.* Development

With MVC 3 comes the IDependencyResolver interface. This is designed to allow Service Providers to be registered with the framework. Service providers can range from IOC Containers such as Castle Windsor / Spring .NET, View Engines or Filter Providers.

The IDependencyResolver interface has two methods which must be implemented and looks like this:

public interface IDependencyResolver{

    public object GetService(Type serviceType);

    public IEnumerable<object> GetServices(Type serviceType);
}

The “GetService” method is used when only one service is expected to be returned, such as a controller implementing IControllerFactory. “GetServices” is called when more than one object is expected to be returned such as FilterProviders. If no matches are found GetService must return null and GetServices needs to return an empty set. If anything else is returned in either of these situations an exception will be thrown.

Let’s see how we can create a SpringDependencyResolver that implements the IDependencyResolver interface.

public class SpringDependencyResolver : IDependencyResolver{

    private IApplicationContext springContext;

    public SpringDependencyResolver(IApplicationContext context){
        springContext = context;
    }

    public object GetService (Type serviceType){
        IDictionaryEnumerator di = springContext.GetObjectsOfType(serviceType).GetEnumerator();

        di.MoveNext();
        try{
            return di.Value;
        }catch(InvalidOperationException ioe){
            return null;
        }

    public IEnumerable<object> GetServices(Type serviceType){
        IDictionary dictionary = springContext.GetObjectsOfType(serviceType);
        return dictionary.Cast<object>();
    }
}

It’s fairly straight forward, with only a couple of things to note. Line 10 can return a number of items. This is because I couldn’t seem to find any other way in Spring .NET of returning one object for that type, so instead, if more than one is returned we just select the first, as you can see in line 12. The SpringContext does have a GetObject(string name) method, but as you can see, itself and all it’s overloaded versions require the name of the Object. This isn’t specified in IDependencyResolver, so I got around it by returning the first item.

Lines 20 and 21 involve getting the Dictionary from Spring .NET with all the objects and casting it to a dictionary of “Object” types and then returning.

Once you have your SpringDependencyResolver ready, you’ll want to plug it into your app and get going. To register the SpringDependencyResolver, you would normally need to do this in the Global.asax file in the Application_Start() method. This is normally a simple step, but I encountered a problem. It seems that in IIS 7, when running in Integrated mode, the Request scope is not available in the Application_Start() method and Spring .NET needs this scope to initialise itself. You can read more about this here: http://mvolo.com/blogs/serverside/archive/2007/11/10/Integrated-mode-Request-is-not-available-in-this-context-in-Application_5F00_Start.aspx

So, with the help of Mike Volodarsky in the post above let’s see how we can plug our SpringDependencyResolver into our application. In my Global.asax file I have the following class defined:

class FirstRequestInitialization{
    private static bool s_InitializedAlready = false;
    private static Object s_lock = new Object();

    // Initialize only on the first request
    public static void Initialize(){
        if (s_InitializedAlready){
            return;
        }
        lock (s_lock){
            if (s_InitializedAlready){
                return;
            }
            //Set SpringDependencyResolver as resolver for the framework
            DependencyResolver.SetResolver(
                new SpringDependencyResolver(ContextRegistry.GetContext()));
            s_InitializedAlready = true;
        }
    }
}

Along with this for my Application_BeginRequest() method:

protected void Application_BeginRequest(Object source, EventArgs e){

    /* Attempt to peform first request initialization - To fix issue of
    ** request scope not being available on App start
    */
    FirstRequestInitialization.Initialize();
}

As you can see, the initialization of the SpringDependencyResolver is only fired on the very first request of the application and gets around the issue described in Mike’s post. Of course all your other regular MVC related stuff can go in the Appication_Start() method such as registering filters and routes. It’s only the Spring .NET initialization that needs Mike’s solution.