C# and Rust interop example

Cargo.toml

[package]
name = "nok-internal"
version = "0.1.0"
authors = ["Chanjung Kim <freiyer.paxbun@gmail.com>"]
edition = "2018"

[dependencies]
num = "0.3.0"

[lib]
name = "internal"
path = "src/lib.rs"
crate-type = ["dylib"]

lib.rs

#[repr(C)]
pub struct StringLengths {
    utf8: i32,
    utf16: i32,
}

pub unsafe fn get_str_lens_internal<T: num::PrimInt>(s: *const T) -> i32 {
    let mut rtn = 0;
    let mut ptr = s;

    while !(*ptr).is_zero() {
        ptr = ptr.offset(1);
        rtn = rtn + 1;
    }
    rtn
}

/// Returns the length of the string in UTF8 form.and UTF16 form
#[no_mangle]
pub unsafe extern "C" fn get_str_lens(s1: *const i8, s2: *const i16) -> StringLengths {
    StringLengths {
        utf8: get_str_lens_internal(s1),
        utf16: get_str_lens_internal(s2),
    }
}

Program.cs

using System;
using System.IO;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace Nok
{
    struct StringLengths
    {
        [MarshalAs(UnmanagedType.I4)]
        int utf8;
        [MarshalAs(UnmanagedType.I4)]
        int utf16;

        public override string ToString()
        {
            return $"{{ UTF8: {utf8}, UTF16: {utf16} }}";
        }
    }

    static class StringExtensions
    {
        [DllImport("nok-internal", EntryPoint = "get_str_lens")]
        private extern static StringLengths GetStringLengths(
            [MarshalAs(UnmanagedType.LPUTF8Str)] string utf8,
            [MarshalAs(UnmanagedType.LPWStr)] string utf16);

        public static StringLengths GetStringLengths(this string s)
        {
            return GetStringLengths(s, s);
        }

        public static void PrintStringLengths(this string s)
        {
            Console.WriteLine("UTF8 Length of \"{0}\": {1}", s, s.GetStringLengths());
        }
    }

    class Program
    {
        static string GetCurrentSourcePath([CallerFilePath] string str = null)
        {
            return str;
        }

        static string dllPath = Path.GetFullPath(
                                    Path.Join(
                                        GetCurrentSourcePath(),
                                        "../nok-internal/target/debug/internal.dll"));

        static void Main(string[] args)
        {
            NativeLibrary.SetDllImportResolver(
                typeof(Program).Assembly,
                (libraryName, assembly, searchPath) => NativeLibrary.Load(dllPath)
            );

            string str1 = "Hello, world!";
            string str2 = "안녕, 세계!";

            str1.PrintStringLengths();
            str2.PrintStringLengths();
        }
    }
}