Unit testing singletons

Question

I have a singleton that contains reference to statistics object.

When I run couple of unit test on the program that uses that singleton - the values sustained between the tests.

I though that when I'm doing Program.Main() it all starts over between unit tests, but somehow it remembers the results from last test.

How can I write unit tests that will be isolated from each other (I don't want clean() functions - I want it to start over with new "everything"),

This is one of the problems with singletons :) You can reset the singleton (typically by using reflection to set the _instance pointer to null), or you need to look at loading it into it's own app domain for testing... — forsvarir, May 05 '11 at 12:35
@forsvarir: That's a correct answer. Why post it as a comment? — razlebe, May 05 '11 at 12:43
@razelbe: I'd say it's not an answer, it's an observation and a pointer in the right direction. An answer would have a bit more substance :0) — forsvarir, May 05 '11 at 12:45
@forsvarir: Fair enough. But you don't get rep from my comment upvote. :) — razlebe, May 05 '11 at 12:46
@razelbe: true, but if you get 4 of your friends to upvote it, I'll be a tenth of the way to a pundits badge — forsvarir, May 05 '11 at 12:50

score 25 · Accepted Answer · answered May 05 '11 at 12:47

25

Short version: do not write your singletons as singletons. Write them as normal classes, and call them via an Inversion of Control container, where you have configured the class to be a singleton instead.

That way, you can unit-test the class just fine and if you decide today or tomorrow that singleton is not the right lifestyle for the class, simply modify the configuration of the IoC container.

answered May 05 '11 at 12:47

Roy Dictus

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I'll check this inversion of control container - though I don't have time to invest in that. In the end, there is still a singleton, and I'll verify how this design pattern solve this issue. Thanks! – Eyal May 05 '11 at 13:33

Philippe · Answer 2 · 2019-03-18T14:42:24.210

5

I wrote a post about that here: http://pvlerick.github.io/2017/03/how-to-get-rid-of-a-singleton

TL;DR:

Extract an interface from the Singleton (even if you don't own it) and make your class work against that interface instead of the Singleton's instance;
Depending on whether you own the Singleton or not, you can make it implement that interface or you'll need a simple adapter.

edited Mar 18 '19 at 14:42

answered May 05 '11 at 12:42

Philippe

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Creating a new `AppDomain` per test is quite hard. NUnit, for example, creates a new AppDomain per test session (one pass through all selected tests). NUnit then has to inject its code into that `AppDomain` to run the tests. That's non-trivial. You can't just create a new one and expect things to magically run in it. You have to get your code to run in it. Overall this answer has possibilities but I don't agree with the "which shouldn't be too difficult part". – Mike Two May 05 '11 at 12:50

score 2 · Answer 3 · answered Jan 13 '17 at 18:35

When attempting to test the singleton itself the following could be a possible solution:

public class Singleton
{
    private static Singleton _Instance;

    public static Singleton getInstance() {
        if (_Instance == null)
        {
            _Instance = new Singleton();
        }

        return _Instance;
    }

    private Singleton()
    {
    }

    public static resetForTesting() {
        _Instance = null
    }
}

So, in your unit testing framework you would call Singleton.resetForTesting() before each unit test.

Note: the downside of this approach is that there is no code level restriction that would stop somebody from invoking this method within production code even though it is only meant to be used with testing code. So, you would have to rely on documentation to convey that to other people.

You could add a restriction by making the `resetForTesting` method internal and making internals visible to your assembly containing your test classes. See this [Microsoft documentation](https://learn.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.internalsvisibletoattribute?view=net-5.0). — devklick, Mar 30 '21 at 13:20

score 1 · Answer 4 · edited May 23 '17 at 12:01

1

look at this Unit testing with singletons

also I would reccomand to use the mocking frameworks like Moq

for isolation your test

edited May 23 '17 at 12:01

Community

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answered May 05 '11 at 12:37

Sergey K

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that question you've linked to is about an exception the OP was getting, but the [accepted answer](http://stackoverflow.com/questions/2085953/unit-testing-with-singletons/2085988#2085988) to that question contains a lot of useful links relevant to the question here. You might want to update your hyperlink. – razlebe May 05 '11 at 12:41
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Indeed. I'm trying to help you to help. :) – razlebe May 05 '11 at 12:48

score 0 · Answer 5 · answered Jan 17 '18 at 21:37

0

You can add property setter to your singleton class that will reassign singleton instance. This way in tests you can stub/mock your singleton.

answered Jan 17 '18 at 21:37

tytyryty

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Unit testing singletons

5 Answers5

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