How to Interop between C# and C++

I'm attempting a very basic interoperation between the 2 languages. I basically have some performance intensive code I wanna handle in C++ and then get the result back to my application.

All is going to be compiled in Visual Studio.

i have chosen int to be the input and output type since marshalling can be a bit wonky and not really what i'm dealing with.

C++ i have:

#include "stdafx.h" // default from vs2013, no idea what it is



_declspec(dllexport) int Diu(int p) {

    return p * 2;

}

C# i have:

using System;



namespace Interop {

    public class Program{

        [System.Runtime.InteropServices.DllImport("Hardworker.dll")]

        public static extern int Diu(int p);



        private static void Main(string[] args) {

            Console.WriteLine(Diu(2));

        }

    }

}

So it's a pretty basic example. But I'm getting the exception:

An unhandled exception of type 'System.BadImageFormatException'
occurred in Interop.exe

Additional information: An attempt was made to load a program with an
incorrect format. (Exception from HRESULT: 0x8007000B)

The C++ project is created as a Console Application > Dll in the create dialog.
I checked the C++ dll in the disassembler and i can see a Diu as an exported symbol.

Uh. What did I miss about the setting up interop?