How to do integration testing in .NET with real files?

First of all, I think, it is better to write unit tests to test your logic without touching any external resources. Here you have two options:

1. you need to use abstraction layer to isolate your logic from external dependencies such as the file system. You can easily stub or mock (by hand or with help of constrained isolation framework such as NSubstitute, FakeItEasy or Moq) this abstractions in unit tests. I prefer this option, because in this case tests push you to a better design.
2. if you have to deal with legacy code (only in this case), you can use one of the unconstrained isolation frameworks (such as TypeMock Isolator, JustMock or Microsoft Fakes) that can stub/mock pretty much everything (for instance, sealed and static classes, non-virtual methods). But they costs money. The only "free" option is Microsoft Fakes unless you are the happy owner of Visual Studio 2012/2013 Premium/Ultimate.

In unit tests you don't need to test the logic of external libraries such as MEF.

Secondly, if you want to write integration tests, then you need to write "happy path" test (when everything is OK) and some tests that testing your logic in boundary cases (file or directory not found). Unlike @Sergey Berezovskiy, I recommend creating separate folders for each test case. The main advantages is:

1. you can give your folder meaningful names that more clearly express your intentions;
2. you don't need to write complex (i.e. fragile) setup/teardown logic.
3. even if you decide later to use another folder structure, then you can change it more easily, because you will already have working code and tests (refactoring under test harness is much easier).

For both, unit and integration tests, you can use ordinary unit testing frameworks (like NUnit or xUnit.NET). With this frameworks is pretty easy to launch your tests in Continuous integration scenarios on your Build server.

If you decide to write both kinds of tests, then you need to separate unit tests from integration tests (you can create separate projects for every kind of tests). Reasons for it:

1. unit tests is a safety net for developers. They must provide quick feedback about expected behavior of system units after last code changes (bug fixes, new features). If they are run frequently, then developer can quickly and easily identify piece of code, that broke the system. Nobody wants to run slow unit tests.
2. integration tests are generally slower than unit tests. But they have different purpose. They check that units works as expected with real dependencies.

I don't know your program's architecture to give a good advice, but I will try

1. I believe that you don't need to test real file structure. File access services are defined by system/framework, and they're don't need to be tested. You need to mock this services in related tests.
2. Also you don't need to test MEF. It is already tested.
3. Use SOLID principles to make unit tests. Especially take look at Single Responsibility Principle this will allow you to to create unit tests, which won't be related to each others. Just don't forget about mocking to avoid dependencies.
4. To make integration tests, you can create a set of helper classes, which will emulate scenarios of file structures, which you want to test. This will allow you to stay not attached to machine on which you will run this tests. Such approach maybe more complicated than creating real file structure, but I like it.

I'd go with single test folder. For various test cases you can put different valid/invalid files into that folder as part of context setup. In test teardown just remove those files from folder.

E.g. with Specflow:

Given configuration file not exist
When something
Then foo

Given configuration file exists
And some dll not exists
When something
Then bar

Define each context setup step as copying/not copying appropriate file to your folder. You also can use table for defining which file should be copied to folder:

Given some scenario
| FileName         |
| a.config         |
| b.invalid.config |
When something
Then foobar

You should test as much logic as possible with unit tests, by abstracting calls to the file system behind interfaces. Using dependency injection and a testing-framework such as FakeItEasy will allow you to test that your interfaces are actually being used/called to operate on the files and folders.

At some point however, you will have to test the implementations working on the file-system too, and this is where you will need integration tests.

The things you need to test seem to be relatively isolated since all you want to test is your own files and directories, on your own file system. If you wanted to test a database, or some other external system with multiple users, etc, things might be more complicated.

I don't think you'll find any "official rules" for how best to do integration tests of this type, but I believe you are on the right track. Some ideas you should strive towards:

• Clear standards: Make the rules and purpose of each test absolutely clear.
• Automation: The ability to re-run tests quickly and without too much manual tweaking.
• Repeatability: A test-situation that you can "reset", so you can re-run tests quickly, with only slight variations.

Create a repeatable test-scenario

In your situation, I would set up two main folders: One in which everything is as it is supposed to be (i.e. working correctly), and one in which all the rules are broken.

I would create these folders and any files in them, then zip each of the folders, and write logic in a test-class for unzipping each of them.

These are not really tests; think of them instead as "scripts" for setting up your test-scenario, enabling you to delete and recreate your folders and files easily and quickly, even if your main integration tests should change or mess them up during testing. The reason for putting them in a test-class, is simply to make then easy to run from the same interface as you will be working with during testing.

Testing

Create two sets of test-classes, one set for each situation (correctly set up folder vs. folder with broken rules). Place these tests in a hierarchy of folders that feels meaningful to you (depending on the complexity of your situation).

It's not clear how familiar you are with unit-/integration-testing. In any case, I would recommend NUnit. I like to use the extensions in Should as well. You can get both of these from Nuget:

install-package Nunit
install-package Should

The should-package will let you write the test-code in a manner like the following:

someCalculatedIntValue.ShouldEqual(3); 
someFoundBoolValue.ShouldBeTrue();

Note that there are several test-runners available, to run your tests with. I've personally only had any real experience with the runner built into Resharper, but I'm quite satisfied with it and I have no problems recommending it.

Below is an example of a simple test-class with two tests. Note that in the first, we check for an expected value using an extension method from Should, while we don't explicitly test anything in the second. That is because it is tagged with [ExpectedException], meaning it will fail if an Exception of the specified type is not thrown when the test is run. You can use this to verify that an appropriate exception is thrown whenever one of your rules is broken.

[TestFixture] 
public class When_calculating_sums
{                    
    private MyCalculator _calc;
    private int _result;

    [SetUp] // Runs before each test
    public void SetUp() 
    {
        // Create an instance of the class to test:
        _calc = new MyCalculator();

        // Logic to test the result of:
        _result = _calc.Add(1, 1);
    }

    [Test] // First test
    public void Should_return_correct_sum() 
    {
        _result.ShouldEqual(2);
    }

    [Test] // Second test
    [ExpectedException(typeof (DivideByZeroException))]
    public void Should_throw_exception_for_invalid_values() 
    {
        // Divide by 0 should throw a DivideByZeroException:
        var otherResult = _calc.Divide(5, 0);
    }       

    [TearDown] // Runs after each test (seldom needed in practice)
    public void TearDown() 
    {
        _calc.Dispose(); 
    }
}

With all of this in place, you should be able to create and recreate test-scenarios, and run tests on them in a easy and repeatable way.

---

Edit: As pointed out in a comment, Assert.Throws() is another option for ensuring exceptions are thrown as required. Personally, I like the tag-variant though, and with parameters, you can check things like the error message there too. Another example (assuming a custom error message is being thrown from your calculator):

[ExpectedException(typeof(DivideByZeroException), 
   ExpectedMessage="Attempted to divide by zero" )]
public void When_attempting_something_silly(){  
    ...
}