Running C++ code from C# Dotnet Core

    #ifdef __cplusplus
    extern "C" {
    #endif

If it were C++ interop, I would be able to DllImport a C++ library (without the C linkage) and get a C# wrapper class of the C++ classes automatically.

It's not C#'s fault. C++ is the one without an ABI. To properly DllImport C++ you would need to define the so/dll name in the import and also the specific C++ compiler version and build, since the binary changes underneath, e.g. foo.msvc_x64_debug.dll, foo.gcc_x86_release.dll, etc. Then C# might have a chance of importing a c++ library, but not today.

It should be theoretically be possible to call C++ via interop:

    [DllImport("<MANGLED NAME>")]
    static extern void Foo(IntPtr @this, int param1, ...);