Unit testing method that uses IMemoryCache extension method

Question

I am trying to write a unit test using MSTest and Moq for a method that uses an extension method of IMemoryCache. Situation:

public class ClassToTest
{        
  private IMemoryCache Cache { get; }

  public ClassToTest(IMemoryCache cache)
  {          
    Cache = cache;
  }

  public async Task<SomeType> MethodToTest(string key)
  {
    // Get is an extension method defined in Microsoft.Extensions.Caching.Memory
    var ci = Cache.Get<CachedItem<T>>(key);

    // Do stuff with cached item
  }
}

How can I unit test this?

So far i tried:

[TestMethod]
public void TestMethodToTest()
{
  IServiceCollection services = new ServiceCollection();
  services.AddMemoryCache();

  var serviceProvider = services.BuildServiceProvider();
  var memoryCache = serviceProvider.GetService<IMemoryCache>();

  ClassToTest testClass = new ClassToTest(memoryCache);
}

This gives me the following error: "'IServiceCollection' does not contain a definition for 'AddMemoryCache' and no accessible extension method 'AddMemoryCache' accepting a first argument of type 'IServiceCollection' could be found (are you missing a using directive or an assembly reference?)".

Does anyone know how to unit test this method? Preferably without changing the method itself. Is there a standard way to do this? Any help is appriciated.

Answer 1

I think you have to modify ClassToTest . You can have a property of type Func<string, CachedItem<T>> that gets assigned to use the extension in your class' constructor:

public Func<string, CachedItem<T>> GetFromCache { get; set; }
public ClassToTest(IMemoryCache cache)
{          
    Cache = cache;
    GetFromCache = key => Cache.Get<CachedItem<T>>(key);
}

Then, when you want to test that class, you can override that behaviour by saying:

ClassToTest testClass = new ClassToTest(memoryCache);
testClass.GetFromCache = key => /* something else */;

Generally speaking, extension methods are still just syntactic sugar over static methods, so using them like ClassToTest does will introduce dependencies and make code harder to test in isolation.