Call Rust from C#

Cargo.toml

// Cargo.toml
[package]
name = "c-abi-to-rust"
version = "0.1.0"
authors = ["YakoYakoYokuYoku <gc1000ll@gmail.com>"]
edition = "2018"

[lib]
name = "ttaone"
crate-type = ["cdylib"]

[dependencies]
libc = { version = "0.2.66", default-features = false }

lib.rs

// src/lib.rs
extern crate libc;
use libc::c_double;

#[no_mangle]
extern "C" fn times_two_at_one(f: extern "C" fn(c_double) -> c_double) -> c_double {
    2.0 * f(1.0)
}

// Compile with:
// `cargo build --release`

main.cs

// src/main.cs
using System;
using System.Runtime.InteropServices;

public class Program
{
    [DllImport("ttaone", EntryPoint="times_two_at_one")]
    public static extern double timesTwoAtOne(Func<double,double> f);

    public static void Main(string[] args)
    {
        Console.WriteLine(timesTwoAtOne(Math.Tanh));
    }
}

// Compile with:
// `csc src/main.cs`

// Run in Linux/UNIX with:
// `LD_LIBRARY_PATH=target/release mono main.exe`

// Run in Windows with:
// `path %PATH%;target/release& mono main.exe`