Fitnesse Slim runner cannot load .net 5 assembly: Error extracting manifest import from file (hr = 0x80131018)

Question

Running a test in Fitnesse gives:

Could not complete testing: fitnesse.slim.SlimError: Error SLiM server died before Header Message could be read.

When using Slim RunnerW.exe to debug my test I get an exception:

System.BadImageFormatException: Could not load file or assembly 'file:///c:\path\assemby.exe' or one of its dependencies. The module was expected to contain an assembly manifest.

I used ProcessExplorer to check and RunnerW.exe was running in 64bit mode. My code is compiled with "Any CPU", the only difference with another working project is that it is a .net 5 (core) project using FitSharp 2.8.2.1 NuGet package.

After enabling the FusionLog it was clear that it could not load my main test assembly. This is part of the log:

LOG: This bind starts in LoadFrom load context.
WRN: Native image will not be probed in LoadFrom context. Native image will only be probed in default load context, like with Assembly.Load().
LOG: No application configuration file found.
LOG: Using host configuration file: 
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework64\v4.0.30319\config\machine.config.
LOG: Attempting download of new URL file:///C:/path/assembly.exe.
LOG: Assembly download was successful. Attempting setup of file: C:\path\assembly.exe
LOG: Entering run-from-source setup phase.
ERR: Error extracting manifest import from file (hr = 0x80131018).
ERR: Run-from-source setup phase failed with hr = 0x80131018.
ERR: Failed to complete setup of assembly (hr = 0x80131018). Probing terminated.

I think it is trying to load a .Net 5 assembly into a .Net 4 process?

Any ideas how to resolve this? I thought of adding an app.config file and specifying a runtime, but I'm not sure, it may just be a rabbit hole since it seems we need to be mixing frameworks here?

Answer 1

Some things that finally got me up and running:

1: Do not reference the executable (file ending with .exe) but refer to the .dll instead. No matter how it is compiled, trying to load the the .exe file as an assembly will always throw a System.BadImageFormatException.

2: Fish the .Net Core versions of Runner.exe and RunnerW.exe (and their dependencies) out of the NuGet package and use those instead of the older SLIM .Net runners (if you are migrating). FusionLog does absolutely nothing with .Net Core, so if there is any logging going on then you know that the process is not running .Net Core.

3: If you got the FitSharp projects from GitHub and are linking to the projects instead of using NuGet, then remove the post build actions to copy the files. Instead go to your test project Dependencies -> Projects -> fit open Properties (F4) and set the options Reference Output Assembly, Copy Local and Copy Local Sattelite Assemblies to Yes. Do the same for the FitSharp and Runner projects.