How to test a CacheProvider

Question

I have a Cache class as follows:

public class MyCache : ICacheProvider
{ 
    private readonly IMemoryCache _cache; 
    private readonly MemoryCacheOptions _options;
    private readonly ILogger<InMemoryCacheProvider> _logger;  

    public MyCache(IMemoryCache cache, MemoryCacheOptions options, ILogger<InMemoryCacheProvider> logger)
    {
      //Elided
    }

    public virtual void Set<T>(string key, T value, TimeSpan expiration) where T : class
    {  
        _cache.Set(key, value, expiration); 
    } 

    public virtual T Get<T>(string key) where T : class
    { 
        if (_cache.Get(key) is T result)
        { 
            return result; 
        }  
        return default(T);
    } 
    // removed some code for clarity
 }

ICacheProvider has methods like Set and Get.

Well how can I test this class? I need to test that set method actually sets something to the depencency. With FakeitEasy I have done the following:

    [Fact]
    public void SetTest()
    {
        var cache = A.Fake<MyCache>();
        var item = A.Fake<TestClass>();
        cache.Set("item", item);

        A.CallTo(() => cache.Set("item", item)).MustHaveHappened(); 
    }

But this didnt make too much sense to me.

What I am interested is, when I call the set method, I need to be able to check if there is really an object set in the fake cache or whatever. Same for Get and other methods.

Can you please elaborate?

That works. I understand that a call has happened. But i need to test the side effect. — DarthVader, Jul 02 '18 at 15:57
I'm stuggling to see that the test is achieving, other than testing that the mock framework is working correctly. Why mock it rather than pass a `new MemoryCache("testCache")` to the `MyCache` constructor? — stuartd, Jul 02 '18 at 15:58
Create an actual instance of the class under test and mock/ fake the dependencies to be injected. Invoke the methods under test and assert the desired behavior. Note however that `IMemoryCache` has a lot of extension methods and that you would need to mock the local interface members. Check https://stackoverflow.com/questions/42381018/mock-imemorycache-with-moq-throwing-exception/42387886#42387886 — Nkosi, Jul 02 '18 at 16:04
@Nkosi one of the dependency is actually the MemoryCache that I use for storing items. My Class is a wrapper around it. — DarthVader, Jul 02 '18 at 16:08
For the test you actually only need the interface/abstractions, not the implementation concerns as this is for an isolated unit test not an integration test. — Nkosi, Jul 02 '18 at 16:09
@Nkosi what do you mean? I dont need to test the implementor class of the interface? I need to unit test that it works actually. — DarthVader, Jul 02 '18 at 16:13
Mock the dependencies of `MyCache` and inject them into an actual instance of `MyCache`. You then call the members of `MyCache` and assert that it does what it is suppose to be doing. — Nkosi, Jul 02 '18 at 16:17

Blair Conrad · Accepted Answer · 2018-07-02T16:48:46.447

3

@Nkosi's comment is correct. A mocking framework is used by mocking the system under test's collaborators. Then the system under test can be exercised. Like this:

// mock a collaborator
var fakeCache = A.Fake<IMemoryCache>();

// Create a real system under test, using the fake collaborator.
// Depending on your circumstances, you might want real options and logger,
// or fake options and logger. For this example, it doesn't matter.
var myCacheProvider = new MyCache(fakeCache, optionsFromSomewhere, loggerFromSomewhere);

// exercise the system under test
myCacheProvider.Set("item", item, someExpriation);

// use the fake collaborator to verify that the system under test worked
// As @Nkosi points out, _cache.Set(key, value, expiration)
// from the question is an extension method, so you can't assert on it
// directly. Otherwise you could do
// A.CallTo(() => fakeCache.Set("item", item, expiration)).MustHaveHappened();
// Instead you'll need to assert on CreateEntry, which could be a little trickier

edited Jul 02 '18 at 16:48

answered Jul 02 '18 at 16:32

Blair Conrad

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That is correct. The `Set` however in this case is an extension method. so it is not as simple as you would have expected. – Nkosi Jul 02 '18 at 16:34
Oh? I saw not evidence of that in the question. But yes, in that case, you'd want to assert against whatever method underlies the extension method. – Blair Conrad Jul 02 '18 at 16:37
I see your point. Check the link in my comment and you would see what I am referring to. – Nkosi Jul 02 '18 at 16:38
Ah. That comment was hidden for me. I was agreeing with another of your comments (same gist). Let's update the answer! – Blair Conrad Jul 02 '18 at 16:42

How to test a CacheProvider

1 Answers1