Get IServiceCollection From Host Builder in .NET 6

Problem ¶

Neither .NET generic host’s HostBuilder nor IHost exposes IServiceCollection. You can resolve services via the IHost.Services property but there is no obvious way of accessing dependency registration metadata.

Solution ¶

It can be done using a custom service provider factory:

public class ExposeServiceCollectionFactory : IServiceProviderFactory<IServiceCollection>
{
    private DefaultServiceProviderFactory _defaultFactory = 
      new DefaultServiceProviderFactory();

    public IServiceCollection? ServiceCollection { get; private set; }

    public IServiceCollection CreateBuilder(IServiceCollection services)
    {
       ServiceCollection = services;
       return _defaultFactory.CreateBuilder(services);
    }

    public IServiceProvider CreateServiceProvider(IServiceCollection containerBuilder)
    {
       return _defaultFactory.CreateServiceProvider(containerBuilder);
    }
}

Then the factory can be used as follows:

var builder = new HostBuilder();
var factory =  new ExposeServiceCollectionFactory();

builder.UseServiceProviderFactory(factory);

// your configuration here

builder.Build();

var services = factory.ServiceCollection;

Console.WriteLine($"Got {services.Count} services here");

Why would you need that ¶

Usually you wouldn’t. It may come handy when you have a library consuming an IServiceCollection but you have lots of extensions methods for IHostBuilder rather than IServiceCollection. This way you can reuse your extension methods and pass the resulting IServiceCollection to the library.