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?

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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()                           ); 

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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

Answer 2

Don't reuse your container across tests. Instead, set it to null in the TearDown() and re-initialise it for each actual test.