Dependency Injection to resolve dependency with runtime data

Question

I am using simple injector for my web api project. I have a service which requires a session token in order for it to instantiate.

public class CustomerService
{
   public CustomerService(Auth auth, IRepositoryFactory repositoryFactory)
   {
        // make post call to another web api for validation
        SomeWebApiCallToValidateAuth.vaildate(auth);
   }
}

So for this service, it requires an auth token and a repositoryFactory. I want it to be able to inject the auth parameter (which comes from the http web request) and at the same time to resolve the repository factory with the specified implemented thats registered to the container.

But I am not sure how to register this with simple injector or if there is a way around it. Any help would be great. Thanks.

Answer 1

Your current approach has several downsides:

• You inject runtime data into the constructor of your component, which can lead to complications.
• You make use of an Abstract Factory, which is often not the best abstraction.
• Your constructor invokes validation, while it should do nothing other than storing its incoming dependencies. This way you can compose your object graphs with confidence.

Concerning the factory: Inject an IRepository rather than an IRepositoryFactory. This might require you to hide the real repository behind a proxy, as explained here.

Concerning the Auth value, it depends on the need, but if the Auth value is an important part of the API of CustomerService, this justifies adding Auth as argument on the methods of CustomerService. If it is an implementation detail, inject an IAuthProvider abstraction of some sort that allows you to retrieve the value at runtime (after the object graph is built). Again, this all is described in this article.