Altered UnsafeAccessor support for closed generics

  • Article

.NET 8 introduced the UnsafeAccessorAttribute attribute, which permits access to non-visible members of types (AKA "fast private reflection"). Support for generics in .NET 8 wasn't added due to time constraints. However, in CoreCLR and native AOT, some very narrow and unsupported scenarios involving closed generic types did work. These scenarios should have been blocked, but inadvertently weren't. New restrictions have been added in .NET 9.

For more information and examples, see the remarks for UnsafeAccessorAttribute.

Previous behavior

In .NET 8, a naive signature look-up on types was implemented, and use of generic types was deemed valid in some cases. For example, the following code succeeded:

[UnsafeAccessor(UnsafeAccessorKind.Method, Name = ".ctor")]
extern static void CtorAsMethod(List<int> c);

New behavior

Starting in .NET 9, the fully supported and documented way to consume generic types is to ensure the type parameters of an extern static method match the type parameters of the private method, and the method parameters of an extern static method match the method parameters of the private method. These restrictions are necessary because the runtime performs a strict metadata signature match.

class Accessor<T>
{
    [UnsafeAccessor(UnsafeAccessorKind.Method, Name = ".ctor")]
    public extern static void CtorAsMethod(List<T> c);
}

Version introduced

.NET 9 Preview 6

Type of breaking change

This change is a behavioral change.

Reason for change

In the official .NET 8 release, support for the use of generic types with UnsafeAccessorAttribute was unintentional. Early in development, it was a possibly supported scenario, but was later deferred until .NET 9 because the team encountered complexity issues. The official documentation did not mention generics, nor did it provide any examples using generics. This change corrects the behavior.

Read the updated documentation for the UnsafeAccessorAttribute API and change your code as necessary to match the new restrictions for generic types.

Affected APIs