Recently a FakeItEasy user came to our Gitter
channel asking about easier ways to create their
systems under test and the Fakes that
those systems depend on. They referenced the old
attributes that FakeItEasy
used to provide
for this purpose, but which had been removed when FakeItEasy 5.0.0 was released.
There are some existing libraries that provide this functionality, or something like it, such as
but I thought there was room for improvement. The first two pull along additional libraries with them, either for complete test data generation or for generalized dependency injection. This isn't a problem (and may be a feature) if you're already using those libraries, but could be overwhelming if you're not. FakeItEasy.Auto has a nice light interface, but appears to be abandoned and only supports .NET Framework 4.5.
So my FakeItEasy co-conspirator Thomas Levesque and I chatted about how to design a simple interface to create the system under test. We (or if I'm being honest, mostly he) arrived at an API that
- doesn't require the user to specify any arguments that they don't care about
- doesn't require fields or properties on the test fixture, or use of any attributes
- allows users to provide any values to the SUT's constructor
- makes it easy to retrieve (and later configure) Faked collaborators
Creating a System Under Test using MakeItEasy
MakeItEasy is an attempt to
fulfill the requirements laid out above. The
Make class is its single entry
point and allows you to create your system under test:
That's it. If
VeryNeedySystem has a public constructor whose arguments can
be made from FakeItEasy Dummies, MakeItEasy will make it for you.
It doesn't matter if the constructor has 1 parameter or 14.
Making and Using Fake collaborators
The example above looks great, but more often you'll want to access the Fake object(s) that the SUT will be using. Then you can configure them, or maybe interrogate them after the system under test has been exercised. This is also very easy:
Currently MakeItEasy supports up to 8
out parameters, which will be
populated with Fakes and passed as constructor arguments to the system under
You can even call
Make from a setup method and initialize fields, if you
prefer not to use local variables for the collaborators.
Supplying Arbitrary Constructor arguments
Maybe the system under test's constructor requires some non-Fake additional parameters. MakeItEasy will usually populate these with Dummies, but if you have a particular value you want to supply, you can do that:
You can supply up to 8 constructor arguments.
You can request Fake collaborators and supply arguments at the same time, of course.
As before, you can supply up to 8 arguments and request up to 8 collaborators back.
MakeItEasy doesn't provide a way to customize the Fake before it's passed to the constructor of the system under test. If you need this behaviour, you can always create the Fake "by hand", configure it, and then pass it in. Or maybe you want to share a Fake between systems under test. All this is supported, even in combination.