Replace service registration in ASP.NET Core built-in DI container?

Question

Let us consider a service registration in Startup.ConfigureServices:

public void ConfigureServices(IServiceCollection services)
{
    services.A         


        

Answer 1

It is easy to override ASP.NET Core DI functionality if you know two simple things:

1. ServiceCollection is just a wrapper on top of List:

    public class ServiceCollection : IServiceCollection
    {
        private List _descriptors = new List();
    }

2. When a service is registered, a new descriptor is added to list:

    private static IServiceCollection Add(
        IServiceCollection collection,
        Type serviceType,
        Type implementationType,
        ServiceLifetime lifetime)
    {
        var descriptor = new ServiceDescriptor(serviceType, implementationType, lifetime);
        collection.Add(descriptor);
        return collection;
    }

Therefore, it is possible to add/remove descriptors to/from this list to replace the registration:

IFoo service = services.BuildServiceProvider().GetService();
Assert.True(service is FooA);

var descriptor = services.FirstOrDefault(d => d.ServiceType == typeof(IFoo));
Assert.NotNull(descriptor);
services.Remove(descriptor);

service = services.BuildServiceProvider().GetService();
Assert.Null(service);

We finish with Replace extention method:

services.Replace(ServiceLifetime.Transient);

Its implementation:

public static IServiceCollection Replace(
    this IServiceCollection services,
    ServiceLifetime lifetime)
    where TService : class
    where TImplementation : class, TService
{
    var descriptorToRemove = services.FirstOrDefault(d => d.ServiceType == typeof(TService));

    services.Remove(descriptorToRemove);

    var descriptorToAdd = new ServiceDescriptor(typeof(TService), typeof(TImplementation), lifetime);

    services.Add(descriptorToAdd);

    return services;
}