This repo illustrates an issue that occurs after installing the .NET SDK 9.0.300, when using mono to run unit tests on in a solution with .NET Framework projects.

The problem occurs when the assembly being tested references a different version of the .NET Framework than the one being used by the test runner. In this example, the MonoRepro.csproj file targets net462, while the MonoRepro.Tests.csproj targets net48.

This was not a problem in .NET SDK 9.0.204 and earlier. You can see an example of a much more complex solution that has different versions of the .NET Framework in the SDK and test projects in the sentry-dotnet repository (where this issue was first discovered). Version 5.10.0 of the Sentry SDK is targeting net462, while the tests for the same version are targeting net48 (just as in this minimal repro).