dll import (Unable to find an entry point named "methodname"

Question

I am trying to import a c++ dll into my C# windows project. But i am getting an error "Unable to find an entry point named 'Add' in DLL 'Test.dll'"
 
My c++ source code:

#pragma once
 
using namespace System;
 
namespace Test {
 
    public ref class Class1
    {
    public: int Add(int i, int j)
        {
            return(i + j);
        }
    };
}

 

My C# windows source code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using Test;// my c++ dll refernce

 
namespace WindowsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        [DllImport("Test.dll")]
        public static extern int Add(int a, int b);
        private void button1_Click(object sender, EventArgs e)
        {
            int p = Add(1, 2);
        }
    }
}

 
I placed the c++ dll in c# application output path i e debug folder.
 
Please help to find me the error i made in the c++ or c# code.