Mock IMemoryCache in unit test

Question

I am using asp net core 1.0 and xunit.

I am trying to write a unit test for some code that uses IMemoryCache. However whenever I try to set a value in the IMemoryCache I get an Null reference error.

My unit test code is like this:
The IMemoryCache is injected into the class I want to test. However when I try to set a value in the cache in the test I get a null reference.

public Test GetSystemUnderTest()
{
    var mockCache = new Mock<IMemoryCache>();

    return new Test(mockCache.Object);
}

[Fact]
public void TestCache()
{
    var sut = GetSystemUnderTest();

    sut.SetCache("key", "value"); //NULL Reference thrown here
}

And this is the class Test...

public class Test
{
    private readonly IMemoryCache _memoryCache;
    public Test(IMemoryCache memoryCache)
    {
        _memoryCache = memoryCache;
    }

    public void SetCache(string key, string value)
    {
        _memoryCache.Set(key, value, new MemoryCacheEntryOptions {SlidingExpiration = TimeSpan.FromHours(1)});
    }
}

My question is...Do I need to setup the IMemoryCache somehow? Set a value for the DefaultValue? When IMemoryCache is Mocked what is the default value?

Answer 1

IMemoryCache.Set Is an extension method and thus cannot be mocked using Moq framework.

The code for the extension though is available here

public static TItem Set<TItem>(this IMemoryCache cache, object key, TItem value, MemoryCacheEntryOptions options)
{
    using (var entry = cache.CreateEntry(key))
    {
        if (options != null)
        {
            entry.SetOptions(options);
        }

        entry.Value = value;
    }

    return value;
}

For the test, a safe path would need to be mocked through the extension method to allow it to flow to completion. Within Set it also calls extension methods on the cache entry, so that will also have to be catered for. This can get complicated very quickly so I would suggest using a concrete implementation

//...
using Microsoft.Extensions.Caching.Memory;
using Microsoft.Extensions.DependencyInjection;
//...

public Test GetSystemUnderTest() {
    var services = new ServiceCollection();
    services.AddMemoryCache();
    var serviceProvider = services.BuildServiceProvider();

    var memoryCache = serviceProvider.GetService<IMemoryCache>();
    return new Test(memoryCache);
}

[Fact]
public void TestCache() {
    //Arrange
    var sut = GetSystemUnderTest();

    //Act
    sut.SetCache("key", "value");

    //Assert
    //...
}

So now you have access to a fully functional memory cache.

Answer 2

I had a similar issue but I want to disable caching for debugging occasionally as its a pain to keep having to clear the cache. Just mock/fake them yourself (using StructureMap dependency injection).

You could easily use them in you tests as well.

public class DefaultRegistry: Registry
{
    public static IConfiguration Configuration = new ConfigurationBuilder()
        .SetBasePath(HttpRuntime.AppDomainAppPath)
        .AddJsonFile("appsettings.json")
        .Build();

    public DefaultRegistry()
    {
        For<IConfiguration>().Use(() => Configuration);  

#if DEBUG && DISABLE_CACHE <-- compiler directives
        For<IMemoryCache>().Use(
            () => new MemoryCacheFake()
        ).Singleton();
#else
        var memoryCacheOptions = new MemoryCacheOptions();
        For<IMemoryCache>().Use(
            () => new MemoryCache(Options.Create(memoryCacheOptions))
        ).Singleton();
#endif
        For<SKiNDbContext>().Use(() => new SKiNDbContextFactory().CreateDbContext(Configuration));

        Scan(scan =>
        {
            scan.TheCallingAssembly();
            scan.WithDefaultConventions();
            scan.LookForRegistries();
        });
    }
}

public class MemoryCacheFake : IMemoryCache
{
    public ICacheEntry CreateEntry(object key)
    {
        return new CacheEntryFake { Key = key };
    }

    public void Dispose()
    {

    }

    public void Remove(object key)
    {

    }

    public bool TryGetValue(object key, out object value)
    {
        value = null;
        return false;
    }
}

public class CacheEntryFake : ICacheEntry
{
    public object Key {get; set;}

    public object Value { get; set; }
    public DateTimeOffset? AbsoluteExpiration { get; set; }
    public TimeSpan? AbsoluteExpirationRelativeToNow { get; set; }
    public TimeSpan? SlidingExpiration { get; set; }

    public IList<IChangeToken> ExpirationTokens { get; set; }

    public IList<PostEvictionCallbackRegistration> PostEvictionCallbacks { get; set; }

    public CacheItemPriority Priority { get; set; }
    public long? Size { get; set; }

    public void Dispose()
    {

    }
}