In Castle Windsor 3, override an existing component registration in a unit test

Question

I am attempting to use Castle Windsor in my automated tests like so:

On every test:

• The Setup() function creates a Windsor container, registering default implementations of each component
• The Test function access the components via the method IWindsorContainer.Resolve<T>, and tests their behavior
• The TearDown() function disposes of the Windsor container (and any created components)

For example, I might have 15 tests which accesses components which indirectly results in the creation of an IMediaPlayerProxyFactory component. The SetUp function registers a good-enough implementation IMediaPlayerProxyFactory, so I don't have the maintenance burden of registering this in each of the 15 tests.

However, I'm now writing a test Test_MediaPlayerProxyFactoryThrowsException, confirming my system elegantly handles an error from the IMediaPlayerProxyFactory component. In the test method I've created my special mock implementation, and now I want to inject it into the framework:

this.WindsorContainer.Register(
                                 Component.For<IMediaPlayerProxyFactory>()
                                          .Instance(mockMediaPlayerProxyFactory)
                              );

But Windsor throws a Castle.MicroKernel.ComponentRegistrationException, with the message "There is already a component with that name."

Is there any way that I can make my mockMediaPlayerProxyFactory be the default instance for the IMediaPlayerProxyFactory, discarding the component that's already registered?

---

According to the documentation, Castle Windsor 3 allows for registration overrides, but I could only find one example:

Container.Register(
    Classes.FromThisAssembly()
        .BasedOn<IEmptyService>()
        .WithService.Base()
        .ConfigureFor<EmptyServiceA>(c => c.IsDefault()));

ConfigureFor is a method of the BasedOnDescriptor class. In my case I'm not using the FromDescriptor or BasedOnDescriptor.

Answer 1

There are two things that you have to do to create an overriding instance:

1. Assign it a unique name
2. Call the IsDefault method

So to get the example to work:

this.WindsorContainer.Register(
                             Component.For<IMediaPlayerProxyFactory>()
                                      .Instance(mockMediaPlayerProxyFactory)
                                      .IsDefault()
                                      .Named("OverridingFactory")
                          );

Because I plan to use this overriding patten in many tests, I've created my own extension method:

public static class TestWindsorExtensions
{
    public static ComponentRegistration<T> OverridesExistingRegistration<T>(this ComponentRegistration<T> componentRegistration) where T : class
    {
        return componentRegistration
                            .Named(Guid.NewGuid().ToString())
                            .IsDefault();
    }
}

Now the example can be simplified to:

this.WindsorContainer.Register(
                             Component.For<IMediaPlayerProxyFactory>()
                                      .Instance(mockMediaPlayerProxyFactory)
                                      .OverridesExistingRegistration()
                          );

---

Later Edit

Version 3.1 introduces the IsFallback method. If I register all my initial components with IsFallback, then any new registrations will automatically override these initial registrations. I would have gone down that path if the functionality was available at the time.

https://github.com/castleproject/Windsor/blob/master/docs/whats-new-3.1.md#fallback-components