Bug? GetPEKind varies between .NET 8.0 and .NET 9.x - why?

[TonyValenti]

We are using an SAP-provided DLL that detects "bitness" by using the following call:

Assembly.GetAssembly(typeof(int)).GetModules()[0].GetPEKind(out var _, out var machine);

On upgrading from .NET 8.0 to .NET 9.0, the DLL no longer functions because the return value has changed.

Using this as my CSPROJ:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

and this code:

using System.Reflection;

Assembly.GetAssembly(typeof(int)).GetModules()[0].GetPEKind(out var _, out var machine);

Console.WriteLine(machine);
//on .NET8: AMD64
//on .NET9: I386

You'll see that the output varies from one version to another.

Even if I explicitly set .NET 9.0 to be <PlatformTarget>x64</PlatformTarget>, it still returns I386 instead of AMD64.

Why?

[colejohnson66]

I took a look at the code for .NET 8.0.0 and 9.0.0, and the code is all the same. It looks like it's reading the image type right out of the file.

[TonyValenti]

Do you know why the image type for that core assembly changed between .net 8.0 and 9.0?
If it is being read right out of the file, I kind of think that the .net 9.0 assembly has the wrong value in it because it is an x64 assembly that is returning a 32 bit identifier.

[colejohnson66]

I don't know. I just figured I'd see if the actual code changed, and it didn't. So something in the actual DLLs is different, as @geniuszxy found.

[geniuszxy]

I notice the code will return I386 if the READYTORUN_FLAG_PLATFORM_NEUTRAL_SOURCE (0x1) bit of the core flag is set. For some reason the core flag for System.Private.CoreLib.dll in 9.0 is 4B, while in 8.0 it is 4A. However, most DLLs in version 8.0 set this bit.

[TonyValenti]

@geniuszxy Does this sound like a bug? Should I create an issue from this?

[jkotas]

I386 returned by this legacy API can either mean a I386-specific binary or a portable binary. The value of the PortableExecutableKinds enum indicates which one of these two cases it is.

In .NET 8, corelib was compiled as architecture-specific on some platforms and platform neutral on others. All other core libraries were compiled as platform neutral.

In .NET 9, all core libraries including corelib are compiled as platform neutral everywhere.

This change is related to dotnet/sdk#42556.

using System.Reflection;

Assembly.GetAssembly(typeof(int)).GetModules()[0].GetPEKind(out var peKind, out var machine);
Console.WriteLine($"{peKind} {machine}");

Assembly.GetAssembly(typeof(System.Diagnostics.Process)).GetModules()[0].GetPEKind(out var peKind2, out var machine2);
Console.WriteLine($"{peKind2} {machine2}");

Output on .NET 8:

PE32Plus AMD64
ILOnly I386

Output on .NET 9:

ILOnly I386
ILOnly I386