[Bug]: Fody + .NET 9 project: Hot Reload (EnC) broken due to wrong/duplicate System.Runtime reference (CS7038)

[jaredthirsk]

Describe the bug 🐞

Hot reload fails (both dotnet watch, and in VS2022 Preview) on .NET 9.0 project with ReactiveUI.Fody.Helpers.

If I remove [Reactive] attribute and ReactiveUI.Fody (I think version 19.5.41), hot reload works.

(If I add ReactiveUI.SourceGenerators, and use their different [Reactive] attribute on a field, it works.)

Step to reproduce

1. Create .NET 9 project
2. Add ReactiveUI.Fody nuget
3. Make a class with [Reactive] property
4. Debug in VS2022 Preview, or dotnet watch
5. Try to make a change for hot reload
6. Fails with error:

dotnet watch :x: error CS7038: Failed to emit module 'Xyz.Core': Changing the version of an assembly reference is not allowed during debugging: 'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' changed version to '9.0.0.0'.

7. If I inspect the DLL with dotPeek, I see there are two references for System.Runtime, version 8.0 and 9.0. I suspect this is where the problem is. Microsoft's Hot Reload should ideally detect that both were there before and after so no version was changed, but I suspect this scenario of two versions of the same DLL is not supposed to happen (I'm not an expert though, so I can't say for sure.)

Reproduction repository

No response

Expected behavior

• Expectation: Hot reload should work
• Expectation: DLL shouldn't have two references to System.Runtime (v8, v9)

Screenshots 🖼️

No response

IDE

Visual Studio 2022

Operating system

Windows 10

ReactiveUI Version

19.5.41

Workarounds

1. If I change my project back to .NET 8, hot reload works.
2. If I drop ReactiveUI.Fody and use ReactiveUI.SourceGenerators instead, it works. (Though it has breaking changes to my code.)

Additional information ℹ️

Related issues:

• EnC: Hot Reload fails with unhelpful error message (CS7038) when there are multiple references to the same DLL (System.Runtime) dotnet/roslyn#76284
• .NET hot reload issue with fody  Fody/Fody#1044