ASPHostCentral.com :: Introduction to ASP.NET MVC 3 Service Location

Nov. 18, 2010 - PRLog -- One of the new features in ASP.NET MVC 3 is the ability to register a service locator that will be used by the framework. Prior versions of the MVC framework have offered opportunities for introducing concepts like service location and dependency injection (DI); in MVC 3, we have formalized the process and opened up several new opportunities for developers

This first post in the series will discuss the general strategy for service location in MVC 3. Later posts will discuss specific ways to perform service location and DI with existing and new features

Disclaimer

This blog post talks about ASP.NET MVC 3 Preview 1, which is a pre-release version. Specific technical details may change before the final release of MVC 3. This release is designed to elicit feedback on features with enough time to make meaningful changes before MVC 3 ships

General Strategy

The most important part of the strategy with service location is that it’s going to be optional. This means that if you are not interested in working with a service locator, you won’t be forced to. We will always offer a way to perform customization functions without requiring you to employ a service locator. We will also work to preserve backward compatibility as much as possible when introducing new service location capability into existing features of MVC

When using the registered service locator, MVC will generally employ one of three strategies, depending on the kind of work it’s trying to do

1. Locating a singly registered service

There are several services which MVC uses today which have the ability to register a single instance of such a service. One example is the controller factory, which implements IControllerFactory. There is a single controller factory that’s used for the entire application

When MVC attempts to use a singly registered service, it will first ask the service locator whether it has an instance of that service available. If it does, then MVC will use that; if it does not, then it will fall back to the singleton registration that’s available for non-service locator users

The upside of this means that service locator users won’t be required to fill their locator/container with all the existing default services used by MVC, because it will automatically fall back to these defaults when nothing exists in the locator. The potential downside of this means that it is theoretically possible to register a custom service in both places, but only the one in the locator will be used

2. Locating a multiply registered service
There are several services which MVC uses today which have the ability to register multiple instances of such a service. One example of this is the view engine, which implements IViewEngine. Typically, MVC offers a registration point which acts like a list of such services, and also offers an API which acts as a facade and figures out the appropriate way to do the work. In the case of view engines, this facade (ViewEngines.Engines) takes the form of “go to each view engine on the list and ask it for a view until one of them can provide it”. There are also multiply registered services where the facade uses all the services (ModelValidatorProviders.Providers) and aggregates all the responses together

When MVC attempts to use a multiply registered service, it will continue to ask the collection facade to do the work. The collection facade will use both the statically registered instances of the service as well as any of the instances registered with the service locator, and combines them together in the way that is most appropriate for the facade. Where service order is important (as with view engines, for example), this usually means that the service locator instances will come before the statically registered instances

Like the single registration strategy, the upside of this means that service locator users won’t be required to fill their locator/container with the existing default service instances. The potential downside of this is that most containers don’t offer a native ordering function for multi-registration of services, so where ordering is important, it may be necessary to use the non-service locator APIs. In practice, however, it probably won’t be much of an issue, as most applications don’t generally need to rely on ordering (for example, they will only primarily use a single view engine), especially since service locator services generally go before the existing pre-registered services

3. Creating arbitrary objects

The final way that MVC may use a service locator is in the creation of arbitrary objects. (This is where we stray from the strictly service location functionality and use it more like a dependency injection container.) Where we’ve found it appropriate that an object we create may be in need of dependency injection of services, we will attempt to create that object through the service locator. One example is controller objects, which may want to take service dependencies to do their work

When MVC attempts to create an arbitrary object in this way, it will ask the service locator to create the object on its behalf. If the service locator cannot fulfill this object creation, it will generally fall back to the existing behavior in prior versions of MVC 2; usually, this means calling Activator.CreateInstance

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