calling a C++ dll from C# throwing a SEHException

Question

I am trying to call a dll, which I build in C++, from a C# code. However, I am getting the following error:

Exception thrown: 'System.Runtime.InteropServices.SEHException' in dlltest_client.exe An unhandled exception of type 'System.Runtime.InteropServices.SEHException' occurred in dlltest_client.exe External component has thrown an exception.

I am building the C++ dll using a cpp code which in turn imports a header file:

dlltest.cpp :

#include "stdafx.h"
#include <iostream>
#include <string>
#include "dlltest.h"

using namespace std;

// DLL internal state variables:
static string full_;
static string piece_;

void jigsaw_init(const string full_input, const string piece_input)
{
    full_ = full_input;
    piece_ = piece_input;
}

void findPiece()
{
    cout << full_;
    cout << piece_;
}

where dlltest.h :

#pragma once

#ifdef DLLTEST_EXPORTS
#define DLLTEST_API __declspec(dllexport)
#else
#define DLLTEST_API __declspec(dllimport)
#endif

extern "C" DLLTEST_API void jigsaw_init(
    const std::string full_input, const std::string piece_input);

extern "C" DLLTEST_API void findPiece();

this successfully builds dlltest.dll

My C# code that is ought to utilise the dll is

dlltest_client.cs

using System;
using System.Runtime.InteropServices;

class Program
{
    [DllImport(@"path\dlltest.dll")]
    private static extern void jigsaw_init(string full_input, string piece_input);
    [DllImport(@"path\dlltest.dll")]
    private static extern void findPiece();

    static void Main(string[] args)
    {
        string full = @"path\monster_1.png";
        string piece = @"path\piece01_01.png";
        jigsaw_init(full, piece);
        findPiece();
    }
}

Answer 1

You cannot use C++ std::string for unmanaged interop, which is the reason for your DLL throwing an exception.

Instead use pointer to null-terminated character array to pass strings between your C# code and the unmanaged C++ code.

The other error is that the C++ code uses cdecl calling convention, but the C# code assumes stdcall. You need to make the two sides of the interface match, change one to match the other.