identify the APIs that have historically been prone to failure, based on past bug reports. Write more extensive tests for those APIs.
Every time that you encounter a failing test, drop down a level and write tests for the methods called on that code path. Eventually you'll find that one of those tests is failing. If the bug isn't obvious at that level, drop down a level and repeat.
Rinse, wash, repeat every time you get a new bug report.
The idea here is that you introduce unit-test coverage "as needed" along the code paths that are currently prone to failure. This will harden those code paths for the future, and will gradually expand your unit test coverage over the entire application.
As JSBangs pointed out, those are call Integration test. And I agree that integration test is better thqn unit test for your case. With 1.4 million lines of code, I am not exactly sure what you can unit test out of that code base.
For isolating the data stores, I like to do the following:
Have a set of hard coded data to start off with initially
After awhile, you should have test that actually creates a bunch of test data. Run those test first and you no longer need the hard coded data.
When something goes wrong in production due to a set of "bad data", add those to your test. Eventually, you will have a good set of test data that test most of the cases.
Also, keep in mind that integration test takes longer to run. You might want to run them on a testing machine so it doesn't block your computer. It's pretty normal for a test suites that takes hours to run.
One thing you could look at is using user stories using something like SpecFlow. I 've found that these stories more naturally map to integration tests, which is what you want. The added benefit of using these is it creates a set of almost use cases that could be used by teams that are other than technical (i.e. product manages/business analysts).
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I saw many questions asking 'how' to unit test in a specific language, but no question asking 'what', 'why', and 'when'.
What is it?
What does it do for me?
Why should I use it?
When should I use it (also when not)?
What are some common pitfalls and misconceptions
Unit testing is, roughly speaking, testing bits of your code in isolation with test code. The immediate advantages that come to mind are:
Running the tests becomes automate-able and repeatable
You can test at a much more granular level than point-and-click testing via a GUI
Note that if your test code writes to a file, opens a database connection or does something over the network, it's more appropriately categorized as an integration test. Integration tests are a good thing, but should not be confused with unit tests. Unit test code should be short, sweet and quick to execute.
Another way to look at unit testing is that you write the tests first. This is known as Test-Driven Development (TDD for short). TDD brings additional advantages:
You don't write speculative "I might need this in the future" code -- just enough to make the tests pass
The code you've written is always covered by tests
By writing the test first, you're forced into thinking about how you want to call the code, which usually improves the design of the code in the long run.
If you're not doing unit testing now, I recommend you get started on it. Get a good book, practically any xUnit-book will do because the concepts are very much transferable between them.
Sometimes writing unit tests can be painful. When it gets that way, try to find someone to help you, and resist the temptation to "just write the damn code". Unit testing is a lot like washing the dishes. It's not always pleasant, but it keeps your metaphorical kitchen clean, and you really want it to be clean. :)
Edit: One misconception comes to mind, although I'm not sure if it's so common. I've heard a project manager say that unit tests made the team write all the code twice. If it looks and feels that way, well, you're doing it wrong. Not only does writing the tests usually speed up development, but it also gives you a convenient "now I'm done" indicator that you wouldn't have otherwise.
I don't disagree with Dan (although a better choice may just be not to answer)...but...
Unit testing is the process of writing code to test the behavior and functionality of your system.
Obviously tests improve the quality of your code, but that's just a superficial benefit of unit testing. The real benefits are to:
Make it easier to change the technical implementation while making sure you don't change the behavior (refactoring). Properly unit tested code can be aggressively refactored/cleaned up with little chance of breaking anything without noticing it.
Give developers confidence when adding behavior or making fixes.
Document your code
Indicate areas of your code that are tightly coupled. It's hard to unit test code that's tightly coupled
Provide a means to use your API and look for difficulties early on
Indicates methods and classes that aren't very cohesive
You should unit test because its in your interest to deliver a maintainable and quality product to your client.
I'd suggest you use it for any system, or part of a system, which models real-world behavior. In other words, it's particularly well suited for enterprise development. I would not use it for throw-away/utility programs. I would not use it for parts of a system that are problematic to test (UI is a common example, but that isn't always the case)
The greatest pitfall is that developers test too large a unit, or they consider a method a unit. This is particularly true if you don't understand Inversion of Control - in which case your unit tests will always turn into end-to-end integration testing. Unit test should test individual behaviors - and most methods have many behaviors.
