/*
 Test.h
 It's not JNI compatible yet! But what about C#?
*/

extern "C" {
int Number(int nr);
           }

/*
Test.cpp
*/
#include "Test.h"

int Number(int nr)
{
 nr++;
 nr++;
 return nr;
}

#Makefile

all: compile linking striping dll sharp

compile:
        g++ -fPIC -c Test.cpp -o Test.o

linking:
        g++ -shared -Wl,-soname,libTest.so Test.o -o libTest.so

striping:
        strip libTest.so

dll:
        #Is it correct? Is Interix .so in dll format?
        cp libTest.so Test.dll

sharp:
        #Copy dll in to charpdevelop project binary directory
        cp Test.dll ./InterixTest/bin/Debug

/*
Test.cs
*/
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Reflection.Emit;
using System.Runtime.InteropServices;

namespace InterixTest
{
        public class Test
        {
                public Test(){}

                [DllImport("Test.dll")] protected static extern 
                int Number(int nr);
                
                public int CallNumber(int nr)
                {
                        return Number(nr);
                }
        }
}

Exception System.EntryPointNotFoundException was thrown in debuggee:
Unable to find an entry point named 'Number' in DLL 'Test.dll'.