评论了几句，这里试试:

After several comments, here a try:

C++ 代码 (DLL)，例如.math.cpp，编译成 HighSpeedMath.dll:

C++ Code (DLL), eg. math.cpp, compiled to HighSpeedMath.dll:

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

C# 代码，例如.程序.cs:

C# Code, eg. Program.cs:

namespace HighSpeedMathTest
{
    using System.Runtime.InteropServices;

    class Program
    {
        [DllImport("HighSpeedMath.dll", EntryPoint="math_add", CallingConvention=CallingConvention.StdCall)]
        static extern int Add(int a, int b);

        static void Main(string[] args)
        {
            int result = Add(27, 28);
        }
    }
}

当然，如果入口点已经匹配，则不必指定它.调用约定也一样.

Of course, if the entry point matches already you don't have to specify it. The same with the calling convention.

正如评论中提到的，DLL 必须提供 C 接口.这意味着，外部C"，没有例外，没有引用等.

As mentioned in the comments, the DLL has to provide a C-interface. That means, extern "C", no exceptions, no references etc.

如果您有 DLL 的头文件和源文件，它可能如下所示:

If you have a header and a source file for your DLL, it could look like this:

数学.hpp

#ifndef MATH_HPP
#define MATH_HPP

extern "C"
{
    __declspec(dllexport) int __stdcall math_add(int a, int b);
}

#endif

数学.cpp

#include "math.hpp"

int __stdcall math_add(int a, int b)
{
    return a + b;
}