The greatest misconception is that programmers shouldn't test. Only bad or lazy programmers believe that. Should the guy building your roof not test it? Should the doctor replacing a heart valve not test the new valve? Only a programmer can test that his code does what he intended it to do (QA can test edge cases - how code behaves when it's told to do things the programmer didn't intend, and the client can do acceptance test - does the code do what what the client paid for it to do)
The main difference of unit testing, as opposed to "just opening a new project and test this specific code" is that it's automated, thus repeatable.
If you test your code manually, it may convince you that the code is working perfectly - in its current state. But what about a week later, when you made a slight modification in it? Are you willing to retest it again by hand whenever anything changes in your code? Most probably not :-(
But if you can run your tests anytime, with a single click, exactly the same way, within a few seconds, then they will show you immediately whenever something is broken. And if you also integrate the unit tests into your automated build process, they will alert you to bugs even in cases where a seemingly completely unrelated change broke something in a distant part of the codebase - when it would not even occur to you that there is a need to retest that particular functionality.
This is the main advantage of unit tests over hand testing. But wait, there is more:
unit tests shorten the development feedback loop dramatically: with a separate testing department it may take weeks for you to know that there is a bug in your code, by which time you have already forgotten much of the context, thus it may take you hours to find and fix the bug; OTOH with unit tests, the feedback cycle is measured in seconds, and the bug fix process is typically along the lines of an "oh sh*t, I forgot to check for that condition here" :-)
unit tests effectively document (your understanding of) the behaviour of your code
unit testing forces you to reevaluate your design choices, which results in simpler, cleaner design
Unit testing frameworks, in turn, make it easy for you to write and run your tests.
I was never taught unit testing at university, and it took me a while to "get" it. I read about it, went "ah, right, automated testing, that could be cool I guess", and then I forgot about it.
It took quite a bit longer before I really figured out the point: Let's say you're working on a large system and you write a small module. It compiles, you put it through its paces, it works great, you move on to the next task. Nine months down the line and two versions later someone else makes a change to some seemingly unrelated part of the program, and it breaks the module. Worse, they test their changes, and their code works, but they don't test your module; hell, they may not even know your module exists.
And now you've got a problem: broken code is in the trunk and nobody even knows. The best case is an internal tester finds it before you ship, but fixing code that late in the game is expensive. And if no internal tester finds it...well, that can get very expensive indeed.
The solution is unit tests. They'll catch problems when you write code - which is fine - but you could have done that by hand. The real payoff is that they'll catch problems nine months down the line when you're now working on a completely different project, but a summer intern thinks it'll look tidier if those parameters were in alphabetical order - and then the unit test you wrote way back fails, and someone throws things at the intern until he changes the parameter order back. That's the "why" of unit tests. :-)
Chipping in on the philosophical pros of unit testing and TDD here are a few of they key "lightbulb" observations which struck me on my tentative first steps on the road to TDD enlightenment (none original or necessarily news)...
TDD does NOT mean writing twice the amount of code. Test code is typically fairly quick and painless to write and is a key part of your design process and critically.
TDD helps you to realize when to stop coding! Your tests give you confidence that you've done enough for now and can stop tweaking and move on to the next thing.
The tests and the code work together to achieve better code. Your code could be bad / buggy. Your TEST could be bad / buggy. In TDD you are banking on the chances of BOTH being bad / buggy being fairly low. Often its the test that needs fixing but that's still a good outcome.
TDD helps with coding constipation. You know that feeling that you have so much to do you barely know where to start? It's Friday afternoon, if you just procrastinate for a couple more hours... TDD allows you to flesh out very quickly what you think you need to do, and gets your coding moving quickly. Also, like lab rats, I think we all respond to that big green light and work harder to see it again!
In a similar vein, these designer types can SEE what they're working on. They can wander off for a juice / cigarette / iphone break and return to a monitor that immediately gives them a visual cue as to where they got to. TDD gives us something similar. It's easier to see where we got to when life intervenes...
