Unity automatic injection

Question

I have already done the following:

1. Register an instance of my Logger in unity via a ILogger interface.
2. Created an interface, ILoggableObject, that has a method, Hook(ILogger logger), to inject my logger.

I would like to accomplish this:

Everytime I ask for any resolution from unity, if that object implements ILoggableObject, automatically inject the ILogger interface via the Hook method.

I think this is possible via interception or policies?

Any help would be awesome.

• Steven: Why don't you use the constructor injection, which is the default mechanism? Simply rename the Hook method to a public constructor (with the same arguments).
• Aaron McIver: Why not just pass ILogger into whatever objects ctor that needs to use the logging behavior?
• poindexter12: That is how I was doing it. The problem with that approach is when your objects start using multiple interfaces, you end up with a constructor that has 10 parameters. Then, another developer who wants to inherit from that base class has to supply all of the constructor parameters that he / she has no need to see.
• Saeed Amiri: You can use Named parameter for calling constructor to avoid this.
• Robin Orheden: @poindexter12: You should never have a constructor that accepts 10 parameters, 5-6 at most. If you do, then theres clearly something wrong with it. And a class like that should be refactored into smaller parts.

Answer 1

What you are looking for is TypeInterception in Unity. See here: http://msdn.microsoft.com/en-us/library/ff660861(PandP.20).aspx

Also here http://msdn.microsoft.com/en-us/library/ff660848(v=PandP.20).aspx

You want to intercept the call to the constructor and inject the Logger on behalf of the calling code without them being any wiser.

While I haven't done it before I believe you can do what you want using Intercept.NewInstance() http://msdn.microsoft.com/en-us/library/ff662093(PandP.20).aspx

Answer 2

That is a horrible way to (attempt to) do Dependency Injection. Use Constructor Injection instead and inject (via the constructor) the ILogger into the consumer that right now has the Hook method.

• poindexter12: This doesn't answer my question at all. While I appreciate the feedback, I am converting away from that method because of my comment above.
• Mark Seemann: However, that reason is a fallacy: too many constructor arguments aren't an indication that Constructor Injection is at fault, but rather that your class has too many responsibilities: stackoverflow.com/questions/2420193/…
• poindexter12: Okay, I read your blog post. It's great. I agree with all of your principles. I don't think you are attempting to answer my question though. While you like constructor injection, others may like property or method injection. The principal is the same. I agree that it would be nice for me to aggregate all of my ancillary services into one aggregate service. I will look into doing this immediately. My question still stands. If I have one aggregate service that I would like injected automatically via a method or property, how can I do this automatically?
• Mark Seemann: I'm sorry that I wasn't more constructive, but it looks like you got the answer you were looking for. For the record, though, I don't agree that the choice between Constructor, Property or Method Injection is a matter of taste. They are each appropriate in different situations.

Answer 3

While I really like Unity and IoC/DI, I'm not sure that it is the right tool to accomplish what you want to do.

You might consider looking at aspect-oriented programming using PostSharp or a similar tool. This would allow you to add logging or other cross-cutting concerns without changing the classes being instrumented.