38

Recently there has been quite some hype around all the different mocking frameworks in the .NET world. I still haven't quite grasped what is so great about them. It doesn't seem to be to hard to write the mocking objects I need myself. Especially with the help of Visual Studio I quickly can write a class that implements the interface I want to mock (it auto-generates almost everything for me) and then write an implementation for the method(s) I need for my test. Done! Why going through the hassle of understanding a mocking framework for the sole purpose of saving a few lines of code. Or is a mocking framework not only about saving lines of code?

Jessica
  • 1,621
  • 2
  • 18
  • 34
bitbonk
  • 48,890
  • 37
  • 186
  • 278
  • 3
    Just a caveat...this question is not .NET specific. So feel free to read on if you found yourself here somehow. – smp7d Dec 21 '12 at 16:00

10 Answers10

32

Once I finally got the hang of mock objects, I realized that they're essential for unit testing for the same reason that double blind testing or control groups are essential for scientific trials: they isolate what you're actually testing.

If you're testing a class which has quite a bit of interaction via other interfaces, you not only save the lines of code on having to mock each and every interface, but you also gain the ability to do things like "throw an exception if an unexpected method is called" or "exception if these methods are called out of order". You can get remarkably sophisticated with mock frameworks, and though I'll quickly admit there's a large learning curve, when you get up to speed they'll help make your unit tests more thorough without being bloated.

Art Doler
  • 720
  • 5
  • 15
  • How can methods be called out of order? IMO it's a code smell. – Jimmy T. May 24 '14 at 21:17
  • 1
    Depends on the workflow of the class under mock. If something has state maintained through a known lifecycle (e.g. init(), run(), teardown()) then the class under test needs to respect that state. – Art Doler Jul 15 '14 at 18:30
  • 1
    Isn't that getting it backwards a bit? Making a caller responsible for ensuring proper state of somebody else? Shouldn't the class that requires proper call order be the one to ensure that it is called/used correctly? By throwing exceptions/assertions when it is not? And shouldn't the tests for this "order-requiring-class" check that they are thrown when methods are called in an inappropriate order? Seems to me that this would be a more effective way to go about it than to shuffle responsibility for proper use onto all possible users of that "order-requiring-class". – Marjan Venema Mar 08 '15 at 13:30
12

You actually identified one of the key points of a mock framework in your question. The fact that you code the mocks yourself is not something the developer should be concerned with. The mocking frameworks give you implementations of interfaces programatically, plus they are functional (based on your setup of the mock).

What do you do if you are testing an ICustomerDAO, for example, and you want to test some method 14 times each with different outcomes? Implement 14 different classes manually? I doubt that anyone would want to do that.

Mocks give you the power to define what will happen with parts of your classes when you are not concerned with whether or not they will actually work, like throwing exceptions whenever you want them to, returning zero results and making sure you handle that correctly, etc...

They are a great unit testing tool.

glenneroo
  • 1,908
  • 5
  • 30
  • 49
Mark
  • 14,820
  • 17
  • 99
  • 159
8

Previous questions that may help:
What is a mock and when should you use it?
Mockist vs classical TDD

I find that using a mocking framework allows me to generate tests a lot faster and with better verification that what I expect to happen in the test actually is happengin. I have in the past implemented stubs or fakes myself. I found that I needed to generate stubs specific to the test that I wanted and this took a lot of time. I can create the same test much faster using a mocking framework. The good ones support the generation of fakes, stubs or mocks with straightforward syntax.

It takes a while to get the hang of it, I avoided it for a while but now wouldn't try to work without a mocking framework for the reasons @Chamelaeon states.

Community
  • 1
  • 1
Hamish Smith
  • 8,153
  • 1
  • 34
  • 48
  • 1
    +1 You could almost say a mocking framework provides a DSL - and the benefits of DSL's are well understood. – AJ. Aug 02 '11 at 17:14
4

Roy Osherove had a poll about Mock Frameworks and down in the comment section, there is a discussion (albeit brief) about whether one needs a Mock Framework or not.

I personally have been manually doing exactly as you stated and it has worked well enough, but this has mainly been out of habit rather than a closely-held opinion on mock frameworks in general.

itsmatt
  • 31,265
  • 10
  • 100
  • 164
3

Well I certainly don't think that you NEED a mocking framework. It's a framework like any other, and it's ultimately designed to save you some time and effort. You can also do things like roll your own common data structures like stacks and queues, but isn't it generally easier to just use the ones built into the class libraries that ship with the compiler/IDE of your language of choice?

I'm sure there are other compelling reasons for using mocking frameworks, though I'd leave it to the TDD and unit testing gurus to answer.

Jesse Taber
  • 2,376
  • 3
  • 23
  • 33
2

One thing that troubles me about a mocking framework is that "what a function should o/p given an i/p" via

when(mock.someMethod("some arg")).thenReturn("something");

statement is spread across many unit test classes.

Let me elaborate with an example. Lets say there was a DAO Interface function getEmp(int EmpID) which was returning an Employee Object when passed an Employee ID as a parameter. Assume that this function was being mocked by 10 different unit test classes. Now if in the future, this function were changed to return a newer version of the Employee Object, one would have to go to each of the 10 different classes to update this change.

The disadvantages are as follows...

a) I don't know how to figure out all the classes which mock this function so that I can go update this change.

b) My existing test cases which consumes the mock DAO object continue to be blissfully unaware of the changes that have happened to the DAO Interface because the mock has not changed and hence continue to be green. Ideally, if I were to have coded a single mock class myself and consumed it everywhere, I would have just one place to update for the newer version of the Employee object. Also, once I update at this one place, all my existing test cases which consume the mock would break and I would then know exactly what places I need to go and do an update for the new Employee Object.

Any thoughts on my views..

Vicky
  • 21
  • 1
2

For the same reason you wouldn't try to write unit tests without NUnit. A mocking framework will assist you in verifying state and behavior over hundreds of unit tests. It's worth the 2 weeks or so of pain to get up to speed and really helps you focus on what needs to be tested.

Jason Slocomb
  • 3,030
  • 1
  • 23
  • 25
0

I've use rhino mocks for a mocking framework. I and 5 other developers used it on a large enterprise application that was an 8 month project. We used tdd on the project. Was it worth it? I guess. Was there such a massive huge selling point to using mocks that I have to use it on every project? In my opinion, no. It is not something that is necessary, it is just a tool that you can use if you want to try it out. Some projects you can roll out your own mock classes as some here say they prefer - it is easier. Other projects are larger and may require a mocking framework. The key word (in my opinion) is MAY require... how much code coverage do you require? To me, that is another consideration to using mocks. The project I did with tdd/rhino mocks we were required to have 80% code coverage so the mocks helped us attain that. If our code coverage requirements were less, for example 40%, we probably would have not used a mocking framework and just wrote our own mock classes as others mention they do.

Frekster
  • 1,138
  • 1
  • 14
  • 32
0

One of the good things about a mocking framework is that it allows setting expectations on the objects being mocked. With the expectations I can then set up all sorts of conditions to exercise the code thats being tested.

Darren
  • 961
  • 2
  • 9
  • 17
0

An isolation framework or mocking framework allows you to test the code you want, without its dependencies. It makes for short running tests, allows you to debug quickly, and easily build a safety net of tests around the code. Different frameworks have different features, and as said before - it's a tool, and you should select the right tool for the job.