Question

I’m using Ninject in an MVC project and am trying to implement Domain Events following Udi Dahan’s pattern http://www.udidahan.com/2009/06/14/domain-events-salvation/

In the extract below, the “Container” is used to resolve all the event-handlers for the particular type of event that has been raised.

My question (& apologies if I am missing something basic) is how to do this with Ninject? In other words:

1. How does the “Container” get set in this static class?

2. Once I have a Container (Kernel?) what would be the Ninject syntax to resolve all the event handlers (which I’m assuming I would have to register before-hand in a Service Module)?

I keep reading in posts that only constructor injection should be used and everything recursively get resolved from that, and that accessing the Ninject Kernel is a no-no. So any advice on how to do this will be much appreciated.

Extract from the article

public static class DomainEvents
{ 
  [ThreadStatic] //so that each thread has its own callbacks
  private static List<Delegate> actions; 

  public static IContainer Container { get; set; } //as before

  //Registers a callback for the given domain event
  public static void Register<T>(Action<T> callback) where T : IDomainEvent
  {
     if (actions == null)
        actions = new List<Delegate>();

     actions.Add(callback);
 }

 //Clears callbacks passed to Register on the current thread
 public static void ClearCallbacks ()
 {
     actions = null;
 }

 //Raises the given domain event
 public static void Raise<T>(T args) where T : IDomainEvent
 {
    if (Container != null)
       foreach(var handler in Container.ResolveAll<Handles<T>>())
          handler.Handle(args);

    if (actions != null)
        foreach (var action in actions)
            if (action is Action<T>)
                ((Action<T>)action)(args);
 }
}

Answer 1

How does the “Container” get set in this static class?

You will have to set it during application startup:

DomainEvents.Container = kernel;

what would be the Ninject syntax to resolve all the event handlers:

You can do it like this, for instance:

Container.Get<IEnumerable<Handles<T>>>())

Udi’s static DomainEvents class is an implementation of the Ambient Context anti-pattern (see DI PP&P chapter 5.3). In this case I would rather use dependency injection to inject an IDomainEvents abstraction into code that needs it, instead of letting code depend on a static instance.

The problem however is that your domain objects will need a dependency on the IDomainEvents and constructor injection is (probably) not possible. The trick is to use method injection in that case.

In other words, use constructor injection to inject the IDomainEvents into command handlers or services (or what ever you call your business logic that uses the methods on your domain objects) and pass that dependency into the domain object when calling a method that needs it (method injection).