Address-taken methods should be considered reflection visible

[eerhardt]

ASP.NET Minimal APIs use the parameter names of methods in order to match them with route parameters.

When trying to run an ASP.NET Minimal API app with TrimMode=full, you get an exception at runtime:

System.InvalidOperationException: Encountered a parameter of type 'System.String' without a name. Parameters must have a name.
  at Microsoft.AspNetCore.Http.RequestDelegateFactory.CreateArgument(ParameterInfo, FactoryContext)
  at Microsoft.AspNetCore.Http.RequestDelegateFactory.CreateArguments(ParameterInfo[], FactoryContext)
  at Microsoft.AspNetCore.Http.RequestDelegateFactory.CreateTargetableRequestDelegate(MethodInfo, Expression, FactoryContext, Expression`1 )
  at Microsoft.AspNetCore.Http.RequestDelegateFactory.Create(Delegate, RequestDelegateFactoryOptions )
  at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.CreateRouteEndpointBuilder(RouteEntry, RoutePattern , IReadOnlyList`1 , IReadOnlyList`1 )
  at Microsoft.AspNetCore.Routing.RouteEndpointDataSource.get_Endpoints()

ASP.NET should set the switch --keep-metadata parametername in its SDK to prevent parameter names from being trimmed. However, there is no official supported way of doing it.

We should add a way to allow ASP.NET apps to tell the trimmer to not trim parameter names.

cc @sbomer

[MichalStrehovsky]

Illink will only trim parameter names that are not obvious targets of reflection (and reserves the right to do more with such methods in the future - NativeAOT will in fact completely remove metadata from such method.)

How is ASP.NET obtaining the MethodInfo in the first place? It should be the case that one can only obtain such crippled MethodInfo though code that is generating a warning.

[eerhardt]

How is ASP.NET obtaining the MethodInfo in the first place?

By calling Delegate.Method:

https://github.com/dotnet/aspnetcore/blob/ce60f0d10c0adb79b2b824f5da05e41e1199f6cf/src/Http/Http.Extensions/src/RequestDelegateFactory.cs#L176

The Delegates are passed in to the Minimal API APIs: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/minimal-apis?view=aspnetcore-6.0#route-parameters

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/users/{userId}/books/{bookId}", 
    (int userId, int bookId) => $"The user id is {userId} and book id is {bookId}");

app.Run();

[MichalStrehovsky]

By calling Delegate.Method:

It's this IL Linker bug then: https://github.com/dotnet/runtime/pull/70198#issuecomment-1145755049

We can use this issue to track fixing that. Exposing --keep-metadata parametername as a public switch should not be needed.

[eerhardt]

FYI @agocke - If we care about fully trimming ASP.NET Minimal APIs applications (but not NativeAOT'ing them), then this will need to get fixed.

@davidfowl @DamianEdwards - do you think that is a scenario we want to enable in .NET 8?

[agocke]

Happy to prioritize this if it's blocking. As Michal mentioned, this is already a trimming difference with NativeAOT, so it's definitely a bug we want to fix.

[DamianEdwards]

@eerhardt yes for sure, our goals include enabling customers who can't use NativeAOT to still get the benfits of full trimming.

[vitek-karas]

This has been fixed by https://github.com/dotnet/runtime/pull/85831.