Hi, thanks for sharing this issue.

It's unfortunately because of issues like this one that the decision was eventually made not to have official support for C++ builds under NCrunch. While many use cases still work, there were just too many edge cases (like this one) that seemed to need their own special handling in the engine. In most cases where I've seen people successfully use C++ projects under NCrunch, it's been the C# projects referencing the C++ ones, instead of the other way around.

The only 'easy' way I can suggest to make this work would be to scrap the project reference between the C++ project and the C# project, and use an assembly reference instead that points to the .dll output from your C# project. This means you'll need to perform a foreground build of your solution every time you want NCrunch to run on updated code, and you'll also lose code coverage, which is far from ideal.

The other option is to make changes to the MSBuild .targets involved here. I can't provide specific guidance on this, but all languages under .NET have their own set of specific .targets files usually stored under the MSBuild or .NET installation directory. These targets contain a range of steps that translate the project XML into parameters that are eventually passed into the compiler. Almost all components of the build system can be overridden by replacing factory build targets with your own targets declared within the project file (NCrunch does this quite heavily for its own builds). If you can find the build logic dependent on the AdditionalUsingDirectories declaration, maybe you can make it smarter to include paths from the Reference declarations also.