如何从C＃（Unity）将参数传递给本机插件？

Question

C++ code:

#include "stdafx.h"

class myClass
 {
  __declspec(dllexport) float addMyNum(float a)
   {
     return(a);
   }
};

The above code is compiled into Trail.dll and placed in the Unity project's Plugin folder.

Unity C# code:

using UnityEngine;
using System.Runtime.InteropServices;


public class ndstryscript : MonoBehaviour {

 [DllImport("Trial", EntryPoint = "?addMyNum@myClass@@AEAAMPEAM@Z")]

 private static extern float addMyNum(float a);

 void Start ()
  {
    float b = addMyNum(20);
    Debug.Log(b);
  }

}

The expected output is 20 but I'm always getting 0. Could someone help me resolve this?

Answer 1

On the C++ side, you have to move the addMyNum function out of the myClass class. This should fix your function return 0 issue.

Another thing that can cause this issue is the parameter amount not matching between the C++ and C# functions. For example, when you have this on C++ side:

DLLExport int add(int num1, int num2){ return ...}

but this on the C# side:

[DllImport("FirstDLL")]
public static extern int add(int num1);

You get undefined behavior with this The result will be random because the function on the C# side has one parameter while the one from C++ has two.

---

Unrelated but using EntryPoint = "?addMyNum@myClass@@AEAAMPEAM@Z" really looks weird. Sometimes it's needed but other times, it's not as you will have to find the function name and match it with what you need to call on the C# side. You can just put your C++ function inside extern "C" and it will make sure that the compiler does not change the function name.

extern "C"
{
    __declspec(dllexport) float addMyNum(float a)
    {
        return(a);
    }
}

Then your C# side:

[DllImport("Trial")]
public static extern float addMyNum(float a);