aspnet - on code

Dependency Injection in Asp.net MVC using MVC Contrib

2010-01-07T05:23:10.037

For the sake of creating the sample let's introduce the dependency of a ILinkRepository in our brand new asp.net MVC application. Eventually this repository will be used by our HomeController to display a number of Links and finally we will inject the dependency by StructureMap dependency framework..

Next we will add the dependency of ILinkRepository on our HomeController. Since the default ControllerFactory cannot cope with controllers having no default constructor we will hard code the initialization of this dependency on the default constructor. Finally we will pass the list of links to the view and the view will display it for the user. This is the common scenario.

Since view is strongly typed, it will just iterate over the collection producing a list of links. We are now done with the basic structure of the application. We have just used a type of dependency injection infamously known as Poor Man's dependency injection since the HomeController constructor hard coded the creation of the repository.

Now, it's time to modify the controller for dependency injection. Since we are using dependency injection, controller is not concerned with creating the repository as in previous version. Dependency injection framework will automatically provide the dependent repository to the controller.

Running the application now will definitely produce an error since the default controller factory cannot create controllers having no parameterless constructor as shown below.

No parameterless constructor defined for this object.

Finally we are in a position to introduce the StructureMap dependency injection framework to the MVC application. It could be configured in the Global.asax.cs file as follows and the application will run as expected.

Source code could be downloaded from here.

Asp.net MVC Areas in depth - Part 2

2009-10-12T08:42:07.097

One of the first gotchas that will come across while using the asp.net MVC areas is in the usage of same controller name across different areas and the main project. For example the default asp.net MVC template has a HomeController and if we add the HomeController to one of the areas, accessing the home part of the areas will work fine but accessing the main home url will result in the following error

	The controller name 'home' is ambiguous between the following types: 
        AreasInDepth.Controllers.HomeController 
        AreasInDepth.Areas.Blog.Controllers.HomeController

The easiest solution is to come up with unique controller names. Since the routing for the controllers in the areas works fine in this case and the only thing we have to fix is the main controllers we can use another concept to make this code work. The following code in Global.asax file is all that is required to fix this mess

 protected void Application_Start()
 {
      AreaRegistration.RegisterAllAreas();
      RegisterRoutes(RouteTable.Routes);
      ControllerBuilder.Current.DefaultNamespaces.Add("AreasInDepth.Controllers"); 1
 }

Simply, the above code instructs the DefaultControllerFactory to give priority for the controller inside the main project over those in areas and do not throw the previous InvalidOperationException if the controller name is happened to be unique among these default name spaces 1.

It's time to continue exploring the areas in asp.net MVC 2 preview 2

public Route MapRoute(string name, string url, object defaults, object constraints, string[] namespaces) 
{
    if (namespaces == null && Namespaces != null) {
        namespaces = Namespaces.ToArray();
    }

    Route route = Routes.MapRoute(name, url, defaults, constraints, namespaces); 1
    route.DataTokens["area"] = AreaName;2

    bool useNamespaceFallback = (namespaces == null || namespaces.Length == 0);
    route.DataTokens["UseNamespaceFallback"] = useNamespaceFallback; 3

    return route;
}

MapRoute function of the AreaRegistrationContext class, discussed in the previous post, creates and adds 1 a Route to the global static RouteTable.Routes. It also places two data tokens namely area 2 and UseNamespaceFallback 3 which are later utilized in the route mapping workflow. If namespaces are not provided UseNameSpaceFallBack data token will have the value of true and the DefaultControllerFactory will probe the rest of the namespaces to get a valid controller type. This is a useful hook to prevent the DefaultControllerFactory from searching for controller in unintended areas. Rest of the magic happens during the request processing and we have to look the code inside DefaultControllerFactory, and here it is

protected internal virtual Type GetControllerType(RequestContext requestContext, string controllerName) {
    if (String.IsNullOrEmpty(controllerName)) {
        throw new ArgumentException(MvcResources.Common_NullOrEmpty, "controllerName");
    }

   
    object routeNamespacesObj;
    Type match;