I think it was Fowler who said: "Imperfect tests, run frequently, are much better than perfect tests that are never written at all". I interprete this as giving me permission to write tests where I think they'll be most useful even if the rest of my code coverage is woefully incomplete.
TDD helps in all kinds of surprising ways down the line. Good unit tests can help document what something is supposed to do, they can help you migrate code from one project to another and give you an unwarranted feeling of superiority over your non-testing colleagues :)
This presentation is an excellent introduction to all the yummy goodness testing entails.
I would like to recommend the xUnit Testing Patterns book by Gerard Meszaros. It's large but is a great resource on unit testing. Here is a link to his web site where he discusses the basics of unit testing. http://xunitpatterns.com/XUnitBasics.html
I use unit tests to save time.
When building business logic (or data access) testing functionality can often involve typing stuff into a lot of screens that may or may not be finished yet. Automating these tests saves time.
For me unit tests are a kind of modularised test harness. There is usually at least one test per public function. I write additional tests to cover various behaviours.
All the special cases that you thought of when developing the code can be recorded in the code in the unit tests. The unit tests also become a source of examples on how to use the code.
It is a lot faster for me to discover that my new code breaks something in my unit tests then to check in the code and have some front-end developer find a problem.
For data access testing I try to write tests that either have no change or clean up after themselves.
Unit tests aren’t going to be able to solve all the testing requirements. They will be able to save development time and test core parts of the application.
This is my take on it. I would say unit testing is the practice of writing software tests to verify that your real software does what it is meant to. This started with jUnit in the Java world and has become a best practice in PHP as well with SimpleTest and phpUnit. It's a core practice of Extreme Programming and helps you to be sure that your software still works as intended after editing. If you have sufficient test coverage, you can do major refactoring, bug fixing or add features rapidly with much less fear of introducing other problems.
It's most effective when all unit tests can be run automatically.
Unit testing is generally associated with OO development. The basic idea is to create a script which sets up the environment for your code and then exercises it; you write assertions, specify the intended output that you should receive and then execute your test script using a framework such as those mentioned above.
The framework will run all the tests against your code and then report back success or failure of each test. phpUnit is run from the Linux command line by default, though there are HTTP interfaces available for it. SimpleTest is web-based by nature and is much easier to get up and running, IMO. In combination with xDebug, phpUnit can give you automated statistics for code coverage which some people find very useful.
Some teams write hooks from their subversion repository so that unit tests are run automatically whenever you commit changes.
It's good practice to keep your unit tests in the same repository as your application.
LibrarIES like NUnit, xUnit or JUnit are just mandatory if you want to develop your projects using the TDD approach popularized by Kent Beck:
You can read Introduction to Test Driven Development (TDD) or Kent Beck's book Test Driven Development: By Example.
Then, if you want to be sure your tests cover a "good" part of your code, you can use software like NCover, JCover, PartCover or whatever. They'll tell you the coverage percentage of your code. Depending on how much you're adept at TDD, you'll know if you've practiced it well enough :)
Unit-testing is the testing of a unit of code (e.g. a single function) without the need for the infrastructure that that unit of code relies on. i.e. test it in isolation.
If, for example, the function that you're testing connects to a database and does an update, in a unit test you might not want to do that update. You would if it were an integration test but in this case it's not.
So a unit test would exercise the functionality enclosed in the "function" you're testing without side effects of the database update.
Say your function retrieved some numbers from a database and then performed a standard deviation calculation. What are you trying to test here? That the standard deviation is calculated correctly or that the data is returned from the database?
In a unit test you just want to test that the standard deviation is calculated correctly. In an integration test you want to test the standard deviation calculation and the database retrieval.
Unit testing is about writing code that tests your application code.
The Unit part of the name is about the intention to test small units of code (one method for example) at a time.
xUnit is there to help with this testing - they are frameworks that assist with this. Part of that is automated test runners that tell you what test fail and which ones pass.
They also have facilities to setup common code that you need in each test before hand and tear it down when all tests have finished.
You can have a test to check that an expected exception has been thrown, without having to write the whole try catch block yourself.
