ASP.NET Core Anatomy – How does UseStartup work? Exploring how UseStartup results in your Startup methods being registered and executed.

I was recently explaining to someone the basics of the program flow for an ASP.NET Core application. One of the things included in the templates for ASP.NET Core and used very often is the UseStartup<T> extension method on the IWebHostBuilder. This gets called from our Program.cs when initialising the application. UseStartup allows us to set the Startup class which defines the services and middleware pipeline for an ASP.NET Core application.

During my explanation I realised that while I know the result of calling this method, I didn’t know how things are wired up under the hood; so I decided to investigate!

NOTE 1: This content is valid as at the 2.0.0 release codebase. I don’t expect that the fundamentals will change dramatically in the future but I have seen some commits which tweak the code a little for 2.1!

NOTE 2: This is a deep dive blog post looking at internal ASP.NET Core code. You don’t need to know this to use ASP.NET Core to build applications – please don’t let this scare you off! This is intended for those of you, who like me, have a curious mind about the internals of ASP.NET Core. I’m conscious that this may get quite hard to follow as we get deep into the guts of the code as there’s a lot of use of delegates that makes explaining the flow quite challenging. I’ll try my best to make it clear!

How are Startup methods registered and executed?

The generic UseStartup<TStartup> method calls down to the main IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType) extension, passing in the Type for the Startup class. That method looks like this (full source on GitHub):

The ConfigureServices method on the IWebHostBuilder is called which expects an Action<IServiceCollection> parameter. In this case it’s defined as a lambda expression which acts on the IServiceCollection. The WebHostBuilder class has a private List<Action<WebHostBuilderContext, IServiceCollection>> field named _configureServicesDelegates. The call to ConfigureServices in the code above will add the lambda as a new item to this list which will be used later to construct an instance of IStartup.

At this stage a list of delegates will have been registered within the WebHostBuilder. The main process begins when Build() is called on the WebHostBuilder. I won’t cover everything that Build does, since it’s not all relevant to the scope of this post. Here’s the code of that method (full source on GitHub):

The Build method calls a private method BuildCommonServices (full source on GitHub) which as the name suggests will add some common framework services into the current ServiceCollection. I’ll skip over most of the code in this method. Unless we’ve changed the WebHostOptions to define different assemblies to load Startup from, the execution flow will eventually hit code which looks over the _configureServicesDelegates List, calling each delegate in turn. That piece of code looks like this:

In the sample application which I used in order to debug through the Hosting codebase, I have two delegates registered, one from a FakeServer (needed so that the WebHostBuilder doesn’t throw an exception) and one from the UseStartup call. The first delegate simply registers the FakeServer as the implementation for IServer inside the ServiceCollection. The second delegate will now execute the lambda expression which was registered in the UseStartup method. Let’s remind ourselves what that looked like:

If our Startup class implements IStartup directly, it can and will be registered as the implementation type for IStartup directly. In my sample (which is based on the default ASP.NET Core templates) our Startup class does not implement IStartup and will rely on conventions instead. In this case an AddSingleton overload is used which takes the Func<IServiceProvider, object) as it’s implementation factory. This Func delegate will be called when the first concrete IStartup implementation is requested from the DI container.

At this point the registration of IStartup is included in the ServiceCollection and ready to be called by the framework. The WebHostBuilder.Build method continues to execute and constructs a new WebHost instance, which includes passing in an IServiceCollection (a clone of the current hostingServices variable). It also passes in a ServiceProvider, built using the current state of the hostingServices ServiceCollection. This represents the application services which have been registered so far by the framework.

Once we have a WebHost instance, its Initialize method is called. This calls down to a private BuildApplication method (full source on GitHub). Hold tight, lots of stuff starts to happen at this stage. I’ll try to pick out the parts I think are relevant to the use of our Startup class.

The BuildApplication method does some basic checks to make sure the relevant services are available for the application to start. One of the checks is that the ServiceProvider which was passed in includes an implementation for IStartup. This particular check happens inside a helper method called EnsureStartup()

The call to _hostingServiceProvider.GetRequiredService<IStartup>(); will trigger the DI framework to construct an instance of IStartup as per its registration. Due to the use of delegates, we need to head back to the UseStartup method to look at the lambda we passed in for the Func<IServiceProvider, object) implementationFactory. As a reminder, here’s the service registration that was used:

We can see that we’re going to get returned a newly constructed ConventionBasedStartup instance as the implementation for IStartup. The ConventionBasedStartup constructor accepts a StartupMethods object (full source on GitHub) as its parameter. A static StartupLoader.LoadMethods method (full source on GitHub) is used to generate a StartupMethods instance. This object acts as a holder for three properties.

The most important of these properties for this discussion are the delegates for the ConfigureServices and Configure. These will be setup with the code which should execute when the framework calls these methods further down in the WebHost initialisation. Ultimately the code for these delegates is expected to execute methods on our our Startup class.

The first delegate it will try to find is the ConfigureDelegate. This will be used to build the middleware pipeline for the application. Internally the StartupLoader uses a helper method called FindMethod to do most of the work. This is called from a FindConfigureDelegate method (full source on GitHub). The FindMethod is as follows (full source on GitHub):

This method will first work out the method name(s) it should be looking for on the Startup class based on the methodName parameter passed to it. The convention is that the method which defines the middleware pipeline should be called Configure. There is a lesser known convention in the Startup class which in addition to providing the standard “Configure” method, you can choose to include environment specific version(s) in your Startup class too. By convention, if a Configure{EnvironmentName} is found (e.g. “ConfigureProduction”) for the current environment, that method will be used in preference to the general Configure method.

