C++ DLL imported into a C# application

I am trying to do a basic C++ DLL in order to use it in C#.. the following classes are used:

My cpp file

#include "stdafx.h"    
#include "MathFuncsAssembly.h"

namespace MathFuncs
{
    double MyMathFuncs::Add(double a, double b)
    {
        return a + b;
    }

    double MyMathFuncs::Subtract(double a, double b)
    {
        return a - b;
    }

    double MyMathFuncs::Multiply(double a, double b)
    {
        return a * b;
    }

    double MyMathFuncs::Divide(double a, double b)
    {
        if (b == 0)
        {
            throw gcnew DivideByZeroException("b cannot be zero!");
        }

        return a / b;
    }
}

My header file

using namespace System;

namespace MathFuncs
{
    public ref class MyMathFuncs
    {
    public:
        static double Add(double a, double b);

        static double Subtract(double a, double b);

        static double Multiply(double a, double b);

        static double Divide(double a, double b);
    };
}

and in my C# application where I am invoking the library

[DllImport("MathFuncsAssembly.dll")]
public static extern double Add(double a, double b);

static void Main(string[] args)
{
   Console.WriteLine(Add(10.0, 11.0));
   Console.ReadLine();
}

An exception occurs at the (Add(10.0,11.0)) part.. The following exception is being thrown: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B).. Any Ideas? Also, i copied the .dll file in the bin of the c# application...

thanks!

If you have a ref class (and hence a C++/CLI dll), you don't need DllImport at all: just add a reference to the dll in the C# project and call the functions 'the C# way' i.e. MathFuncs.Add() (note you still need to make sure the platform matches, and that all native dlls the CLI dll depends on are in the path)