How to debug .DLL Plugins

I am using visual studio c++ Build my path-finding .dll file for unity plugins which saved in Assets->Plugins folder. However I can't Debug the .dll file in MonoDevelop. Does anyone know how to do that? Thanks

This is my PlugImport.cs

using UnityEngine; using System.Collections; using System; using System.Runtime.InteropServices; using IntPtr = System.IntPtr;

public class PluginImport : MonoBehaviour { //Lets make our calls from the Plugin [DllImport ("ASimplePlugin")] private static extern int PrintANumber();

 [DllImport ("ASimplePlugin")]
 private static extern IntPtr PrintHello();
 
 [DllImport ("ASimplePlugin")]
 private static extern int AddTwoIntegers(int i1,int i2);

 [DllImport ("ASimplePlugin")]
 private static extern float MultiTwoFloats(float f1,float f2);    
 
 [DllImport ("ASimplePlugin")]
 private static extern float GetNodes(Vector4[] vertices, int[] trangles, int m_vCount);