Sitecore Dependency Injection - register two abstractions to same singleton instance of an implementation

Question

When using the native Sitecore Dependency Injection I have 2 abstrations:

• IService
• IExtendedService

I want them both to be registered to same singleton instance of a ServiceImplementation

How do I register the implementation with my IoC Container to do that?

Answer 1

Same instance for multiple services

I believe this can only be achieved with code registration:

var implementation = new ServiceImplementation();

serviceCollection.AddSingleton<IService>(provider => implementation);
serviceCollection.AddSingleton<IExtendedService>(provider => implementation);

In case ServiceImplementation has its own dependencies, you will need to instantiate it from the container as well. Here's a trick you can use:

serviceCollection.AddSingleton<IService, ServiceImplementation>();
serviceCollection.AddSingleton<IExtendedService>(
    provider => (ServiceImplementation)provider.GetService(typeof(IService)));

---

Below you can find more general details that I typed in before fully understanding the question :)

XML registration

If you're using XML registration, just add a .config patch file with the following contents:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <services>
      <register
        serviceType="[Namespace].IService, [Assembly]"
        implementationType="[Namespace].ServiceImplementation, [Assembly]"
        lifetime="Singleton" />
      <register
        serviceType="[Namespace].IExtendedService, [Assembly]"
        implementationType="[Namespace].ServiceImplementation, [Assembly]"
        lifetime="Singleton" />
    </services>
  </sitecore>
</configuration>

Code registration

You should use Sitecore configurators to register dependencies from code.

There are many available registration methods. Here's an example with a singleton dependency:

serviceCollection.AddSingleton<IExtendedService, ServiceImplementation>;

A transient example:

serviceCollection.AddTransient<IService>(provider => new ServiceImplementation());

To see the complete list of available registration methods, decompile the class ServiceCollectionServiceExtensions in the assembly Microsoft.Extensions.DependencyInjection.Abstractions. The main variations are:

• AddTransient<...>(...)
• AddScoped<...>(...)
• AddSingleton<...>(...)
• Add(Type, Type, ServiceLifetime)
• Add(Type, Func<IServiceProvider, object>, ServiceLifetime)

Dependency lifetime

Regardles of which registration method you choose, remember to specify an appropriate lifetime scope. The supported options are:

• Singleton
• Scoped
• Transient

Further reading

• https://docs.asp.net/en/latest/mvc/controllers/dependency-injection.html
• http://kamsar.net/index.php/2016/08/Dependency-Injection-in-Sitecore-8-2/

Answer 2

You should be able to register the two interfaces to the same singleton instance implementation, but only in code, not in config.

ServiceImplementation impl = new ServiceImplementation();
serviceCollection.AddSingleton<IService>(provider => impl);
serviceCollection.AddSingleton<IExtendedService>(provider => impl);

This kinda sucks because you're newing up the singleton instance here....but it'll work.

I originally attempted this in config, however this DOES NOT WORK.

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
    <sitecore>
        <services>
            <register 
                serviceType="MyContracts.MyNamespace.IService, MyContracts" 
                implementationType="MyProject.MyNamespace.ServiceImplementation, MyProject" 
                lifetime="Transient" />
            <register serviceType="MyContracts.MyNamespace.IExtendedService, MyContracts" 
                implementationType="MyProject.MyNamespace.ServiceImplementation, MyProject" 
                lifetime="Transient" />
        </services>
    </sitecore>
</configuration>