如何从 .NET Core 3.1 中的 Autofac 容器解析已注册的依赖项

Question

There's a project I'm trying to upgrade to .NET Core 3.1 from 2.2. My major issue is at the startup.cs where I'm registering dependencies with Autofac. I have studied the documentation for registering dependencies in 3.0 and I understand it. But there is a requirement in ConfigureServices method to register an object as an instance and it requires the container to resolve a registered service in the same method as seen in the code below

Startup.cs (2.2)

public IServiceProvider ConfigureServices(IServiceCollection services)
{

    services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    services.AddSingleton<IExecutionContextAccessor, ExecutionContextAccessor>();

    //other services registered

    return CreateAutofacServiceProvider(services);
}

private IServiceProvider CreateAutofacServiceProvider(IServiceCollection services)
{
    var containerBuilder = new ContainerBuilder();

    containerBuilder.Populate(services);

    containerBuilder.RegisterModule(new MyModule());

    var container = containerBuilder.Build();

    var httpContextAccessor = container.Resolve<IHttpContextAccessor>();
    var executionContextAccessor = new ExecutionContextAccessor(httpContextAccessor);

    var emailsConfiguration = new EmailsConfiguration(_configuration["EmailsConfiguration:FromEmail"]);

    // executionContextAccessor is used to initialize some modules here

    return new AutofacServiceProvider(container);
}

In .net core 3.1, I couldn't find a clear means of replicating the above with the new Autofac approach.

Startup (3.1)

public ILifetimeScope AutofacContainer { get; private set; }

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    services.AddSingleton<IExecutionContextAccessor, ExecutionContextAccessor>();

    // other services registered

    var httpContextAccessor = this.AutofacContainer.Resolve<IHttpContextAccessor>();
    var executionContextAccessor = new ExecutionContextAccessor(httpContextAccessor);

    var emailsConfiguration = new EmailsConfiguration(_configuration["EmailsConfiguration:FromEmail"]);

    // executionContextAccessor is used to initialize some modules here
}

public void ConfigureContainer(ContainerBuilder containerBuilder)
{
    containerBuilder.RegisterModule(new MyModule());
}

public void Configure(IApplicationBuilder app, IHostEnvironment env)
{
    this.AutofacContainer = app.ApplicationServices.GetAutofacRoot();

    // other stuff
}

this.AutofacContainer throws null reference at runtime. I am not surprised by this because ConfigureServices executes before Configure method where I get references to the registered dependencies in the container. I can't seem to find a way to resolve IHttpContextAccessor from the container in ConfigureServices and use it as it is in netcore 2.2.

For clarity, Please see how the executionContextAccessor is used in the class that does the initialization as done in netcoreapp2.2:

public class ModuleStartup
{
    private static IContainer _container;

    public static void Initialize(
        IExecutionContextAccessor executionContextAccessor,
        EmailsConfiguration emailsConfiguration)
    {
        ConfigureCompositionRoot(
            executionContextAccessor,
            emailsConfiguration);
    }

    private static void ConfigureCompositionRoot(
        IExecutionContextAccessor executionContextAccessor,
        EmailsConfiguration emailsConfiguration)
    {
        var containerBuilder = new ContainerBuilder();

        containerBuilder.RegisterModule(new EmailModule(emailsConfiguration)); 

        containerBuilder.RegisterInstance(executionContextAccessor);

        _container = containerBuilder.Build();

        ModuleCompositionRoot.SetContainer(_container);
    }

    Internal static class ModuleCompositionRoot
    {
        private static IContainer _container;

        internal static void SetContainer(IContainer container)
        {
            _container = container;
        }

        internal static ILifetimeScope BeginLifetimeScope()
        {
            return _container.BeginLifetimeScope();
        }
    }
}

Answer 1

You could do the following in Configure although I wouldn't recommend it as you're splitting the responsibility of registering your dependencies across multiple parts of your startup. This will resolve the IHttpContextAccessor through IApplicationBuilder.GetServce<TService>() which you can then use for subsequent registrations.

public void Configure(IApplicationBuilder app, IHostEnvironment env)
        {
            var httpContextAccessor = app.ApplicationServices.GetService<IHttpContextAccessor>();
            var executionContextAccessor = new ExecutionContextAccessor(httpContextAccessor);
            var emailsConfiguration = new EmailsConfiguration(_configuration["EmailsConfiguration:FromEmail"]);

            ModuleStartup.Initialize(executionContextAccessor, emailsConfiguration);
        }

Likewise, you could also resolve the IExecutionContextAccessor in order to add registrations like so.

        public void Configure(IApplicationBuilder app, IHostEnvironment env)
        {
            var httpContextAccessor = app.ApplicationServices.GetService<IHttpContextAccessor>();
            var executionContextAccessor = new ExecutionContextAccessor(httpContextAccessor);

            var fooToRegister = new Foo(executionContextAccessor);

            var registration = RegistrationBuilder
                .ForDelegate<IAnotherInterface>((c, p) => fooToRegister)
                .InstancePerLifetimeScope()
                .CreateRegistration();

            app.ApplicationServices
                .GetAutofacRoot().ComponentRegistry
                .Register(registration);
        }