In our sample, we only have the standard Configure method defined. Reflection is used to find a method matching the expected name on our Startup type. There are various checks in place to ensure that the expected members on our class are valid (i.e. we don’t have more than one Configure method defined with the same name) and that it has the expected Void return type.

Once we have the MethodInfo for the matching method, it is passed as the parameter into the constructor for a new ConfigureBuilder instance (full source on GitHub). This is stored in a variable in the LoadMethods method to be used a little later on.

A very similar process occurs to find and store the MethodInfo for ConfigureServices from our Startup class which is stored in a local variable called servicesMethod. Finally, the same approach is used looking for a ConfigureContainerDelegate. This is  an optional method which we can include on our Startup class to interact with 3rd party dependency injection containers such as AutoFac. We won’t look at this here.

Next, inside LoadMethods, a static ActivatorUtilities.GetServiceOrCreateInstance is called to get or create an instance of our Startup class. Here’s a compressed version of that LoadMethods method for reference (full source on GitHub):

As an implementation instance of IStartup is not currently stored in the DI container, GetServiceOrCreateInstance will create an instance of our Startup class by calling it’s constructor. In my sample (which matches the default Startup class in a new ASP.NET Core application template) it expects an IConfiguration object be passed in. The DI framework will have access to an implementation for this and will inject it in for us. Here’s my Startup constructor for reference:

Next the method on the ConfigureServicesBuilder is set as a callback variable. It gets passed the newly created Startup instance as its parameter. The same occurs to store a callback for ConfigureContainer. Next a Func<IServiceCollection, IServiceProvider> is setup using a lambda expression (which I’ve excluded from the code above for now). We’ll look at this when we see how this gets called a little later.

At this point, the callback delegates are passed into the constructor for a new StartupMethods instance which is then returned as the result of LoadMethods. This is then passed into the constructor for the new ConventionBasedStartup instance. At this point we have a concrete implementation of IStartup registered with the DI framework. Back inside the WebHost.EnsureApplicationServices method, the ConfigureServices method from the IStartup interface is called. Ready to start navigating some delegates!?

The ConfigureServices method on the ConventionBasedStartup instance calls the ConfigureServicesDelegate property on its StartupMethods member.

This executes the lambda defined code from the StartupMethods.LoadMethods method which in-turn invokes the Func<IServiceCollection, IServiceProvider> delegate returned from the ConfigureServicesBuilder.Build method, which ultimately calls the private ConfigureServicesBuilder.Invoke method (full source on GitHub).

The Invoke method uses reflection to get and inspect the parameters required by the ConfigureServices method defined on our Startup class. By convention this method can be either parameterless or take a single parameter of type IServiceCollection.

If the ConfigureServices method on our Startup class expects the IServiceCollection parameter, this is set using the IServiceCollection which was passed into the Invoke method. Once the method is configured via reflection it is invoked and the returned value will either be Void or an IServiceProvider. It’s at this point that we are actually executing the code contained in our ConfigureServices method on our Startup class. Our class can use the IServiceCollection extensions to register services and their implementations with the DI container.

At this point the lambda expression (in StartupLoader) wants to return a ServiceProvider. If our Startup.ConfigureServices method returned an IServiceProvider directly, this then gets returned immediately. If not, an IServiceProviderFactory is requested from the hostingServiceProvider and used to construct the ServiceProvider. This is the application level ServiceProvider that will be used to resolve dependencies in our code base.

The final point I’d like to show within WebHost.BuildApplication is how the final RequestDelegate is built. We won’t cover this in depth here, but in short, the RequestDelegate is defined as “A function that can process an HTTP request.” This is what the framework will actually use to process each request through our application. This will be setup to include all of the middleware as defined in our applications pipeline.

The relevant code in side of WebHost.BuildApplication is (full source on GitHub):

An IApplicationBuilderFactory is used to build up and finally surface our RequestDelegate. This is one of the services registered earlier in the WebHostBuilder.BuildCommonServices method.

The ApplicationServices property on the builder is set with the ServiceProvider that was just created. The next detail is something I’ll gloss over slightly as it goes a bit too far off the flow I want to explore. In short an IEnumerable of IStartupFilters may have been registered with the DI framework. In my sample I haven’t registered any so only the default AutoRequestServicesStartupFilter (full source on GitHub) will be returned from the ServiceProvider.

An Action<IApplicationBuilder> delegate variable is created holding a wrapped set of Configure methods from each IStartupFilter, the final one being the delegate for our Startup.Configure method. At this point, the Configuration chain is called which first hits the AutoRequestServicesStartupFilter.Configure method. This holds our delegate chain as its next action and so this will call down into the ConventionBasedStartup.Configure method. This will call the ConfigureDelegate on its local StartupMethods object.

Invoking that Action will call the private ConfigureBuilder.Invoke method (full source on GitHub) which looks like this:

This will prepare to call our Startup.Configure method sending in the appropriate parameters which get resolved from the ServiceProvider. Our Configure method can add middleware into the application pipeline using the IApplicationBuilder. The final RequestDelegate is built and returned from the IApplicationBuilder and the WebHost initialisation then completes.


This has been a quite long and deeply technical post. If you’ve stuck with it; well done! I hope I’ve interpreted everything correctly and lifted the curtain on some of the “magic” behind the scenes that makes ASP.NET Core work. It was quite difficult to explain everything due to the layers of delegates involved here. Hopefully I did a good enough job for you to get the gist of things. I find it really useful to dig into the code like this and gain a better understanding of the internals. If you want to explore the code yourself, check out the ASP.NET Core Hosting repository on GitHub.

Other posts in this series

Visit the ASP.NET Core Anatomy Index post to see the other deep dives covered in this series.