NUnit Mocking not working for Singleton Method

Question

Bear with me, I'm new to NUnit. I come from the land of Rails, so some of this is new to me.

I have a line of code that looks like this:

var code = WebSiteConfiguration.Instance.getCodeByCodeNameAndType("CATALOG_Brands_MinQty", item.Catalog);

I'm trying to mock it, like this (assume code is already initialized):

var _websiteConfigurationMock = new DynamicMock(typeof(WebSiteConfiguration));
_websiteConfigurationMock.ExpectAndReturn("getCodeByCodeNameAndType", code);

When I debug the test, getCodeByCodeNameAndType is returning null , instead of the expected code . What am I doing wrong?

NUnit version: 2.2.8

Answer 1

I'm sorry, but I've never used NUnit.Mocks - but I do have some experience with NMock and Moq [which, by the way, I highly recommend]. Typically, you use a mocking library to generate proxies for Interface definitions, and I presume NUnit.Mocks operates the same way.

Therefore, if you would like to mock your singleton, you will likely have to do the following,

a. Create an interface, say

// All methods you would like to mock from this class, should 
// be members of this interface
public interface IWebSiteConfiguration
{
    // Should match signature of method you are mocking
    CodeType getCodeByCodeNameAndType (
        string codeString, 
        CatalogType catalogType);
}

b. "Implement" interface

// You've already written the method, interface matches signature,
// should be as easy as slapping interface on class declaration
public class WebSiteConfiguration : IWebSiteConfiguration { }

c. Consume interface

alright, so step c. is where most of your work will be. Logically, if you are mocking your singleton, you are actually unit testing the consumer [which you have left out of your sample]. For c. simply add a parameter to the consumer's ctor, or add a publicly accessible property of Type 'IWebSiteConfiguration', and then internally, reference the instance member and invoke your methods against this new interface. Consider this,

was

public class MyClass
{
    public MyClass () { }

    public void DoSomething ()
    {
        // bad singleton! bad boy! static references are bad! you
        // can't change them! convenient but bad!
        code = WebSiteConfiguration.Instance.getCodeByCodeNameAndType (
            "some.string", 
            someCatalog)
    }
}

becomes

public class MyClass
{
    private readonly IWebSiteConfiguration _config = null;

    // just so you don't break any other code, you can default
    // to your static singleton on a default ctor
    public MyClass () : this (WebSiteConfiguration.Instance) { }

    // new constructor permits you to swap in any implementation
    // including your mock!
    public MyClass (IWebSiteConfiguration config) 
    {
        _config = config;
    }

    public void DoSomething ()
    {
        // huzzah!
        code = _config.getCodeByCodeNameAndType ("some.string", someCatalog)
    }
}

In your unit test, create the mock, pass a reference of the mock to the consumer, and test the consumer.

[Test]
public void Test ()
{
    IWebSiteConfiguration mockConfig = null;
    // setup mock instance and expectation via
    // NUnit.Mocks, NMock, or Moq

    MyClass myClass = new MyClass (mockConfig);
    myClass.DoSomething ();

    // verify results
}

---

This also serves as a practical introduction to Dependency Injection [DI]. It's simply the practice of passing, or "injecting", references of services [eg your web site configuration class] to the consumer, rather than having the consumer invoke the service directly [eg via static singleton class].

Hope this helps :)

Answer 2

A DynamicMock creates a new object in-memory that represents the interface, or marshallable (inherits from MarshalByRef) class you want to mock.

Try this:

var _websiteConfigurationMock = new DynamicMock(typeof(WebSiteConfiguration));
_websiteConfigurationMock.ExpectAndReturn("getCodeByCodeNameAndType", code);
WebSiteConfiguration conf = (WebSiteConfiguration)_websiteConfigurationMock.MockInstance;
var x = conf.getCodeByCodeNameAndType("CATALOG_Brands_MinQty", item.Catalog);

Note that the third line there will not work unless WebSiteConfiguration inherits from MarshalByRef.

What you typically do is mock an interface and get a new object that implements this interface, but behaves the way you've configured it to do, without having to go and make a concrete type for it, so I'm not entirely sure what you're doing is going to work unless you employ a better isolation framework, like TypeMock that can intercept calls to static methods/properties in existing objects.

Answer 3

Seems there is a kind of solution for this using reflection, or maybe I totally misunderstood this.

It is discussed here: http://www.geekbeing.com/2010/05/23/how-to-unit-test-singleton-hack-in-c

Could it really works?

public class TestableSingleton : SingletonClass
{
  public TestableSingleton ()
  {
    FieldInfo fieldInfo = typeof(SingletonClass)
        .GetField("_instance",
        BindingFlags.Static | BindingFlags.NonPublic);
    fieldInfo.SetValue(Instance, this);
  }
}

Project availabe on https://github.com/rbabreu/TestableSingleton

Actually I could not compile it on Visual Studio since the SingletonClass would have a private constructor. If someone get it to work would be great to avoid the overhead of adapter pattern.