How to Resolve type based on end-user configuration value?

Question

I have an interface (call it IAcmeService) that has multiple implementations.

FileSystemAcmeService
DatabaseAcmeService
NetworkAcmeService

Answer 1

Defining and implementing and Abstract Factory is the standard solution to this type of problem. If you will pardon my use of C#, you can define an IAcmeServiceFactory interface like this:

public interface IAcmeServiceFactory
{
    IAcmeService Create(string serviceName);
}

You can now write a concrete implementation like this one:

public class AcmeServiceFactory : IAcmeServiceFactory
{
    private readonly IAcmeService fsService;
    private readonly IAcmeService dbService;
    private readonly IAcmeService nwService;

    public AcmeServiceFactory(IAcmeService fsService,
        IAcmeService dbService, IAcmeService nwService)
    {
        if (fsService == null)
        {
            throw new ArgumentNullException("fsService");
        }
        if (dbService == null)
        {
            throw new ArgumentNullException("dbService");
        }
        if (nwService == null)
        {
            throw new ArgumentNullException("nwService");
        }

        this.fsService = fsService;
        this.dbService = dbService;
        this.nwService = nwService;
    }

    public IAcmeService Create(string serviceName)
    {
        switch case serviceName
        {
            case "fs":
                return this.fsService;
            case "db":
                return this.dbService;
            case "nw":
                return this.nwService;
            case default:
                throw new ArgumentException("serviceName");
        }
    } 
}

You can make it more general-purpose if you want to be able to create an arbitrary number of IAcmeService instances, but I will leave that as an exercise to the reader :)

This will require you to register the Factory with Unity as well. In any place where you need an IAcmeService based on a name, you take a dependency on IAcmeServiceFactory instead of IAcmeService itself.