新建C++win32空项目

//main.h 创建头文件

#pragma once

//#ifdef _MSC_VER && __cplusplus
//#define SDK_API  extern "C" __declspec(dllexport)
//#else
//#define SDK_API extern "C"
//#endif

__declspec(dllexport) void key(); 

//main.c 要封装的源文件

#include "main.h" //添加头文件

void key()
{
	//主函数
}

修改项目属性：

输出目录：之后创建的C#项目位置
KeyAuthCSharpDemo\KeyAuthCSharpDemo\bin\Debug

新建C#项目-windows窗体应用

//Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace KeyAuthCSharpDemo
{
    public partial class Form1 : Form
    {
        [DllImport("KeyAuthDll.dll", CallingConvention = CallingConvention.Cdecl)]
        public extern static void key();


        public Form1()
        {
            InitializeComponent();
            key();
        }
    }
}

---------------------------------------------------------------------------------------------------------
//补充：调用dll函数时要传入浮点型参数
namespace Demo
{
    public partial class Form3 : Form
    {
        public Form3()
        {
            InitializeComponent();
        }

        [DllImport("keyStrokeRec.dll", CallingConvention = CallingConvention.Cdecl)]
        public extern static int StopHook(ref float feature); // ref 传入传出

		//某个事件调用stophook函数
        private void tb_password_Leave(object sender, EventArgs e)
        {
            float[] feature = new float[4]; //声明浮点型数组，长度为4
            StopHook(ref feature[0]); //将数组传入函数
        }

修改项目属性：

生成DLL项目解决方案
生成C#项目解决方案

感谢我伟大的同事手把手教我呜呜呜！

//DLL中有函数
extern "C" __declspec(dllexport) BOOL StopHook(float* feature); 
//C#引用时应写为
[DllImport("keyStrokeRec.dll", CallingConvention = CallingConvention.Cdecl)]
public extern static int StopHook(ref float feature); //ref
//在C#中调用
float[] feature = new float[4];
StopHook(ref feature[0]); 
//feature为空数组，作为参数传入
//在dll中的stophook函数中改变后C#中的该参数也改变了
//因为C不能直接返回数组

C#中有多个窗体需要调用Dll时，无需重复声明

//新建一个C#空类定义 DllFun.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace KDemo //注意这里
{
    //dll接口类，不用重复声明
    class DllFun
    {
        [DllImport("keyStrokeRec.dll", CallingConvention = CallingConvention.Cdecl)]
        public extern static int StartHook();
    }
}

//在调用Dll的窗体中加入
using KDemo;
//调用函数时应写
DllFun.StartHook();