RuntimeInformation.OSArchitecture returns X64 unexpectedly on arm64-Windows system

[JensNordenbro]

Description

On Windows ARM (Surface Pro X) RuntimeInformation.OSArchitecture differs between net481 and net10.0 when RID win-x86 is forced.

Reproduction Steps

1. Add this code to Program.cs on a Console app:

Architecture osArchitecture = RuntimeInformation.OSArchitecture;
Console.WriteLine($"OS Architecture: {osArchitecture}");

2. compilie using dotnet publish -f net481 -r win-x86 ...

3. Run executable

Expected behavior

"OS Architecture: Arm64" as console output, just like when publishing for framework net10.0 with the same rid.

Actual behavior

"OS Architecture: X64" as console output.

Regression?

Yes, we haad this working before but hard to reproduce.

Known Workarounds

No response

Configuration

No response

Other information

No response

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[huoyaoyuan]

RID is not applicable to .NET Framework. .NET Framework produces AnyCPU exe and switches architecture via manifest configuration.

The default for .NET Framework on ARM64 is always x64 emulation, which emulates a full x64 system. You can print ProcessArchitecture to confirm.

To change the behavior, you can set following properties in csproj:

• <PreferNativeArm64>true</PreferNativeArm64>

ProcessArchitecture: ARM64 OSArchitecture: ARM64
Process runs with native arm64.

• <Prefer32Bit>true</Prefer32Bit>

ProcessArchitecture: x86 OSArchitecture: ARM64
Process runs with x86 emulation.

• No option:

ProcessArchitecture: x64 OSArchitecture: x64
Process runs with x64 emulation.

[huoyaoyuan]

The comment of .NET 10 also intentionally reports x64 OS for emulated x64:

runtime/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/RuntimeInformation.Windows.cs

Line 42 in 75fa507

// If we are running an x64 process on a non-x64 windows machine, we will report x64 as OS architecture.

Closing as by design.

[JensNordenbro]

Hmm...

@huoyaoyuan , I dont think this exaplains it all.

I get this scenario inside Windows Sandbox (on the host it works correctly) :

This time I built using .NET Framework SDK more true to the scenario of our real world application that experiences problem.

How is this explained?

[huoyaoyuan]

I tested again and observed different behavior under different conditions:

If launched via F5 under VS, it reports OS as x64:

If launched via Ctrl+F5, or double clicking directly, it reports OS as ARM64:

There may be more detail that I missed. This repo is not the best place to ask .NET Framework-specific questions. You can ask at Visual Studio Developer Community and the developers may answer you the behavior behind.

Anyway, the ARM64 emulator tend to hide itself for processes that's non-native ARM64.

[JensNordenbro]

I would say, that this is a bug.
Sandbox should not behave differently than host machine.

Then again it might not be for this repo but notice that this behavior is true when .NET SDK is used to target net481 so...

[huoyaoyuan]

SDK is totally unrelated to the behavior. It just sets corresponding flag/manifest and lets the runtime/OS to determine the behavior.

Further issues should be reported to Developer Community.