How do I test an abstract class's protected abstract method?

Question

I've been working on the best way to test an abstract class named TabsActionFilter. I've guranteed that classes that inherit from TabsActionFilter will have a method called GetCustomer. In practice this design seems to work well.

Where I've had some issues is figuring out how to test the OnActionExecuted method of the base class. This method relies upon the implementation of the the protected abstract GetCustomer method. I've tried mocking the class using Rhino Mocks but can't seem to mock the return of a fake customer from GetCustomer. Obviously, flipping the method to public will make mocking available, but protected feels like the more appropriate accessibility level.

For the time being in my test class I've added a concrete private class that inherits from TabsActionFilter and returns a faked Customer object.

• Is a concrete class the only option?
• Is there a simple mechanism of mocking that I'm missing that would allow Rhino Mocks to provide a return for GetCustomer?

As a note Anderson Imes discusses his opinions on this in an answer about Moq and I could be missing something key, but it doesn't seem applicable here.

Class that needs to be tested

public abstract class TabsActionFilter : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        Customer customer = GetCustomer(filterContext);

        List<TabItem> tabItems = new List<TabItem>();
        tabItems.Add(CreateTab(customer, "Summary", "Details", "Customer",
            filterContext));
        tabItems.Add(CreateTab(customer, "Computers", "Index", "Machine",
            filterContext));
        tabItems.Add(CreateTab(customer, "Accounts", "AccountList",
            "Customer", filterContext));
        tabItems.Add(CreateTab(customer, "Actions Required", "Details",
            "Customer", filterContext));

        filterContext.Controller.ViewData.PageTitleSet(customer.CustMailName);
        filterContext.Controller.ViewData.TabItemListSet(tabItems);
    }

    protected abstract Customer GetCustomer(ActionExecutedContext filterContext);
}

Test Class and Private Class for "mocking"

public class TabsActionFilterTest
{
    [TestMethod]
    public void CanCreateTabs()
    {
        // arrange
        var filterContext = GetFilterContext(); //method omitted for brevity

        TabsActionFilterTestClass tabsActionFilter =
            new TabsActionFilterTestClass();

        // act
        tabsActionFilter.OnActionExecuted(filterContext);

        // assert
        Assert.IsTrue(filterContext.Controller.ViewData
            .TabItemListGet().Count > 0);
    }

    private class TabsActionFilterTestClass : TabsActionFilter
    {
        protected override Customer GetCustomer(
            ActionExecutedContext filterContext)
        {
            return new Customer
            {
                Id = "4242",
                CustMailName = "Hal"
            };
        }
    }
}

Answer 1

I think the problem you are currently running into is that your class is not testable, or is not testable enough. This is of course assuming that you have correctly identified that GetCustomer is indeed something that needs to be mocked in order to properly test in isolation.

If GetCustomer is something that needs to be mocked in order to properly isolate and test TabsActionFilter, you will somehow need to make the implementation of GetCustomer a composable part of the class, and not an inherited method. The most common way to achieve this is to use Inversion of Control/Dependency Injection.

All that said, you COULD use something like TypeMock to achieve this. However, when you encounter a situation like this where a class is hard to test, it is usually a signal that your class has too many responsibilities and needs to be broken into smaller components.

(I am not a fan of using TypeMock FWIW).

Answer 2

Phil's answer is correct. If instead of having an abstract class, you had a class that required the injection of a customer getter (or factory, or whatever) then you'd be in good shape to test. Abstract classes are the enemy of testing (and good design).

public class TabsActionFilter : ActionFilterAttribute 
{ 
    private readonly ICustomerGetter _getter;
    public TabsActionFilter(ICustomerGetter getter)
    { _getter = getter; }

    public override void OnActionExecuted(ActionExecutedContext filterContext) 
    { 
        Customer customer = _getter.GetCustomer(filterContext); 

        ...
    } 
} 
public interface ICustomerGetter
{ 
     Customer GetCustomer(ActionExecutedContext filterContext);
}

in your test you instantiate the now non-abstract TabsActionFilter and give it a Mock getter, which should be trivial to mock.

EDIT: seems there is concern that you must have a parameterless constructor. this is easy. given the above code, you then have your "real" filters implemented as

public class MyFilter : TabsActionFilter
{
    public MyFilter() : base(new MyGetter()) {}
}

you can still test your base class the same.