System.AccessViolationException : Attempted to read or write protected memory in C++ dll

Question

So I have been scratching my head over this for a while. I have built a C++ dll to use in a VB.net project. The C++ source code is shown below.

C++

    #include "stdafx.h"
    #include "mydll.h"
    #include <vector>
    #include <string>
    #include <algorithm>
    #include <sstream>
    #include <fstream>
extern "C" __declspec(dllexport) void __cdecl ExtractVolumeDataC(std::string path, std::string fileName, bool chkVol, std::string txtKeyName)
{

    std::string line;
    std::vector<std::vector<std::string>> ValuesCSV;
    std::replace(path.begin(), path.end(), '\\', '/'); //replace backslash with forward slash
    std::ifstream in(path + fileName);
    while (std::getline(in, line)) {
    std::string phrase;
    std::vector<std::string> row;
    std::stringstream ss(line);
    while (std::getline(ss, phrase, ',')) {
    row.push_back(std::move(phrase));
    }
    ValuesCSV.push_back(std::move(row));
    }
}

The code i am using in VB.net is the following

VB.net

    Public Class Form1

    <Runtime.InteropServices.DllImport("mydll.dll")> _
    Public Shared Sub ExtractVolumeDataC(ByVal path As String, ByVal fileName As String, ByVal txtKeyName As String)

    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        ExtractVolumeDataC("C:\\users\me\\documents\\", "VOL.CSV", "01100872")
    End Sub
End Class

One observation I made is that I don't get this error with primitives, but including STL elements like string and vector. I get the error. I am sorry if this a dumb question I haven't looked at a C++ code in like 15 years.

Answer 1

VB.net doesn't pass strings as c++ std::string, ByVal Strings are passed as pointers to chars (i.e. old C style strings). You can use LPCSTR as the parameter type for this. In your case this would be:

extern "C" __declspec(dllexport) void __cdecl ExtractVolumeDataC(LPCSTR path, LPCSTR fileName, bool chkVol, LPCSTR txtKeyName)

See This Microsoft support page for more info.