I think the point that you don't understand is that unit testing frameworks like NUnit (and the like) will help you in automating small to medium-sized tests. Usually you can run the tests in a GUI (that's the case with NUnit, for instance) by simply clicking a button and then - hopefully - see the progress bar stay green. If it turns red, the framework shows you which test failed and what exactly went wrong. In a normal unit test, you often use assertions, e.g. Assert.AreEqual(expectedValue, actualValue, "some description") - so if the two values are unequal you will see an error saying "some description: expected <expectedValue> but was <actualValue>".
So as a conclusion unit testing will make testing faster and a lot more comfortable for developers. You can run all the unit tests before committing new code so that you don't break the build process of other developers on the same project.
Use Testivus. All you need to know is right there :)
Unit testing is a practice to make sure that the function or module which you are going to implement is going to behave as expected (requirements) and also to make sure how it behaves in scenarios like boundary conditions, and invalid input.
xUnit, NUnit, mbUnit, etc. are tools which help you in writing the tests.
Test Driven Development has sort of taken over the term Unit Test. As an old timer I will mention the more generic definition of it.
Unit Test also means testing a single component in a larger system. This single component could be a dll, exe, class library, etc. It could even be a single system in a multi-system application. So ultimately Unit Test ends up being the testing of whatever you want to call a single piece of a larger system.
You would then move up to integrated or system testing by testing how all the components work together.
First of all, whether speaking about Unit testing or any other kinds of automated testing (Integration, Load, UI testing etc.), the key difference from what you suggest is that it is automated, repeatable and it doesn't require any human resources to be consumed (= nobody has to perform the tests, they usually run at a press of a button).
I went to a presentation on unit testing at FoxForward 2007 and was told never to unit test anything that works with data. After all, if you test on live data, the results are unpredictable, and if you don't test on live data, you're not actually testing the code you wrote. Unfortunately, that's most of the coding I do these days. :-)
I did take a shot at TDD recently when I was writing a routine to save and restore settings. First, I verified that I could create the storage object. Then, that it had the method I needed to call. Then, that I could call it. Then, that I could pass it parameters. Then, that I could pass it specific parameters. And so on, until I was finally verifying that it would save the specified setting, allow me to change it, and then restore it, for several different syntaxes.
I didn't get to the end, because I needed-the-routine-now-dammit, but it was a good exercise.
What do you do if you are given a pile of crap and seem like you are stuck in a perpetual state of cleanup that you know with the addition of any new feature or code can break the current set because the current software is like a house of cards?
How can we do unit testing then?
You start small. The project I just got into had no unit testing until a few months ago. When coverage was that low, we would simply pick a file that had no coverage and click "add tests".
Right now we're up to over 40%, and we've managed to pick off most of the low-hanging fruit.
(The best part is that even at this low level of coverage, we've already run into many instances of the code doing the wrong thing, and the testing caught it. That's a huge motivator to push people to add more testing.)
This answers why you should be doing unit testing.
The 3 videos below cover unit testing in javascript but the general principles apply across most languages.
Unit Testing: Minutes Now Will Save Hours Later - Eric Mann - https://www.youtube.com/watch?v=_UmmaPe8Bzc
JS Unit Testing (very good) - https://www.youtube.com/watch?v=-IYqgx8JxlU
Writing Testable JavaScript - https://www.youtube.com/watch?v=OzjogCFO4Zo
Now I'm just learning about the subject so I may not be 100% correct and there's more to it than what I'm describing here but my basic understanding of unit testing is that you write some test code (which is kept separate from your main code) that calls a function in your main code with input (arguments) that the function requires and the code then checks if it gets back a valid return value. If it does get back a valid value the unit testing framework that you're using to run the tests shows a green light (all good) if the value is invalid you get a red light and you then can fix the problem straight away before you release the new code to production, without testing you may actually not have caught the error.
So you write tests for you current code and create the code so that it passes the test. Months later you or someone else need to modify the function in your main code, because earlier you had already written test code for that function you now run again and the test may fail because the coder introduced a logic error in the function or return something completely different than what that function is supposed to return. Again without the test in place that error might be hard to track down as it can possibly affect other code as well and will go unnoticed.
