Using UnsafeAccessor to access List<T>._items

[sfiruch]

I'm trying to access the internal _list field of List<T>. It doesn't appear to work. Here's my experiment on Sharplab:

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Runtime.CompilerServices;

var list = new List<string>() { "ahoyhoy" };

//reflection works
var internalItemsOld = (string[])list.GetType().GetField("_items", BindingFlags.NonPublic | BindingFlags.Instance)!.GetValue(list)!;
Console.WriteLine(internalItemsOld.Length);

//UnsafeAccessor doesn't appear to work?
var internalItemsNew = GetListItems(list);
Console.WriteLine(internalItemsNew.Length);


[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_items")]
extern static ref T[] GetListItems<T>(List<T> list);

The reflection-based way works fine but the UnsafeAccessor throws this exception:

System.MissingFieldException: Field not found: 'System.Collections.Generic.List`1._items'.

UnsafeAccessor-with-generics had a bumpy start, but since #89439 (.NET 9) it should be working, no? Am I doing something wrong?

[KeterSCP]

The answer is the same as in #110054 (comment)

In your case, T should be type parameter and not method parameter, as in List<T> T - is a type parameter.
Following code works as expected:

using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

var list = new List<string>() { "ahoyhoy" };

var internalItemsNew = Accessors<string>.GetListItems(list);
Console.WriteLine(internalItemsNew.Length);

class Accessors<T>
{
    [UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_items")]
    public extern static ref T[] GetListItems(List<T> list);
}

[Clockwork-Muse]

.... Do you really need the underlying array? There's also CollectionsMarshal.AsSpan, which would give you a Span back.

[sfiruch]

Yes, I want to use the same code paths for List<T> and T[], and can't use Span<T>. I described the use case in #105179.

[hamarb123]

Just call AsSpan once on each thread. It's pretty cheap. And you can use spans in lambdas, you just can't pass them into the lambdas like that as they're not boxable and that's how it gives the state implicitly.