AsImplementedInterfaces.cs

 /// <summary>
 /// Register all interfaces for a type.
 /// </summary>
 /// <returns>The original <see cref="T:Microsoft.Extensions.DependencyInjection.IServiceCollection" />.</returns>
 public static IServiceCollection AsImplementedInterfaces(this IServiceCollection services)
 {
     var lastRegistration = services.LastOrDefault();
     if (lastRegistration != null)
     {
         var implementationType = GetImplementationType(lastRegistration);

         // We except that this method is called when a service is registered as a concrete type.
         // If this is not the case, we bail out
         if (lastRegistration.ServiceType != implementationType)
             return services;

         var interfaces = lastRegistration.ServiceType.GetInterfaces();
         foreach (var @interface in interfaces)
         {
             // Register all interfaces proxying to our specific registration
             services.Add(new ServiceDescriptor(
                 @interface,
                 provider => provider.GetService(implementationType),
                 lastRegistration.Lifetime));
         }

         return services;
     }

     return services;
 }

 private static Type? GetImplementationType(ServiceDescriptor descriptor)
 {
     if (descriptor.ImplementationType != null)
     {
         return descriptor.ImplementationType;
     }

     if (descriptor.ImplementationInstance != null)
     {
         return descriptor.ImplementationInstance.GetType();
     }

     if (descriptor.ImplementationFactory != null)
     {
         return descriptor.ImplementationFactory.GetType().GenericTypeArguments[1];
     }

     return null;
 }