Why we need to call BuildServiceProvider to get dependencies everytime?

Question

After registering instances on my IServiceCollection, I need to register an IAutomapperProvider that depends on an IAssemblyProvider that was registered before this method call

public static IServiceCollection RegisterAutomapperConfiguration(this IServiceCollection container, ServiceLifetime lifeTime = ServiceLifetime.Scoped)
{
        //creating the provider to get the IAssemblyProvider for my IAutomapperProvider
        var prov = container.BuildServiceProvider();
        var assemblyProvider = prov.GetService<IAssemblyProvider>();
        container.Register<IAutomapperProvider>(aProv => new AutomapperProvider(assemblyProvider), lifeTime);
        var autoMapperProvider = prov.GetService<IAutomapperProvider>();
        var mapperConfig = autoMapperProvider.GetMapperConfiguration();
        ...
}

If right after the call of container.Register<IAutomapperProvider>(aProv => new AutomapperProvider(assemblyProvider), lifeTime); I don't call BuildServiceProvider again, then I would not get the IAutomapperProvider I registered before

public static IServiceCollection RegisterAutomapperConfiguration(this IServiceCollection container, ServiceLifetime lifeTime = ServiceLifetime.Scoped)
{
        //creating the provider to get the IAssemblyProvider for my IAutomapperProvider
        var prov = container.BuildServiceProvider();
        var assemblyProvider = prov.GetService<IAssemblyProvider>();
        container.Register<IAutomapperProvider>(aProv => new AutomapperProvider(assemblyProvider), lifeTime);
        prov = container.BuildServiceProvider();
        var autoMapperProvider = prov.GetService<IAutomapperProvider>();
        var mapperConfig = autoMapperProvider.GetMapperConfiguration();
        ...
}

On the AspNetCore code when you call BuildServiceProvider extension method they use the same IServiceCollection that can change over the time adding more elements, at the end you are pointing to the same reference

public static ServiceProvider BuildServiceProvider(this IServiceCollection services)
{
    return BuildServiceProvider(services, ServiceProviderOptions.Default);
}

Then why I need to call it again to get a new instance that knows how to resolve my Service?

To avoid confusions, the Register method is an extension I created but internally calls the AddSinglenton or Add...

  public static IServiceCollection Register<TService>(this IServiceCollection container, Func<IServiceProvider, TService> implementationFactory, ServiceLifetime lifeTime)
        where TService : class
    {

    if (container == null)
        throw new ArgumentNullException(nameof(container));
    if (implementationFactory == null)
        throw new ArgumentNullException(nameof(implementationFactory));

    switch (lifeTime)
    {
        case ServiceLifetime.Scoped:
            container.AddScoped(typeof(TService), (Func<IServiceProvider, object>)implementationFactory);
            break;
        case ServiceLifetime.Transient:
            container.AddTransient(typeof(TService), (Func<IServiceProvider, object>)implementationFactory);
            break;
        default:// ServiceLifetime.Singleton
            container.AddSingleton(typeof(TService), (Func<IServiceProvider, object>)implementationFactory);
            break;
    }

    return container;
}

Answer 1

BuildServiceProvider does not have to be invoked so often.

That is what the implementation factory delegates are for. They provide access to a service provider for that purpose.

It is still uncertain why you have to build the provider in the first instance but based on currently provided code use the service provider within the implementation factory delegate when registering your provider.

public static IServiceCollection RegisterAutomapperConfiguration(this IServiceCollection services, 
    ServiceLifetime lifeTime = ServiceLifetime.Scoped) {

    services.Register<IAutomapperProvider>(p => //<-- p is IServiceProvider
         //use the provider to get the IAssemblyProvider for my IAutomapperProvider
        new AutomapperProvider(p.GetService<IAssemblyProvider>()), lifeTime);
    prov = services.BuildServiceProvider();
    var autoMapperProvider = prov.GetService<IAutomapperProvider>();
    var mapperConfig = autoMapperProvider.GetMapperConfiguration();
    //...
}

When BuildServiceProvider is invoked, the created service provider will only be aware of types that were in the collection when it was built. It will not be aware of any additional types, which is why is it usually done at the end of registering all types.