Unit testing code with a file system dependency

Question

I am writing a component that, given a ZIP file, needs to:

1. Unzip the file.
2. Find a specific dll among the unzipped files.
3. Load that dll thro

Answer 1

One way would be to write the unzip method to take InputStreams. Then the unit test could construct such an InputStream from a byte array using ByteArrayInputStream. The contents of that byte array could be a constant in the unit test code.

Answer 2

Assuming that "file system interactions" are well tested in the framework itself, create your method to work with streams, and test this. Opening a FileStream and passing it to the method can be left out of your tests, as FileStream.Open is well tested by the framework creators.

Answer 3

You should not test class interaction and function calling. instead you should consider integration testing. Test the required result and not the file loading operation.

Answer 4

Yay! Now it's testable; I can feed in test doubles (mocks) to the DoIt method. But at what cost? I've now had to define 3 new interfaces just to make this testable. And what, exactly, am I testing? I'm testing that my DoIt function properly interacts with its dependencies. It doesn't test that the zip file was unzipped properly, etc.

You have hit the nail right on its head. What you want to test is the logic of your method, not necessarily whether a true file can be addressed. You don´t need to test (in this unit test) whether a file is correctly unzipped, your method takes that for granted. The interfaces are valuable by itself because they provide abstractions that you can program against, rather than implicitly or explicitly relying on one concrete implementation.

Answer 5

As others have said, the first is fine as an integration test. The second tests only what the function is supposed to actually do, which is all a unit test should do.

As shown, the second example looks a little pointless, but it does give you the opportunity to test how the function responds to errors in any of the steps. You don't have any error checking in the example, but in the real system you may have, and the dependency injection would let you test all the responses to any errors. Then the cost will have been worth it.