Also the fact that you have a computer program that runs through your code and tests it instead of you manually doing it in the browser page by page saves time (unit testing for javascript). Let's say that you modify a function that is used by some script on a web page and it works all well and good for its new intended purpose. But, let's also say for arguments sake that there is another function you have somewhere else in your code that depends on that newly modified function for it to operate properly. This dependent function may now stop working because of the changes that you've made to the first function, however without tests in place that are run automatically by your computer you will not notice that there's a problem with that function until it is actually executed and you'll have to manually navigate to a web page that includes the script which executes the dependent function, only then you notice that there's a bug because of the change that you made to the first function.
To reiterate, having tests that are run while developing your application will catch these kinds of problems as you're coding. Not having the tests in place you'd have to manually go through your whole application and even then it can be hard to spot the bug, naively you send it out into production and after a while a kind user sends you a bug report (which won't be as good as your error messages in a testing framework).
It's quite confusing when you first hear of the subject and you think to yourself, am I not already testing my code? And the code that you've written is working like it is supposed to already, "why do I need another framework?"... Yes you are already testing your code but a computer is better at doing it. You just have to write good enough tests for a function/unit of code once and the rest is taken care of for you by the mighty cpu instead of you having to manually check that all of your code is still working when you make a change to your code.
Also, you don't have to unit test your code if you don't want to but it pays off as your project/code base starts to grow larger as the chances of introducing bugs increases.
Unit-testing and TDD in general enables you to have shorter feedback cycles about the software you are writing. Instead of having a large test phase at the very end of the implementation, you incrementally test everything you write. This increases code quality very much, as you immediately see, where you might have bugs.
The product I have been working on has been in development for the past six years. It started as a generic data entry portal into an insanely complex part WPF/part legacy application. The system has been developed for all these years without a single Unit test in its fold. Now, the point has been raised for a comprehensive unit testing framework. I have been recruited recently to work on this product and have been tasked to get the 'Testing' in order. Since the team that worked on the product for the last six years adopted 'Agile', the project lacks any documentation of the business rules or any design documents.
I have been trying to write unit tests for some of the modules. But I am not sure what to Mock, how to setup my Test fixture and eventually what to Test for, since a casual glance of the methods does not reveal its intentions. Also, it has come to my attention that the code was not developed with a particular methodology in mind.
Given the situation, I was wondering if the good people of Stackoverflow could provide me with some advise on how to salvage this situation. I have heard about the book 'Working with Legacy Code' that has something to say about this general situation but I was thinking about getting some pointers from individuals who have encountered similar situations within the technology stack(C#,VB,C++,.NET 3.5,WCF,SQL Server 2005).
In my opinion the best way is to start by "stabilizing" the current code functionallity using integration tests. Try to create tests that has start point which is not likely to change later. Using the integration tests you can gain confidence that refactoring that'll come later for the unit tests are not breaking anything.
The next step is to unit test the code. If you're free to refactor the code you can start separating logic to classes (e.g. extra logic in view layer) and add unit tests to them. Using this process you also get to know better the code of the product.
It is very recommended read Working with Legacy Code, many of the problems you're going to encounter already have solutions :)
Unit testing legacy code can be a challenge sometimes, depending on the existing code and on how much you can change the code. You can use some tools, for example for writing integration tests you can use White framework to automate the GUI. Another tool you can use for writing the unit tests without forcing major changes in the code is Typemock Isolator (disclaimer - I work at Typemock), it allows faking most of dependencies without changing the production code. There are many other tools which can ease the process, try to find and make best use of them :)
#sc_ray: I know this may sound pretty obvious, but I believe before you start writing tests against the existing code base, you should focus on making sure you are using an MVVM approach when interacting with your UI. Being a legacy app does not mean you have code updating the UI directly with if statements, but the older the project the easier it is for people to bypass more modern software development styles. All I'm saying is that I would make sure I am optimizing the use of binding, commanding, and all the wonderful infrastructure facilities WPF provides. Otherwise important pieces of your business logic would not be able to be tested and you could be potentially writing tests against less relevant code...