How to use c++ classes and functions in c#?

//MomentOfInertia.h
#ifdef COMPILE_MYLIBRARY
 #define DllExport   __declspec( dllexport )
#else
 #define DllExport   __declspec( dllimport )
#endif

class DllExport a
{
public:
double MOIRectangle(double b, double h);
double MOISemiCircle(double r);
double MOITriangle(double b, double h, int);
};

// MomentOfInertia.cpp : 
#include "stdafx.h"
// Calculation of the moment of inertia
#include "MomentOfInertia.h"
double a::MOIRectangle(double b, double h)
{
return b * h * h * h / 3;
}
double a::MOISemiCircle(double r)
{
const double PI = 3.14159;
return r * r * r * r * PI / 8;
}
double a::MOITriangle(double b, double h, int)
{
return b * h * h * h / 12;
}