The .NET sdk doesn't include an f# compiler exe anymore (just fsc.dll). However Visual Studio does as part of a tools folder. NCrunch uses this when running in Visual Studio.
I've found a workaround by adding this Property to Directory.Build.Props to use the Visual Studio fsc.exe
Code:
<FscToolPath Condition="'$(NCrunch)' == '1'">C:\Program Files\Microsoft Visual Studio\2022\Professional\Common7\IDE\CommonExtensions\Microsoft\FSharp\Tools</FscToolPath>
This work for me as I already have Visual studio installed, but obviously not ideal to have this dependency for running in Rider.
When I build F# projects in Rider I can see in the logs that in order to run the f# compiler it runs
Code:
C:\Program Files\dotnet\dotnet.exe "C:\Program Files\dotnet\sdk\8.0.204\FSharp\fsc.dll"
I couldn't work out exactly what MSBuild property Rider is using to enable this build behavior, but I think if this could made the default for NCrunch Rider it would remove the need for Visual Studio workaround.