As a general rule, anything you can easily call from C will be callable from Julia. For example, there was some early discussion on calling Rust from Julia via ccall over in Interfacing rust? - #5 by Rory-Finnegan which was apparently quite successful.
I haven’t used C#, but as long as it’s possible to generate a library that you can call from C, you should be able to use that same library from Julia with ccall. Perhaps something like Calling C# from C - Stack Overflow would work?