Creating a System.Web.Caching.Cache object in a unit test

Question

I'm trying to implement a unit test for a function in a project that doesn't have unit tests and this function requires a System.Web.Caching.Cache object as a parameter. I've been trying to create this object by using code such as...

System.Web.Caching.Cache cache = new System.Web.Caching.Cache();
cache.Add(...);

...and then passing the 'cache' in as a parameter but the Add() function is causing a NullReferenceException. My best guess so far is that I can't create this cache object in a unit test and need to retrieve it from the HttpContext.Current.Cache which I obviously don't have access to in a unit test.

How do you unit test a function that requires a System.Web.Caching.Cache object as a parameter?

Answer 1

I think your best bet would be to use a mock object (look into Rhino Mocks).

Answer 2

When I've been faced with this sort of problem (where the class in question doesn't implement an interface), I often end up writing a wrapper with associated interface around the class in question. Then I use my wrapper in my code. For unit tests, I hand mock the wrapper and insert my own mock object into it.

Of course, if a mocking framework works, then use it instead. My experience is that all mocking frameworks have some issues with various .NET classes.

public interface ICacheWrapper
{
   ...methods to support
}

public class CacheWrapper : ICacheWrapper
{
    private System.Web.Caching.Cache cache;
    public CacheWrapper( System.Web.Caching.Cache cache )
    {
        this.cache = cache;
    }

    ... implement methods using cache ...
}

public class MockCacheWrapper : ICacheWrapper
{
    private MockCache cache;
    public MockCacheWrapper( MockCache cache )
    {
        this.cache = cache;
    }

    ... implement methods using mock cache...
}

public class MockCache
{
     ... implement ways to set mock values and retrieve them...
}

[Test]
public void CachingTest()
{
    ... set up omitted...

    ICacheWrapper wrapper = new MockCacheWrapper( new MockCache() );

    CacheManager manager = new CacheManager( wrapper );

    manager.Insert(item,value);

    Assert.AreEqual( value, manager[item] );
}

Real code

...

CacheManager manager = new CacheManager( new CacheWrapper( HttpContext.Current.Cache ));

manager.Add(item,value);

...

Answer 3

A very useful tool for unit testing legacy code is TypeMock Isolator. It'll allow you to bypass the cache object entirely by telling it to mock that class and any method calls you find problematic. Unlike other mock frameworks, TypeMock uses reflection to intercept those method calls you tell it to mock for you so you don't have to deal with cumbersome wrappers.

TypeMock is a commercial product, but it has free versions for open-source projects. They used to have a "community" edition that was a single-user license but I don't know if that's still offered.