1,编译生成dll时可能出现错误，解决办法：pch.h文件头部，添加声明
#define _CRT_SECURE_NO_WARNINGS

2, c++头文件声明
extern "C" __declspec(dllexport)  char* getvalue(const char * param1, const char * param2);

3, c++方法实现
 char* getvalue(const char * param1, const char * param2) {
     //process，省略
    //return
    std::string value = "result";
    char* cStr = new char[value.length() + 1]; // 加1是为了存储结束的空字符
    strcpy(cStr, value.c_str());
    return cStr;
}

4, c#端

 [DllImport("test.dll", EntryPoint = "getvalue", CallingConvention = CallingConvention.Cdecl)]
 static extern IntPtr getvalue([MarshalAs(UnmanagedType.LPStr)] string param1, [MarshalAs(UnmanagedType.LPStr)] string param2);
 
//调用
try
{
    string param1 = "param1";
    string param2 = "param2";
    IntPtr retPtr = getvalue(param1, param2);
    if (retPtr != IntPtr.Zero)
    {
        try
        {
            // 将指针转换为托管字符串
            string result = Marshal.PtrToStringAnsi(retPtr);                        
            Console.WriteLine("getvalue result: " + result);
            Console.WriteLine();
        }
        finally
        {
            // 释放C++分配的内存
            //Marshal.FreeCoTaskMem(retPtr);
        }
    }
}
finally
{
    // 清理分配的内存
    //Marshal.FreeHGlobal(ptr);
    //Marshal.FreeHGlobal(ptr2);
}

5，参考文档
(1),C++封装成DLL并调用
https://blog.csdn.net/kenjianqi1647/article/details/102833477
https://blog.csdn.net/weixin_44244190/article/details/123504926
(2),VS中用C#调试DLL
https://www.cnblogs.com/lzhu/p/9268280.html
 
(3),生成32位dll注意事项 
 取消使用预编译头
属性-》配置属性-》C/C++ -》预编译头-》预编译头-》
参考资料
https://blog.csdn.net/qq_34059233/article/details/113837245