ASP.NET Core Anatomy (Part 3) - UseMvc - Steve Gordon

In parts one and two of this series I looked at the two main IServiceCollection extensions (AddMvcCore and AddMvc) in ASP.NET Core. These are used to add in the required MVC services when you plan to use the MVC middleware in your application.

The next thing you are required to do to enable MVC in your ASP.NET Core application is to include the UseMvc IApplicationBuilder extension from the Configure method of your Startup class. This method registers the MVC middleware into your application pipeline so that the MVC framework can handle requests and return the appropriate response (usually a view result or some JSON). In this post I will take a look at what happens when the UseMvc method is called during the application startup.

As with the earlier parts of this series, I’m currently using the rel/1.1.2 codebase from the MVC repository for my investigations. I’m still using the slightly older project.json based code as at the moment there don’t seem to be simple ways to debug into multiple ASP.NET Core source code assemblies in VS2017.

UseMvc is an extension on the IApplicationBuilder which takes an Action delegate of IRouteBuilder. The IRouteBuilder will be used to configure the routing for MVC. There is a more basic variant of the UseMvc method which does not required any parameters. This method simply calls down into the main version, passing in an empty delegate like so…

The main UseMvc method looks like this…

Let’s dissect what this method does. First it does a check to see if the IServiceProvider has an MvcMarkerService service registered in it. To do this it uses the ApplicationServices property which exposes the current IServiceProvider. Calling GetService on the service provider will try to locate a registered implementation for the requested Type. The MvcMarkerService is registered when AddMvcCore executes and as a result, if it is not found, it indicates that either AddMvc or AddMvcCore was not called during ConfigureServices. Marker services like this are used in quite a few places to help calling code verify proper registration of fundamental dependencies before they try to execute. The MvcMarkerService is just an empty class with no actual code.

Next, UseMvc requests a MiddlewareFilterBuilder from the ServiceProvider and sets its ApplicationBuilder using the IApplicationBuilder.New() method. The ApplicationBuilder creates and returns a new instance copy of itself when this method is called.

Next, UseMvc initialises a new RouteBuilder, setting the default handler to the registered MvcRouteHandler. At this point, the DI magic really kicks in and a bunch of dependencies start getting instantiated. MvcRouteHandler is requested and because it has some dependencies in its constructor, various other classes get initialised. Each constructor for these classes can also require additional dependencies which then also get created. This is a great example of how the DI system works. While all of the interfaces and concrete implementations are registered in the container during ConfigureServices, the actual objects are only created at the point they are required to be injected into something. By new-ing up the RouteBuilder, this snowball of object creation begins to occur, until all of the required dependencies have been constructed. Some of these are registered as singletons and will then live for the lifetime of the application, being returned whenever they are requested from the ServiceProvider. Others, might be constructed for each request or in some cases, constructed uniquely every time they are required. Understanding the details of how dependency injection works and specifically the ASP.NET Core ServiceProvider is out of scope for this post.

After creating the RouteBuilder, the Action<IRouteBuilder> delegate action is called which takes the RouteBuilder as its only parameter. In the MvcSandbox sample that I’m using to debug and dissect MVC we call the UseMvc method, passing in a delegate function via a lambda. This function maps a single route named “default” as follows.

This calls the MapRoute method which is an extension on the IRouteBuilder. The main MapRoute method looks like this:

This requests an instance of an IInlineConstraintResolver which is resolved by the ServiceProvider to a DefaultInlineConstraintResolver. This class takes an IOptions<RouteOptions> in its constructor.

The Microsoft.Extensions.Options assembly, as part of its work, calls into the MvcCoreRouteOptionsSetup.Configure method which takes a RouteOptions parameter. This adds a new constraint map of type KnownRouteValueConstraint to the Dictionary of constraint maps. This Dictionary is initialised with a number of default constraints when the RouteOptions is first constructed. I’ll look into the routing code in a little more detail in a future post.

A new Route object is constructed using the name and template provided. In our case this is added the RouteBuilder.Routes List. By this point, running the sample MvcSandbox application, our route builder now has a single route added.

Note that the MvcApplicationBuilderExtensions class also includes an extension method called UseMvcWithDefaultRoute. This method includes a call to UseMvc, passing in a hardcoded default route.

This route is defined with the same name and template as defined in the MvcSandbox application. Therefore it could have been used as a slight code saver within the MvcSandbox Startup class. For the most basic of MVC applications it’s possible that using this helper extension method might be enough for your application routing requirements. In most real-world cases I suspect you’ll end up passing in a more complete customised set of routes. It’s a nice shorthand though if you want to start with the basic routing template. It’s also possible to call UseMvc() without a parameter and instead use the attribute routing approach.

Next, the static AttributeRouting.CreateAttributeMegaRoute method is called and the resulting route is added to the Routes List (at index zero) on the RouteBuilder. CreateAttributeMegaRoute is documented with the summary “Creates an attribute route using the provided services and provided target router.” and it looks like this:

This method, creates a new AttributeRoute which implements the IRouter interface. Its constructor looks like this:

In its constructor it takes an IActionDescriptorCollectionProvider, an IServiceProvider and a Func which takes an array of ActionDescriptor’s and which returns an IRouter. By default, with the AddMvc registered services, it will get an instance of ActionDescriptorCollectionProvider which is a singleton object registered with the ServiceProvider. ActionDescriptors represent the available MVC actions that have been created and discovered within the application. We’ll look at those another time.

The code (inside the CreateAttributeMegaRoute method) which creates the new AttributeRoute uses a lambda to define the code for the Func<ActionDescriptor[], IRouter>. In this case the delegate function requests an MvcAttributeRouteHandler from the ServiceProvider. Because this is registered as transient, the ServiceProvider will return a new instance of an MvcAttributeRouteHandler each time one is requested. The delegate code then sets the Actions property (an array of ActionDescriptions) on the MvcAttributeRouteHandler using the incoming ActionDescriptions array and finally the new handler is returned.

Back to UseMvc; it finishes by calling the UseRouter extension on the IApplicationBuilder. The object passed into UseRouter is created by calling Build on the RouteBuilder. This method will add the routes into the RouteCollection. Internally the RouteCollection keeps track of which routes are named and which are unnamed. In our MvcSandbox example we end up with a named “default” route and an unnamed AttributeRoute.

UseRouter has two signatures. In this case we’re passing in a IRouter so the following method is called:

This does a check for a RoutingMarkerService within the ServiceProvider. Assuming this is registered as expected it adds the RouterMiddleware into the middeware pipeline. It is this middleware which will use the IRouter to try and match up requests to the controller and action within MVC which should handle them. The details of this process will feature in a future blog post.

At this point the application pipeline is configured and the application is ready to start receiving requests. That’s a good point to stop for this post.

Summary

In this post we’ve looked at UseMvc and seen that it sets up the MiddlewareFilterBuilder which will be used later. It then also gets an IRouter via a RouteBuilder and much of the work at this stage is around registering routes. Once everything is setup, the routing middleware is registered into the pipeline. It’s this code that knows how to inspect incoming requests and map the paths to the appropriate controllers and actions that can handle them.