Krzysztof Koźmic's blog

As we're near­ing the release date of Cas­tle Wind­sor 3.0 (code­name Wawel) I will be blog­ging about some of the new fea­tures and improve­ments intro­duced in the new version.

One of the fea­tures that were intro­duced in cur­rent ver­sion 2.5 was sup­port for debug­ger diag­nos­tic views in Windsor.

In Wawel one of the improve­ments is addi­tion of a new diag­nos­tic — detec­tion of cases where the con­tainer is being used as Ser­vice Loca­tor. For those unfa­mil­iar with what Ser­vice Loca­tor is, it's an approach that breaks the basic rule of Inver­sion of Con­trol, by explic­itly call­ing out to the con­tainer from within application.

Here's where I tell you Ser­vice Loca­tor is bad

I spent last two days read­ing through the Stack­Over­flow archive of IoC related ques­tions and one of the most com­mon sources of prob­lems is that con­tainer is being used as Ser­vice Locator.

I wrote why Ser­vice Loca­tor (par­tic­u­larly when imple­mented via IoC con­tainer) is a bad idea on few occa­sions (for exam­ple here). Also Mark has a good overview of draw­backs of this approach so I won't rehash it again.

Good news is — even if some­one from your team starts using the con­tainer as Ser­vice Loca­tor, Wind­sor will now help you detect those cases.

Here's where I give you an example

Take a look at the fol­low­ing class:

[Singleton]
public class ComponentFactory
{
    private readonly IKernel kernel;

    public ComponentFactory(IKernel kernel)
    {
        this.kernel = kernel;
    }

    public object Create(String name)
    {
        return kernel.Resolve<object>(name);
    }
}

It is part of the appli­ca­tion layer (and not infra­struc­ture layer) and as such it should not be aware of the con­tainer, yet it depends on IKer­nel. Also quick look at its Cre­ate method is enough to see that it most likely is passed around through­out the appli­ca­tion and used to pull com­po­nents from the con­tainer with­out giv­ing it much thought, which as you're surely guess­ing by now is a recipe for trouble.

Here's where I show you how it looks

If you nav­i­gate in debug mode to a con­tainer instance where com­po­nents like the one described above exists a new entry in the debug­ger view will appear list­ing those components.

Here's where I tell you how it works

Wind­sor doesn't have the whole pic­ture of your appli­ca­tion, so it can only detect a sub­set of cases of Ser­vice Loca­tor usage. In par­tic­u­lar it is unable to detect the case when a Ser­vice Loca­tor is built com­pletely on top of the con­tainer as com­monly found, where con­tainer is assigned to a pub­lic field of a sta­tic class and accessed through that field.

Since Wind­sor only knows about the com­po­nents you reg­is­ter with it, it looks for com­po­nents that depend on the con­tainer and are not extend­ing infra­struc­ture of the con­tainer itself (like for exam­ple Inter­cep­tors are). All such com­po­nents are flagged as poten­tial ser­vice loca­tor and pre­sented to you.

I hope you'll find this fea­ture useful.

Here's where you tell me what you think

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