Unable to find an entry point named 'xxx' in DLL 'xxx.dll' exception?

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C++

C#

WinForms

Hey everyone.

I have a question to ask.

I'm trying to create a library in C++ and use it in C#.

Here's the guidance that I'm using to achieve:
How to[^]

I created the files in C++(source and header) as follows:

// MathFuncsDll.h

namespace MathFuncs
{
    class MyMathFuncs
    {
    public:
        // Returns a + b
        static __declspec(dllexport) double Add(double a, double b);

        // Returns a - b
        static __declspec(dllexport) double Subtract(double a, double b);

        // Returns a * b
        static __declspec(dllexport) double Multiply(double a, double b);

        // Returns a / b
        // Throws DivideByZeroException if b is 0
        static __declspec(dllexport) double Divide(double a, double b);
    };
}

// MathFuncsDll.cpp
// compile with: /EHsc /LD

#include "MathFuncsDll.h"

#include <stdexcept>

using namespace std;

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 new invalid_argument("b cannot be zero!");
        }

        return a / b;
    }
}

I build the files, create a dll.
After that I put the dll file in my C# solution(with the same folder of .exe file)

Then, I import the dll in my C# project:

[DllImport("SimpleDll.dll", CallingConvention=CallingConvention.Cdecl)]
public static extern double Add(double a, double b);

static void Main(string[] args)
{
    double a = Add(1.0, 3.0));
}

When I excecute the program, it gives me an exception.

Why does it happen? Any ideas?

My best regards...

Note: I've tried to add extern "C", but linkage problems occured.