The simplest Interop case HelloWorld I can create, give it a try and see what happens:

>cat countbyone.cpp

extern "C" int SomeMethod(int num) {
  return num++;
}

Compile your shared library:

• Linux: gcc -g -shared -fPIC countbyone.cpp -o libcountbyone.so
• OS-X: clang -dynamiclib countbyone.cpp -o libcoutbyone.dylib

>cat interop.cs

using System;
using System.Runtime.InteropServices;
namespace InteropDemo
{
    class MainClass
    {
        [DllImport("countbyone")]
        private static extern int SomeMethod(int num);

        public static void Main (string[] args)
        {
            var x = SomeMethod(0);
            Console.WriteLine(x);
        }
    }
}

Compile your .Net/Mono app:

>mcs interop.cs

Run it:

>mono interop.exe
1

Output should be 1 and no errors…

If the shared (native) library is not found, you receive:

XXXXX failed to initialize, the exception is: System.DllNotFoundException

If you have a entry point mismatch you would receive a:

XXXXX failed to initialize, the exception is: System.EntryPointNotFoundException

From my answer on Stackoverflow