Proxy-factory to single instance per lifetime, if possible

[TheConstructor]

Currently e.g.

public interface IServiceTag
{
}

[RegisterSingleton(Tags = "Client,FrontEnd")]
public class ServiceTag : IServiceTag
{
}

yields two independent registrations, so that provider.GetService<IServiceTag>() and provider.GetService<ServiceTag>() return two independent instances, that are individually tracked and disposed. Introducing a 'factory' that resolves ServiceTag (implementation-type), if it is registered, and non-open-generic, to resolve IServiceTag solves this. Now only a single instance is created.

Generated code for the registration above with this PR is

            if (tagSet.Count == 0 || tagSet.Intersect(new[] { "Client", "FrontEnd" }).Any())
            {
                global::Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAdd(
                    serviceCollection,
                    global::Microsoft.Extensions.DependencyInjection.ServiceDescriptor.Singleton<global::Injectio.Tests.Library.IServiceTag>((serviceProvider) => global::Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService<global::Injectio.Tests.Library.ServiceTag>(serviceProvider))
                );

                global::Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAdd(
                    serviceCollection,
                    global::Microsoft.Extensions.DependencyInjection.ServiceDescriptor.Singleton<global::Injectio.Tests.Library.ServiceTag, global::Injectio.Tests.Library.ServiceTag>()
                );

            }

[coveralls]

Pull Request Test Coverage Report for Build 14622968813

Details

• 29 of 35 (82.86%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 83.27%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Injectio.Generators/ServiceRegistrationGenerator.cs	14	20	70.0%

Files with Coverage Reduction	New Missed Lines	%
src/Injectio.Generators/ServiceRegistrationGenerator.cs	1	83.81%

Totals
Change from base Build 14602062882:	0.3%
Covered Lines:	547
Relevant Lines:	627

---

💛 - Coveralls

[Cologler]

This was previously discussed in issue #138

[TheConstructor]

@Cologler while the result is similar, I would argue, that you haven't really stated why this was desirable. I think people expect that one attribute yields one instance in their chosen lifetime, and so I created this PR. I am totally aware it isn't perfect, and may need changes or be rejected completely

[TheConstructor]

Just realized, that I probably don't need to supress the second generic argument when registering a factory. Guess I will push a new version later.

[pwelter34]

I've been considering this idea as well. Its especially important for singleton registrations. I like the idea that its an option, though I'm concerned about switch the default for backward compatibility. Then again, we are working toward a new version 5 that it could be acceptable.

[pwelter34]

This enhancement could address the default behavior issue.

#217

[hdocsek]

I need to support initialization of multiple components which have this interface:

interface IInitialize
{
    void Initialize();
}

I tried to use
[RegisterSingleton(Duplicate = DuplicateStrategy.Append, Registration = RegistrationStrategy.SelfWithInterfaces)]
and expected that only one instance exists for both class and all of its interfaces.
However, this didn't work because it produced individual registrations.

I tried it it out with RegistrationStrategy.SelfWithProxyFactory from this PR instead and it worked like expected.

I am looking forward to getting this merged.

Special thanks to @TheConstructor