Alex Rivera | Logout

Lazy Dependency Injection

Asked 2012-09-05T15:09:42.367
20

I have a project where the Ninject is used as IoC container. My concern is that a lot of classes have such kind of constructors:

[Inject]
public HomeController(
    UserManager userManager, RoleManager roleManager, BlahblahManager blahblahManager) {
   _userManager = userManager;
   _roleManager = roleManager;
   _blahblahManager = blahblahManager;
}

What if I don't want to have all instances of these classes at once?

The way, when all this classes are wrapped by Lazy<T> and passed to constructor is not exactly what I need. The T instances are not created yet, but Lazy<T> instances are already stored in memory.

My colleague is suggesting me to use Factory pattern to have control over all instantiations, but I'm not sure that IoC have such great design bug.

Is there a workaround for this situation or IoC really have such big defect in it's design? Maybe I should use another IoC container?

Any suggestions?

c# .net design-patterns inversion-of-control ninject
Edit
Report

1 Answer

54

Seems to me that you are doing premature optimization: don't do it.

The constructors of your services should do nothing more than storing the dependencies that it takes in private fields. In that case the creation of such an object is really light weight. Don't forget that object creation in .NET is really fast. In most cases, from a performance perspective, it just doesn't matter whether those dependencies get injected or not. Especially when comparing to the amount of objects the rest of your application (and the frameworks you use) are spitting out. The real costs is when you start using web services, databases or the file system (or I/O in general), because they cause a much bigger delay.

If the creation is really expensive, you should normally hide the creation behind a Virtual Proxy instead of injecting a Lazy<T> in every consumer, since this allows common application code to stay oblivious to the fact that there is a mechanism to delay the creation (both your application code and test code are becoming more complex when you do this).

Chapter 8 of Dependency Injection: Principle, Practices, Patterns contains a more detailed discussion about lazy and Virtual Proxies.

However, a Lazy<T> just consumes 20 bytes of memory (and another 24 bytes for its wrapped Func<T>, assuming a 32bit process), and the creation of a Lazy<T> instance is practically free. So there is no need to worry about this, except when you’re in an environment with really tight memory constraints.

And i

answered 2012-09-05T16:01:43.100

Your Answer