     1
    if (requestContext != null && requestContext.RouteData.DataTokens.TryGetValue("Namespaces", out routeNamespacesObj)) 
    {
        IEnumerable routeNamespaces = routeNamespacesObj as IEnumerable;
        if (routeNamespaces != null) {
            HashSet nsHash = new HashSet(routeNamespaces, StringComparer.OrdinalIgnoreCase);
            match = GetControllerTypeWithinNamespaces(controllerName, nsHash);

            
            if (match != null || false.Equals(requestContext.RouteData.DataTokens["UseNamespaceFallback"])) {
                
                return match; 2
            }
        }
    }

    
    HashSet nsDefaults = new HashSet(ControllerBuilder.DefaultNamespaces, StringComparer.OrdinalIgnoreCase);
    match = GetControllerTypeWithinNamespaces(controllerName, nsDefaults); 3
    if (match != null) {
        return match;
    }

    
    return GetControllerTypeWithinNamespaces(controllerName, null /* namespaces */); 4
}

GetControllerType, first searches 1 the route namesapaces for the matching controller type. It will exit the search if a match is found or the namespace fallback option 2 is set to false. If the previous two conditions were not satisfied the function will search for the type in the application's default namespaces collection 3. This is the scenario explained at the beginning of this post. Finally there will be a fallback option to search all of the namespaces for valid controller type. 4

Asp.net MVC Areas in depth

2009-10-04T12:10:05.617

protected void Application_Start()
{
     AreaRegistration.RegisterAllAreas(); 1
     RegisterRoutes(RouteTable.Routes);
}

In the Application_Start event, the Asp.net MVC application registers the areas by calling a static function on the AreaRegistration abstract class 1.

public static void RegisterAllAreas() 
{
      RegisterAllAreas(null);
}

public static void RegisterAllAreas(object state) 
{
                                 2                        3
      RegisterAllAreas(RouteTable.Routes, new BuildManagerWrapper(), state);
}

RegisterAllAreas function was finally invoked with a RouteCollection 2 and an IBuildManager instance 3.

Buildmanager provides the methods to manage the Asp.net compilation. GetReferencedAssemblies method of the Buildmanager will return a collection of assemblies which includes those specified in assemblies element of web.config, assemblies built from App_Code folder, and assemblies from other top level folders.

internal static void RegisterAllAreas(RouteCollection routes, IBuildManager buildManager, object state) 
{
                                                                                               4
        List areaRegistrationTypes = TypeHelpers.FilterTypesInAssemblies(buildManager, IsAreaRegistrationType);
        foreach (Type areaRegistrationType in areaRegistrationTypes) 
        {
            AreaRegistration registration = (AreaRegistration)Activator.CreateInstance(areaRegistrationType);5
            registration.CreateContextAndRegister(routes, state);
        }
}

From the referenced assemblies, the application filters, using a predicate 4, those of AreaRegistration Type and creates an instance for each item found 5. In asp.net MVC preview 2 a class that is derived from AreaRegistration Type should be present in the corresponding area's folder and the instances of these classes are the ones created in the above step.

public class Routes: AreaRegistration 6
{
                                           7
    public override void RegisterArea(AreaRegistrationContext context)
    {
        context.MapRoute("blog_default", "Blog/{controller}/{action}/{id}",
                             new {controller = "Post", action = "index", id = ""});8
    }

    public override string AreaName 5
    {
        get { return "blog"; }
    }
}

CreateContextAndRegister method of AreaRegistration instance packages the RouteCollection, object state, and area name 5 (which is obtained from the user defined AreaRegistration derived class 6) in to an AreaRegistrationContext and passes the context 7 to the RegisterArea method which is implemented in the user defined AreaRegistration class. The custom defined AreaRegistration class then can add the required routes 8 to the RoutesCollection and the internals of these will be further explored in the next post.