`ObjectFactory<T>` delegate should be covariant

[alrz]

Current workaround is to use the non-generic ObjectFactory and cast but that shouldn't be necessary.

ObjectFactory<A> a = ...;
ObjectFactory<B> b = ...;
ObjectFactory<Base> f = c ? a : b;
return f(...);

API Proposal

namespace Microsoft.Extensions.DependencyInjection;
- public delegate T ObjectFactory<T>(IServiceProvider serviceProvider, object?[]? arguments);
+ public delegate T ObjectFactory<out T>(IServiceProvider serviceProvider, object?[]? arguments);

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[stephentoub]

runtime/src/libraries/Microsoft.Extensions.DependencyInjection.Abstractions/src/ObjectFactoryT.cs

Line 15 in ee93104

public delegate T ObjectFactory<T>(IServiceProvider serviceProvider, object?[]? arguments);

[alrz]

Would it be possible to use params ReadOnlySpan<object> for the arguments here?
(I suppose this would be considered as a part of new params span set of APIs though.)

[stephentoub]

Would it be possible to use params ReadOnlySpan<object> for the arguments here? (I suppose this would be considered as a part of new params span set of APIs though.)

That would need to be an entirely different delegate and an entirely different set of APIs that take that delegate... just changing the delegate itself would be a binary breaking change. Feel free to open a separate issue with such a proposal.

[alrz]

Opened #101889

[steveharter]

@terrajobst is this on the backlog for API review?

[terrajobst]

Yeah, it's number 16 though. https://apireview.net/?q=objectfactory