[API Proposal]: `ActivatorUtilities.CreateFactoryOf<TDelegate>`

[alrz]

Background and motivation

As an alternative to #101889 this API would be only exposed as a single method taking the delegate as a type arg. This will avoid array allocation and boxing for the args, and also typechecks that the callsite matches the factory.

API Proposal

namespace Microsoft.Extensions.DependencyInjection
{
  public static class ActivatorUtilities
  {
+    public static T CreateFactoryOf<T>() where T : Delegate;
  }
}

API Usage

// IServiceProvider is specially recognized
var f1 = ActivatorUtilities.CreateFactoryOf<Func<IServiceProvider, TArg, TResult>>();
// but it's optional when not using DI
var f2 = ActivatorUtilities.CreateFactoryOf<Func<TArg, TResult>>(); 
// Just like Lambda<T> you could choose to use a custom delegate type
var f3 = ActivatorUtilities.CreateFactoryOf<CustomDelegate>();

Alternative Designs

No response

Risks

No response

[MichalPetryka]

I assume this will be rejected for same reasons as #36194.

[steveharter]

@alrz can you provide a prototype of this to verify the expected perf gains? I assume no boxing and no array allocation would be needed.

[alrz]

@alrz can you provide a prototype of this to verify the expected perf gains? I assume no boxing and no array allocation would be needed.

To be clear that's not the main goal here, it's the typechecking between construction and the callsite.