ServiceCollectionExtensionForClassicDecorators.cs

public static IServiceCollection Decorate<TInterface, TDecorator>(this IServiceCollection services)
    where TInterface : class
    where TDecorator : class, TInterface
{
    var objectFactory = ActivatorUtilities.CreateFactory(
        typeof(TDecorator),
        new[] { typeof(TInterface) });

    // Save all descriptors that needs to be decorated into a list.
    var descriptorsToDecorate = services
        .Where(s => s.ServiceType == typeof(TInterface))
        .ToList();

    if (descriptorsToDecorate.Count == 0)
    {
        throw new InvalidOperationException($"Attempted to Decorate services of type {typeof(TInterface)}, " +
                                            "but no such services are present in ServiceCollection");
    }

    foreach (var descriptor in descriptorsToDecorate)
    {
        // Create new descriptor with prepared object factory.
        var decorated = ServiceDescriptor.Describe(
            typeof(TInterface),
            s => (TInterface)objectFactory(s, new[] { s.CreateInstance(descriptor) }),
            descriptor.Lifetime);

        services.Remove(descriptor);
        services.Add(decorated);
    }

    return services;
}

private static object CreateInstance(this IServiceProvider services, ServiceDescriptor descriptor)
{
    if (descriptor.ImplementationInstance != null)
    {
        return descriptor.ImplementationInstance;
    }

    if (descriptor.ImplementationFactory != null)
    {
        return descriptor.ImplementationFactory(services);
    }

    return ActivatorUtilities.GetServiceOrCreateInstance(services, descriptor.ImplementationType);
}