"无法在dll中找到名为[function]的入口点"(c ++到c#类型转换)

#include<iostream>

extern "C" {
    __declspec(dllexport) int add(int a, int b) {
        return a + b;
    }
}

using System;
using System.Runtime.InteropServices;

class Program {
    [DllImport("mydll.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto)]
    public static extern int add(int a, int b);

    static void Main(string[] args) {
        int result = add(1, 2);
        Console.WriteLine("Result: " + result);
    }
}