Open-generic registrations can not be decorated

[dotnetjunkie]

Let me start by saying that I am quite amazed to see what you've been able to build on top of the DI abstraction. The omission of being able to apply decorators (and batch-registration) makes the built-in container IMO a worthless tool for any reasonably sized code base.

That said, I stumbled upon a limitation of your current implementation. Although you're able to apply open-generic decorators to closed-generic registrations, it seems impossible to apply open-generic decorators to open-generic registrations.

This might very well be a limitation of the API you are building upon, but I'll let you be the judge of that.

Here's a failing unit test:

[Fact]
public void CanDecorateOpenGenericTypeBasedOnOpenGenericRegistration()
{
    var provider = ConfigureProvider(services =>
    {
        services.AddTransient(typeof(IQueryHandler<,>), typeof(QueryHandler<,>));
        services.Decorate(typeof(IQueryHandler<,>), typeof(LoggingQueryHandler<,>));
        services.Decorate(typeof(IQueryHandler<,>), typeof(TelemetryQueryHandler<,>));
    });

    var instance = provider.GetRequiredService<IQueryHandler<MyQuery, MyResult>>();

    var telemetryDecorator = Assert.IsType<TelemetryQueryHandler<MyQuery, MyResult>>(instance);
    var loggingDecorator = Assert.IsType<LoggingQueryHandler<MyQuery, MyResult>>(
        telemetryDecorator.Inner);
    Assert.IsType<MyQueryHandler>(loggingDecorator.Inner);
}

[khellang]

This might very well be a limitation of the API you are building upon, but I'll let you be the judge of that.

Yes. This is definitely a limitation of the container itself.

At first, I was convinced that it was impossible to implement this (given the existing constraints), but @nicholasham came up with a way where we could leverage the existing (closed) registrations in order to create decorated registrations.

Because there's no way to get information about the requested generic type arguments, I can't see a way for us to create closed generic types to decorate 😢

[khellang]

I don't think there's anything we can do here, unfortunately 😞

[julealgon]

I don't think there's anything we can do here, unfortunately 😞

This is incredibly sad to hear. As I mentioned in #49, open decorators are an incredible feature that I'm honestly not sure if I can live without.

Are you 100% sure it is impossible to make it work now? Can we consider asking Microsoft for the necessary tooling to create this feature in Scrutor?

I understand they would probably not add decorator registrations themselves, but maybe they could add what we need in order to do that in this library here.

[dotnetjunkie]

Asking Microsoft will have little effect. Any time microsft adds a feature to their DI Container, they are introducing a breaking change that might affect every adapter to their abstraction.

The solution, however, is rather simple: just use an alternative DI Container. Most libraries do support the feature you are looking for. For sure I know Autofac, StructureMap and Simple Injector support this.

[khellang]

The solution, however, is rather simple: just use an alternative DI Container.

Yes, do this 👍

[akovanev]

Any updates here?

[dotnetjunkie]

Any updates here?

@akovanev, Microsoft didn't add any feature in .NET 5 or .NET 6 to MS.DI that would allow @khellang to implement this feature in Scrutor. That status of this feature request is, therefore, unchanged.

[akovanev]

I don't know if my workaround will be helpful to anyone, as it has a few significant disadvantages, but let me put the link to the project here CqsDecorator.