Question

Our team has just started unittesting and mocking, and we have run into some discussion considering extension methods. The question is what is a good approach to testing classes, that make use of extension methods. I.e. we have an Enum like this..

public enum State
{
    [LangID(2817)]
    Draft = 0,
    [LangID(2832)]
    Booked = 1,
    [LangID(1957)]
    Overdue = 2,
    [LangID(2834)]
    Checked = 3,
}

Which makes use of the extension method:

public static string GetDescription(this Enum _enum)
{
    Type type = _enum.GetType();
    MemberInfo[] memInfo = type.GetMember(_enum.ToString()); 
    if (memInfo != null && memInfo.Length > 0)
    {
        object[] attrs = memInfo[0].GetCustomAttributes(typeof(LangID), false);
        if (attrs != null && attrs.Length > 0)
            return LanguageDB.GetString(((LangID)attrs[0]).ID);
    }
    return _enum.ToString();
}

Which again will be called by the class under test, like so ..

public class SUT(){

  public void MethodUnderTest(){
      string description = SomeObject.Status.GetDescription();//Status is Type State
  }
}

In this example the enum is getting a description in the language of the user, via LanguageDB, which unfortunately is not injected inside the class since it’s static. We could naturally refractor the lot, but this would be a big investment, considering the code is working almost flawless.
Any good suggestion?

Answer 1

If you’re using MS’ test suite you can do a very simple refactoring and be able to use Accessors to inject a mock into your static type object.

Let’s say you’ve got code like this:

public static void Save(this Entity data)
{
  Repository.Instance.Save(data);
}

A static within a static… hard to test? Not really. Modify the extension class thusly:

private static Repository Instance
{
  get
  {
    return _repository ?? Repository.Instance;
  }
}
private static Repository _repository = null;

public static void Save(this Entity data)
{
  Instance.Save(data);
}

Simple refactor. Now, you can use the accessor to set your mock at test time…

[TestInitialize(), DebuggerStepThrough]
public void Setup()
{
  MyEntityExtensions_Accessor._repository = new Mock<IRepository>();
}