Using UnsafeAccessorAttribute (.NET 8) on struct throws 'System.BadImageFormatException' occurred in Console.dll: 'Invalid usage of UnsafeAccessorAttribute.'

[VidarErmesjo]

Description

I am trying to access private members from an instance of a Guid (struct) but doing so throws the error supplied in the title.

I have discoved that using Unsafe.As<>() I can cast the struct to a dummy class and the extraction will work, though with some allocation overhead.

Reproduction Steps

var guid = Guid.NewGuid();

var dummy = Unsafe.As(guid);

ref var a = ref UnsafeAccessorClass.GetInt32FieldA(dummy); // Works

ref var alsoA = ref UnsafeAccessorClass.GetInt32FieldA(guid); // Throws

sealed class Dummy
{
private readonly int _a; // Field must be declared.
}

sealed class UnsafeAccessorClass
{
[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_a")]
public extern static ref int GetInt32FieldA(Dummy dummy);

[UnsafeAccessor(UnsafeAccessorKind.Field, Name = "_a")]
public extern static ref int GetInt32FieldA(Guid guid);

}

Expected behavior

No errors.

Actual behavior

Throws 'System.BadImageFormatException' occurred in Console.dll: 'Invalid usage of UnsafeAccessorAttribute.'

Regression?

No response

Known Workarounds

Cast struct to class using Unsafe.As()

Configuration

`dotnet --info
.NET SDK:
Version: 8.0.204
Commit: c338c7548c
Workload version: 8.0.200-manifests.9f663350

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19045
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\8.0.204\

.NET workloads installed:
There are no installed workloads to display.

Host:
Version: 8.0.4
Architecture: x64
Commit: 2d7eea2

.NET SDKs installed:
8.0.204 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 8.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found`

Other information

No response

[MichalStrehovsky]

Accessing fields on structs requires passing them as ref, so try public extern static ref int GetInt32FieldA(ref Guid guid); instead.

[VidarErmesjo]

A better mitigation for this use case is perhaps ditching the UnsafeAccessorAttribute and do something like this.

`
private readonly record struct DummyGuid
(
int A,
short B,
short C,
byte D,
byte E,
byte F,
byte G,
byte H,
byte I,
byte J,
byte K
);

var guid = Guid.NewGuid();

var data = Unsafe.BitCast<Guid, DummyGuid>(guid);
`

This will extract the data with no allocations.

[VidarErmesjo]

Accessing fields on structs requires passing them as ref, so try public extern static ref int GetInt32FieldA(ref Guid guid); instead.

Beautiful! That absolutely works (and makes sense when I actually think about it ..).

Closing the issue.