Add interface to allow container implementors to expose a more native service lookup for required services

[davidfowl]

We're going to add this interface to allow container implementors to directly call into their intrinsics without checking for existence and returning null. It also lets GetRequiredService fail with container specific diagnostics instead of our own. This interface would be optional and we would light it up here:

https://github.com/aspnet/DependencyInjection/blob/dev/src/Microsoft.Extensions.DependencyInjection.Abstractions/ServiceProviderExtensions.cs#L38

If the interface exists, we would call it directly, otherwise we'll use the existing behavior.

See the original discussion here #238.

public interface IGetRequiredServiceAware
{
   object GetRequiredService(Type serviceType);
}

Name suggestions are welcome.

/cc @jeremydmiller

[jeremydmiller]

Thank you. This will make it easier for StructureMap users to correct common registration problems that could be hidden behind the previously default TryGetInstance<T> mechanics.

Consider the case where a users is using some kind of conventional scanning to register types, and it finds two different concrete types of IService and registers each. In StructureMap, we will not choose either the first or last registration as the "default", in the case of multiples you have to explicitly choose one or the other. In the case of asking for IService through IContainer.GetInstance<IService>(), StructureMap will tell you it has no default because more than one is registered, with a complete listing of each. That's a common problem in SM usage and one that would be much easier to solve for users w/ this new GetRequiredService() feature.

[davidfowl]

/cc @dadhi @seesharper @tillig @khellang

[khellang]

Nice! 👍

[kichalla]

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