Define an initialization order of WebActivator.PreApplicationStartMethod classes

Question

I have several WebActivator.PreApplicationStartMethod decorated classes.

One is for Ninject, another class for AwesomeMVC and a third one is for background task scheduler.

The problem is that the scheduler class needs to take advantage of the dependecies, that are resolved by IoC container.

My questions are:

1. Can I have several WebActivator.PreApplicationStartMethod classes?
2. Can I define order, in which they are initialized, so that IoC, being the most important, comes first?
3. Can WebActivator.PreApplicationStartMethod static class instances rely on IoC container to resolve their constructor-defined dependencies?

Pemberton

Answer 1

If you know that PreAppStart method A needs to run after PreAppStart method B, then the only way to achive that is to explicitly add a call to B inside the body of A.

For that strategy to work correctly you should also make sure that your PreAppStart method implementations are indempotent i.e. they can safely be called multiple times. Usually this can be achieved by keeping track of whether the method has already been called in a static boolean variable and not doing anything if that vale is true.

Peneus

Are you saying that even if I have one PreAppStat method, it can get called several times? Why is it so? – Pemberton

The framework will only call the method once. But say you have 3 components: A, B, and C. A and B both depend on C (and therefore as per this guideline, they both call C's PreAppStart method in their own methods). But A and B are independent and don't know about each other and so cannot coordinate how many times C's PreAppStart method got called. That's why C's method needs to be idempotent. – Peneus

You can order it (see other comment). Use "Order" parameter in WebActivator attribute – Hyonhyoscine

Answer 2

Yes, you can have as many classes as you want which have a WebActivator.PreApplicationStartMethod assembly attribute pointing to them. Many NuGet packages use this technique to enable them to bootstrap into your application without editing Global.asax.

You can define the order too. You can pass a named parameter, Order in the PreApplicationStartMethod call. The WebActivator framework will ensure that the methods are called in the order specified. For example, to make your IoC framework register first, do something like this:

[assembly: WebActivator.PreApplicationStartMethod(typeof(MyApp.App_Start.NinjectWebCommon), "Start", Order=1]
[assembly: WebActivator.PreApplicationStartMethod(typeof(MyApp.App_Start.BGScheduler), "Start", Order=2]

Because WebActivator classes are static classes, I don't see how you can use constructor injection in them. You can however, use the service locator (anti?)-pattern by registering your IoC resolver as Mvc's default service locator, using System.Web.Mvc.DependencyResolver.SetResolver(IDependencyResolver resolver).

I don't particularly want to go into the benefits and drawbacks of the service locator pattern here though!

Mirthamirthful

Just worth to mention that a WebActivatorEx package needs to be used instead of WebActivator. – Barquentine