Question

Looking at this answer on SO, I am a bit confused by the following “principle”:

Apply the Hollywood Principle

The Hollywood Principle in DI terms says: Don’t call the DI Container, it’ll call you.

Never directly ask for a dependency by calling a container from within
your code. Ask for it implicitly by using Constructor Injection.

But what if I have a repository class in my DAL, and I want to supply this instance to an object which is created when a TCP/IP client connects? At what place should I make the injection?

Right now, I have something like:

// gets created when a new TCP/IP client is connected
class Worker
{
    private readonly IClient client;
    public Worker(IClient client)
    {
        // get the repository
        var repo = IoC.GetInstance<IClientMessagesRepo>();

        // create an object which will parse messages
        var parser = new MessageParser(client);

        // create an object which will save them to repo
        var logger = new MessageLogger(parser, repo);
    }
}

I obviously cannot create this instance when my app is started. So where do I inject the repo?

Thanks a lot!

Answer 1

You should strive to only call IoC.GetInstance() once.

Since you cannot create the Worker at startup, you should instead create a WorkerFactory and have the DI container inject the dependency into that:

public class WorkerFactory
{
    private readonly IClientMessagesRepo clientMessagesRepo;
    public WorkerFactory(IClientMessagesRepo clientMessagesRepo)
    {
        this.clientMessagesRepo = clientMessagesRepo;
    }

    public Worker Create(IClient client)
    {
        return new Worker(client, clientMessagesRepo);
    }
}