Exception: Assembly with same name is already loaded

[kudima03]

Describe the bug
Running mutation testing causes System.IO.FileLoadException: Assembly with same name is already loaded.

Logs
log-20251110.txt

Expected behavior
Mutation testing should complete successfully without compilation errors.

Desktop (please complete the following information):

• OS: Windows 11
• Type of project: .NET 9 (ASP.NET Web API + class libraries)
• Framework Version: .NET 9
• Stryker Version: 4.8.1

Additional context
Based on the logs, the issue occurs during the mutation phase when Stryker.NET attempts to load analyzer or source generator assemblies from the .NET 9 SDK.
The following warnings appear:

• Assembly with same name is already loaded
• The analyzer 'Microsoft.CodeAnalysis.Razor.Compiler' references a newer version (5.0.0.0) of the compiler than the one used by Stryker.NET
• Errors related to missing interceptors in generated ASP.NET Core code (Microsoft.AspNetCore.Http.RequestDelegateGenerator)

It seems Stryker.NET 4.8.1 is using an older Roslyn compiler version that is not fully compatible with the new source generator and interceptor features introduced in .NET 9.

[dupdob]

Thanks for opening this issue.
The short answer is: it looks like you are trying to work with beta versions of Roslyn.
What happens here is that Microsoft.CodeAnalysis.Razor.Compiler expects a compiler (Microsoft.CodeAnalysis.CSharp) with a version equal or above to 5.0.
Currently, Stryker ships with a 4.14 compiler version, which is the latest stable version. As of today, Microsoft.CodeAnalysis.CSharp V5.0 is still in pre-release.

You need to wait for a Stryker release integrating Roslyn 5.0.