Question

Suppose I have an interface for fetching data, and an implementation of it:

interface IResourceProvider
{
  string Get( Uri uri );
}

class HttpResourceProvider : IResourceProvider
{
  public string Get( Uri uri )
  {
    // Download via HTTP.
  }
}

I can register this in Castle Windsor as follows:

container.Register
  ( Component.For<IResourceProvider>().ImplementedBy<HttpResourceProvider>()
  );

(Which is all fine).

If I then decided I wanted a caching implementation as follows:

class CachingResourceProvider : IResourceProvider
{
  public CachingResourceProvider( IResourceProvider resourceProvider )
  {
    _resourceProvider = resourceProvider;
  }

  public string Get( Uri uri )
  {
    // Return from cache if it exists.
    // Otherwise use _resourceProvider and add to cache.
  }

  private readonly IResourceProvider _resourceProvider;
}

How would I register these nested dependencies? i.e., I want to say an IResourceProvider is implemented by a CachingResourceProvider, except where in the constructor, where it’s a HttpResourceProvider.

Answer 1

Just register CachingResourceProvider before HttpResourceProvider – e.g.

container.Register(Component
    .For<IResourceProvider>()
    .ImplementedBy<CachingResourceProvider>());
container.Register(Component
    .For<IResourceProvider>()
    .ImplementedBy<HttpResourceProvider>());

BTW – this is know as the Decorator design pattern.