How to resolve instance of base interface with Simple Injector

Question

I have an interface that inherits from another one

public interface ISpecificHandler : IHandler

With Simple Injector I register my type and implementation

container.RegisterSingleton<ISpecificHandler, SpecificHandlerImpl>();

But how can I just resolve IHandler instead of ISpecificHandler ?

//Expect SpecificHandlerImpl as handler
IHandler handler = ServiceLocator.GetInstance<IHandler>(); //error

This throws a InvalidOperationException that the IHandler type is not registered

Answer 1

There are several solutions, but it really depends on what your application really needs. Here are some suggestions.

You just register the implementation twice:

container.Register<IHandler, SpecificHandlerImpl>(Lifestyle.Scoped);
container.Register<ISpecificHandler, SpecificHandlerImpl>(Lifestyle.Scoped);

But perhaps you want to resolve multiple handlers, in which case you will have to probably register them as a collection:

var reg1 = Lifestyle.Singleton.CreateRegistration<SpecificHandlerImpl>();
var reg2 = Lifestyle.Singleton.CreateRegistration<AnotherHandlerImpl>();

container.RegisterCollection<IHandler>(new[] { reg1, reg2 });

You might run into troubles however when using a non-generic IHandler interface, because you will usually have one specific implementation that should be executed in a certain scenario (in other words, you might be violating the Liskov Substitution Principle ). So you'd often be better of using a generic IHandler<T> abstraction where T is a parameter object that contains the values of the handler. In that case there is always a one-to-one mapping between a closed IHandler<T> and an implementation. This article gives a good explanation of the advantages of such design.

But without knowing the problem you're trying to solve, it's hard to say what would be the right solution for you.