如何从C＃中的托管C ++ DLL中获取字符串（使用包装器类）

Question

i have done this using unmanaged dll but having some difficulty using managed dll.

I want to pass a string to my managed c++ wrapper class which process it and return modified string

the purpose of c++ dll is to return hex code of a file(and later modifies it to do some complicated tasks within dll) which i pass it as a string, for this approcach i have used managed c++ dll instead of unmanaged one.

my c++ class is as follows :

using namespace std;

//main function - entry point of program __declspec(dllexport)
 const char* hexclass:: getfilename(char* filename)
{
//to open file
ifstream::pos_type size;
string virusScanStatus;
char* fileAccessError;
string errorDes ="Exception has Occured With Current File Operation";
//hexcode logic goes here
fileAccessError = new char[errorDes.size()];
errorDes.copy(fileAccessError,errorDes.size());
return fileAccessError;
}

my c++ wrapper class is as follows : header file including c++ file here (not shown for code readabitily)

 using namespace System;

 namespace CWrapperHexValue {

public ref class cWrapperHexValue
{
    public:
        cWrapperHexValue();
        const char* hexValue;
        const char* getHexValue(char* fileName);
    private:
        hexclass* pHexClass;

};
}

and my wrapper class is as follows :

// This is the main DLL file.
 #pragma once

  #include "stdafx.h"

   #include "CWrapperHexValue.h"

   #include "D:\Projects\program10\program10\hexclass.cpp"
   #include "D:\Projects\program10\program10\hexclass.h"

    CWrapperHexValue::cWrapperHexValue::cWrapperHexValue()
    {
    pHexClass = new hexclass();
     }

     const char* CWrapperHexValue::cWrapperHexValue::getHexValue(char* fileName)
    {
    hexValue= pHexClass -> getfilename(fileName);
return hexValue;
     }

and finally my c# code to send filename is as follows :

//my c++ dll name is CWrapperHexValue
        CWrapperHexValue.cWrapperHexValue objHexClass = new CWrapperHexValue.cWrapperHexValue();
        byte[] fileNameBytes = Encoding.ASCII.GetBytes(fileNameForHexScan);

        unsafe 
        {
            fixed (byte* p= fileNameBytes)
            {
                sbyte* sp = (sbyte*)p;
                sbyte* returnSp = objHexClass.getHexValue(sp);
            }
        }

now how do i get back the returnSp value as a string or any other better way to pass and get string, please provide useful code because i have not much experience with c++/c# cli conversion

please advice how can i improve my code for better memory management becuase i have to pass a whole lot of system files one by one and get their hex code

Answer 1

Depending on what

// hexcode logic

is, it might be better/easier to implement it in C# and forget about the marshaling.

If you really need to marshal strings from C++ to C#, this topic will likely help you quite a bit: How to marshall c++ char* to C# string using P/INVOKE (code is here)

Basically just build a native DLL out of your native code and you can just pinvoke it from C# rather than have the intermediate managed C++ class if all you need is a string.

In terms of memory management, if you can move the hexcode logic into C#, that would eliminate the need to marshal strings and likely save you some processing, but more importantly, be less painful to debug. Namely, why can't you do this?

C#

public void SomeFunction()
{
    string filepath = "foo.bar"
    string hexVal = HexCodeLogicInManaged(filepath);
    ... other code ...
}

private string HexCodeLogicInManaged(string filepath)
{
    // hexcode logic converted from native to managed
}