The Injectionist is a dependency injection tool that can be used to e.g. configure something.

It came to life when Rebus needed a flexible built-in IoC container-like thing that supported decorators. Rebus uses it to register stuff throughout the Configure.With(...) spell, finally calling injectionist.Get<IBus>() at the end to resolve the bus and have all of its dependencies injected.

First, create an Injectionist instance:

var injectionist = new Injectionist();

Then, register a type-to-func mapping in it:

injectionist.Register<ISomething>(c => new ImplementationOfSomething());

where ImplementationOfSomething, obviously, is an implementation of ISomething.

If a class has one or more dependencies and needs to have stuff injected, get that stuff from the IResolutionContext passed to the func:

injectionist.Register(c => new HomeController(c.Get<ISomething>()));

Now, let's pretend that I want to decorate ImplementationOfSomething with another implementation of ISomething - do this:

injectioninst.Decorate<ISomething>(c => {
    var something = c.Get<ISomething>();

    return new SomethingDecorator(something);
});

The Injectionist will always resolve decorators first, in the order that they're registered in, recursively moving closer to the primary implementation for each nested call to c.Get<ISomething>().

When the time comes to actually create an instance of something, just Get it:

var result = injectionist.Get<HomeController>();

which will give you a ResolutionResult<HomeController> whose Instance property will give you the HomeController instance you asked for, and the TrackedInstances property will be an IEnumerable of all the objects that were built in order to build the object you asked for.

var instance = result.Instance; //< this is the HomeController

var disposables = result.TrackedInstances.OfType<IDisposable>();

This way, you can do whatever is necessary to properly dispose disposables that may have been built in order to satisfy some remote transitive dependency of HomeController.

It's pretty simple, but it's still kind of neat.

Injectionist is MIT-licensed