Question

So, let’s say I have an interface IThingFactory:

public interface IThingFactory
{
    Thing GetThing(int thingId);
}

Now, let’s say I have a concrete implementation that retrieves Things from a database. Now, let’s also say I have a concrete implementation that wraps an existing IThingFactory and checks for a Thing‘s presence in, say, an in-memory cache before hitting the wrapped IThingFactory. Something like:

public class CachedThingFactory : IThingFactory
{
    private IThingFactory _wrapped;
    private Dictionary<int, Thing> _cachedThings;

    public CachedThingFactory(IThingFactory wrapped)
    {
        this._wrapped = wrapped;
        _cachedThings = new Dictionary<int,Thing>();
    }

    public Thing GetThing(int thingId)
    {
        Thing x;
        if(_cachedThings.TryGetValue(thingId, out x))
            return x;

        x = _wrapped.GetThing(thingId);

        _cachedThings[thingId] = x;

        return x;
    }
}

How would I deal with a scenario like this using dependency injection with something like, say, Ninject, so that I could configure the DI container so that I can inject or remove a caching proxy like this, or, say, something that does logging, or (insert here)?

Answer 1

You can do something along the lines of:

Bind<IThingFactory> ().To<DefaultThingFactory> ().WhenInjectedInto<CachedThingFactory> ();
Bind<IThingFactory> ().To<CachedThingFactory> ();

This will let consumers not need to specify a name attribute, and is still relatively easy to further enhance. If you later wanted to add an additional “decorator” layer for logging, you could do:

Bind<IThingFactory> ().To<DefaultThingFactory> ().WhenInjectedInto<LoggingThingFactory> ();
Bind<IThingFactory> ().To<LoggingThingFactory> ().WhenInjectedInto<CachedThingFactory> ();
Bind<IThingFactory> ().To<CachedThingFactory> ();

Not the prettiest, but it works.