Sitecore MVC Internals: IControllerActivator

When we previously looked into IControllerFactory implementations in both Sitecore MVC and standard ASP.NET MVC, we noticed that Microsoft’s default IControllerFactory implementation, DefaultControllerFactory, doesn’t actually handle the instantiation of new IController instances. To enable dependency injection at the point of object creation, DefaultControllerFactory hands off instation of IController objects to another player – an object which implements IControllerActivator.

How we call an IControllerActivator implementation

Let’s look back at the code for the GetControllerInstance method of Microsoft’s DefaultControllerFactory class (see previous post for a more in-depth discussion)

protected internal virtual IController GetControllerInstance(RequestContext requestContext, Type controllerType)
{
    //...
    return this.ControllerActivator.Create(requestContext, controllerType);
}

This method crucially takes in the Type of a controller, which we have determined by looking carefully at the requested URL and matching it to a known route. We see that DefaultControllerFactory already has an implementation of IControllerActivator to work with, referenced by this.ControllerActivator.

What is Microsoft’s default implementation?

In standard ASP.NET MVC sites, this implementation is Microsoft’s DefaultControllerActivator. Let’s look at the DefaultControllerActivator.Create() method in full:

public IController Create(RequestContext requestContext, Type controllerType)
{
    try{
            return (IController) (this._resolverThunk().GetService(controllerType) ?? Activator.CreateInstance(controllerType));
    }
    catch (Exception ex)
    {
            throw new InvalidOperationException(string.Format((IFormatProvider) CultureInfo.CurrentCulture, MvcResources.DefaultControllerFactory_ErrorCreatingController, new object[1]{(object) controllerType}), ex);
    }
}

Ignoring boilerplate, the line we’re left with is:

(IController) (this._resolverThunk().GetService(controllerType) ?? Activator.CreateInstance(controllerType));

Don’t be discouraged by the odd looking _resolverThunk(). This is a reference to the chosen Dependency Resolver for your project. Remembering that ASP.NET MVC was built for extendability at every point, a Dependency Resolver lets us have full control over how any object is created – typically using a Dependency Injection framework. We’ll look at Dependency Resolvers in a future post.

Back to the line above. If you have a Dependency Resolver in place, the GetService() method will use this resolver to return an instance of the IController you need to fulfil the request. If not, we fall back to good old Activator.CreateInstance(), which is the .NET frameworks vanilla way of creating new objects at runtime.

Does Sitecore implement IControllerActivator?

No, Sitecore doesn’t utilist IControllerActivator. There’s a simple reason – once Sitecore has determined the type of Controller you would like to create, it hands off the job of creating the controller to the mvc.createController Pipeline:

IController controller = PipelineService.Get().RunPipeline<CreateControllerArgs, IController>(“mvc.createController”, 
        new CreateControllerArgs(requestContext, controllerName), 
        (Func<CreateControllerArgs, IController>) (args => args.Result));

That’s it for IControllerActivator, happy hacking!

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