What's the difference between registering using ICakeContainerRegistrar or IServiceCollection?

[paulomorgado]

I different methods of registering dependencies:

namespace Cake.Frosting
{
    /// <summary>
    /// The Cake host.
    /// </summary>
    public sealed class CakeHost
    {
///...
        /// <summary>
        /// Adds a delegate for configuring additional services for the host.
        /// </summary>
        /// <param name="services">A delegate for configuring the <see cref="IServiceCollection"/>.</param>
        /// <returns>The same <see cref="CakeHost"/> instance so that multiple calls can be chained.</returns>
        public CakeHost ConfigureServices(Action<IServiceCollection> services)
        {
            services(_services);
            return this;
        }
///...

namespace Cake.Core.Composition
{
    /// <summary>
    /// Represents a module responsible for
    /// registering types and instances.
    /// </summary>
    public interface ICakeModule
    {
        /// <summary>
        /// Performs custom registrations in the provided registrar.
        /// </summary>
        /// <param name="registrar">The container registrar.</param>
        void Register(ICakeContainerRegistrar registrar);
    }
}

Which one is preferable?
Which one is more "Cake idiomatic"?
What are the differences?

[devlead]

ICakeModule is for extending Cake from external assemblies, i.e., replacing core functionality i.e. replacing a logger/filesystem/etc. implementation via a NuGet package; some docs about it here
https://cakebuild.net/docs/extending/modules/

Modules are IoC agnostic, so works with Cake.Tool which uses Autofac as Cake predates MS IOC, Frosting, and SDK both use Microsoft.Extensions.DependencyInjection.

An example of a module is Cake.BuildSystems.Module which enhances Cake's interaction with CI/CD providers.

You could in Frosting use something like

CakeHost()
        // Register all modules from Cake.Buildsystems.Module
        .UseModule<YourModule>()

But internally for your code, I would use ConfigureServices, and if I want to share something across multiple Cake projects, then a module could be the way to go. It all depends on your use case and audience.