unit-testingcastle-windsor

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


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

On every test:

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.


Solution

  • 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