ASP.NET Core 3.0 - ConfigureContainer magic

Last week I blogged about the changes I had to make to let Autofac work with ASP.NET Core 3.0. Inside my Startup.cs file I had to use the .ConfigureContainer() method:

	public void ConfigureContainer(ContainerBuilder builder)
	{
	builder.RegisterModule(new AutofacModule());
	}

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But where is this method coming from? Let’s dig into the ASP.NET Core source code to find out…

The source of all magic is the StartupLoader class: https://github.com/aspnet/Hosting/blob/rel/1.1.0/src/Microsoft.AspNetCore.Hosting/Internal/StartupLoader.cs.

This class uses reflection to find the following 3 methods in the Startup.cs file:

• a Configure() method
• a ConfigureServices() method
• a ConfigureContainer() method

	var configureMethod = FindConfigureDelegate(startupType, environmentName);
	var servicesMethod = FindConfigureServicesDelegate(startupType, environmentName);
	var configureContainerMethod = FindConfigureContainerDelegate(startupType, environmentName);

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If you want environment-specific setup you can put the environment name after the Configure part, like ConfigureDevelopment, ConfigureDevelopmentServices, and ConfigureDevelopmentContainer. If a method isn’t present with a name matching the environment it’ll fall back to the default.

If a ConfigureContainer() method is found, the IServiceProviderFactory<TContainerBuilder> CreateBuilder method is invoked and the created builder is passed as a parameter to the ConfigureContainer()

	// We have a ConfigureContainer method, get the IServiceProviderFactory<TContainerBuilder>
	var serviceProviderFactoryType = typeof(IServiceProviderFactory<>).MakeGenericType(configureContainerMethod.GetContainerType());
	var serviceProviderFactory = hostingServiceProvider.GetRequiredService(serviceProviderFactoryType);
	// var builder = serviceProviderFactory.CreateBuilder(services);
	var builder = serviceProviderFactoryType.GetMethod(nameof(DefaultServiceProviderFactory.CreateBuilder)).Invoke(serviceProviderFactory, new object[] { services });
	configureContainerCallback.Invoke(builder);
	// applicationServiceProvider = serviceProviderFactory.CreateServiceProvider(builder);
	applicationServiceProvider = (IServiceProvider)serviceProviderFactoryType.GetMethod(nameof(DefaultServiceProviderFactory.CreateServiceProvider)).Invoke(serviceProviderFactory, new object[] { builder });

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