ASP.NET Core Pitfalls – Returning a Custom Service Provider from ConfigureServices

In pre-3.1 versions of ASP.NET Core, you could return your own service provider (AutoFac, Ninject, etc) by returning some IServiceProvider-implementing class from the ConfigureServices method. This is no longer supporting, and having code like this results in an NotSupportedException being thrown at startup:

public IServiceProvider ConfigureServices(IServiceCollection services)

{
    //something
    return myCustomServiceProvider;
}

The way to do it now is by registering a service provider factory at bootstrap, something like this:

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .UseServiceProviderFactory(new CustomServiceProviderFactory())
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        });

A custom service provider factory must implement IServiceProviderFactory<T>, for an example, you can see Autfac’s implementation: https://github.com/autofac/Autofac.Extensions.DependencyInjection/blob/develop/src/Autofac.Extensions.DependencyInjection/AutofacServiceProviderFactory.cs and https://github.com/autofac/Autofac.Extensions.DependencyInjection/blob/develop/src/Autofac.Extensions.DependencyInjection/AutofacChildLifetimeScopeServiceProviderFactory.cs.