C# using F# DLL

//-----C# (Main File):

using System;
using fstest;

//Main
class Entry
{
    static int Main( string[] args )
    {
        Console.WriteLine(testfun.addtwo(2, 6));
        return 0;
    }
}

//-----F# (libtest.fs) (compiles to DLL):

namespace fstest

module testfun =

let addtwo x y = x + y

//-----compile.bat:

fsc.exe libtest.fs --standalone --target:library
csc.exe *.cs /r:libtest.dll