First class support for NUnit

[cottsak]

Currently the life cycle management that's baked into this library works well with the way xunit does things however NUnit is different. Internal instances leak from test to test when using NUnit so it would be great to see some first class support for NUnit.

[cottsak]

Attempting to apply with 81a96f6
Didn't work :(

Somehow the tests in this repo worked with the NUnit helpers but in another project I'm testing the integration of this package against, it's still failing. I've proven that there is still leakage between different tests in the same run. Running the tests in isolation will pass them. But when two or more of these tests who take the same dependencies are run together there is some context that is shared/leaked between them causing at least one of the tests to have the wrong context (proven with Console.WriteLine()) and then fail. Setting up the correct context in the failing text explicitly (rather than replying on defaults, which should be enough) passes it. But it's still leaking which is a problem.

[cottsak]

81a96f6 was never going to work! It's only one test. It seems that NUnit does not isolate test runs (in terms of threads test instances) the same as xunit does and so you need to have at least two NUnit tests (in the same test class?) before this problem can be reproduced.

TODO:

• Reproduce this problem in this repository and then fund a fix that way.

[cottsak]

The error at the crux of this problem is "ValueFactory attempted to access the Value property of this instance." which is an InvalidOperationException which this page seems to suggest means another thread is accessing the Value of the Lazy. I don't yet understand why this is the case since the default LazyThreadSafetyMode for Lazy is LazyThreadSafetyMode.ExecutionAndPublication which should mean everything is protected with locks and is thread safe. However in NUnit, when running more than one test in a single run (where you use the Session I think), it's not thread safe.

[cottsak]

xunit uses different instances of the test class for each test run, even though they're in the same thread. NUnit however uses the same thread and the same test class instance. If the NUnit tests are separated into their own test classes they seem to work: the test class instance is obviously different.

So Dispose() is not being called after every test run because NUnit shares the test class instance between tests in the same class (same "run"). This causes the sessionTeardown not to be invoked which shares the Session between tests in the same class/run.

It seems that in a scenario where the store is adequately isolated, that when using NUnit, if the [TearDown] is used to this.Dispose();, we seem to be working ok.

TODO:

• test another project by adding proper store/db isolation and using the [TearDown] to see if the errors are then localised to Session/store-related problems and not with the Lazy instance.

[bradwilson]

You have already determined the salient difference between xUnit.net and NUnit in terms of lifetime. We (https://github.com/xunit) create a new instance of the test class for each test, where NUnit does not. We made this change purposefully, because it helps isolate test runs from each other.

You may want to have separate examples for the two test frameworks which takes this into account.

[cottsak]

@bradwilson thanks mate. And good advice: I'll have different examples for each framework which include the workarounds.