Dependency Injection is a great way to remove dependencies while having an lightweight container (a “service locator”) wiring up the application’s components and resolving instances and relationships. But Ninject, one of the dependency injection solutions for .Net, adds another unwanted dependency: to the container itself.
A dependency to Ninject is needed when initializing the container and obtaining entities, although the latest can be minimized using the Common Service Locator library to create an abstraction to most of the currently available dependency injection implementations. But Ninject also adds a dependency to the classes that will be initialized by the container, this is not desired and should be avoided at all cost and these classes shouldn’t be aware how they’ll be instantiated.
Ninject defines an attribute (Inject) used as an hint by the Ninject kernel to know where to inject the instantiated objects; although this attribute is optional for Constructor Injection is mandatory for Property Injection and Method Injection.
// My class where Ninject
// will create and inject
// an instance of IWeapon
public class Samurai
{
// This is a Ninject attribute,
// if we switch to another
// DI container the code will break
[Inject]
public IWeapon Context { get; set; }
}
Fortunately Ninject is highly customizable and allows to change that behavior, the plan is to create a custom attribute and configure it so that Ninject can use it the same way it uses the Inject attribute. The usage will be similar but all the dependencies will be to local classes, the custom attribute can be something as simple as this:
public class InjectHereAttribute : Attribute
{
}
The class will now look like this:
public class Samurai
{
// Now there's no external dependency,
//
//
[InjectHere]
public IWeapon Context { get; set; }
}
Now Ninject must be configured to use the custom attribute, this can be done by creating an implementation of IInjectionHeuristic that recognizes the custom attribute:
public class CustomInjectionHeuristic : NinjectComponent, IInjectionHeuristic, INinjectComponent, IDisposable
{
public new bool ShouldInject(MemberInfo member)
{
return member.IsDefined(
typeof(InjectHereAttribute),
true);
}
}
And finally add this behavior to the Ninject Kernel using the Components collection, it will run along the existing components, namely the default implementation of IInjectionHeuristic, which means either the default or the custom attribute can be used.
// Add custom inject heuristic kernel.Components.Add<IInjectionHeuristic, CustomInjectionHeuristic>();
