C# compile fail because of “Due to its protection level”

Question

My situation is this.

I have 3 Projects.

PrivacyDetectorDLL PDWrapper WindowsFormsApplication1 (C#)

My goal is to use Native C++ class in C#. So I used C++/Cli wrapper class. But I get error of protection level. I don't get it.

I get this error!

Error   1   'PDWrapper.PDWrapperClass.m_pCPrivacyDetectEngine' is inaccessible due to its protection level  K:\Visual Studio 2010 Project\PrivacyDetecter\WindowsFormsApplication1\Form1.cs 23

My c# code from WindowsFormsApplication1 project is this.

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.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public PDWrapper.PDWrapperClass m_PDWrapper = null;

        public Form1()
        {
            m_PDWrapper = new PDWrapper.PDWrapperClass();

            unsafe
            {
                m_PDWrapper.m_pCPrivacyDetectEngine->initEngine();
            }

            InitializeComponent();
        }
    }
}

My PDWrapper Class looks like this.

// PDWrapper.h

#pragma once

using namespace System;

namespace PDWrapper {

    public ref class PDWrapperClass
    {
        // TODO: Add your methods for this class here.
    public :
        PDWrapperClass();
        ~PDWrapperClass();

    public :
        CPrivacyDetectEngine* m_pCPrivacyDetectEngine;
    };
}

And This is my PrivacyDetectEngine header ( header from PDWrapper Project )

#pragma once
class CPrivacyDetectEngine
{
public: // my func
    CPrivacyDetectEngine(void);
    ~CPrivacyDetectEngine(void);

    void CPrivacyDetectEngine::initEngine();

    void CPrivacyDetectEngine::startEngine(char* currentPath, size_t length);

public: // my util func
    int CPrivacyDetectEngine::bytecmp(unsigned char *a, unsigned char *b, int length);
    void CPrivacyDetectEngine::extToNum(char* fileName, unsigned int* extNum);
    int CPrivacyDetectEngine::checkFileSig(unsigned char* fileSig, int sigLength, char* filePath, char** pFileInternal);
    void CPrivacyDetectEngine::parseMSDocText(char** pFileInternal, unsigned int* compSize, unsigned int* decompSize);
    char* CPrivacyDetectEngine::infData(char* pFileData, int compSize, int decompSize);
    void CPrivacyDetectEngine::privacyDetectXML(char* text, int textLength, int* pItemIndex, char* filePath);
    void CPrivacyDetectEngine::checkSocialNum(char* text);

public: // my var
    int driverCount;            
    char driverName[256];           
    unsigned int parseFileList;     
};

My PrivacyDetectEngine header from PrivacyDetectorDLL is same but does have __declspec(dllexport) between class CPrivacyDetectEngine like

class __declspec(dllexport) CPrivacyDetectEngine

I have no idea what is the problem.... Someone please help me.

Answer 1

Your "wrapper" class isn't doing its job.

C# can't access native C++ classes. This is the source of the error. It doesn't matter if the pointer is stored in a managed ref class or not. Although the pointer is public, the data type it points to is hidden from C# (because it is incompatible).

There is a solution, though. C++/CLI can access native C++ classes just fine. So what you need is:

public ref class PDWrapperClass
{
    // TODO: Add your methods for this class here.
public :
    PDWrapperClass();
    ~PDWrapperClass();

private:
    CPrivacyDetectEngine* m_pCPrivacyDetectEngine;

public:
    void initEngine() { m_pCPrivacyDetectEngine->initEngine(); }
};

You need to do this for each function you want C# to be able to call. A wrapper is more than just a pointer holder. In many cases you can provide one method in the C++/CLI ref class that calls a whole sequence of native functions.

Do consider using a smart pointer, though, to make sure the native object is freed in all cases. For example, I posted a smart pointer on codereview.se . Please do respect the license if you use it; the terms are quite generous but do impose some attribution requirements.