loading c++ dll function having parameter (char * ) into c#

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i have created dll in vc++

init_set(char *)

takes char *

C#

extern "C" MYDLL_API int init_set(char*);
extern "C" MYDLL_API int init_valid();

in c# i want to pass an string to init_set().

C#

[DllImport("MYDLL.dll")]
public static extern int init_set(char[] s);

[DllImport("MYDLL.dll")]
public static extern int init_valid();

in main

C#

static void Main(string[] args)
       {
           string st = "Quad127";
           char[] s = st.ToCharArray();

           init();
            init_set(s);

all void function runs but this gives message

An unhandled exception of type 'System.EntryPointNotFoundException' occurred in usedll.exe

Additional information: Unable to find an entry point named 'init_set' in DLL 'MYDLL.dll'.

Posted 12-Nov-11 0:13am

01.mandar

Updated 12-Nov-11 0:15am

v3

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Comments

Richard MacCutchan 12-Nov-11 6:20am

What is the value of MYDLL_API in your build of the DLL. Also show the actual code for the init_set() function.

01.mandar 14-Nov-11 8:23am

int init_set(char* id)
{
char fire[100]={0};

strcat(fire,"AT+AIPO=");
strcat(fire,id);
// some more strcat parameters

if(sp_writer(fire))
return 0;
else
return 1;
}

Richard MacCutchan 14-Nov-11 12:12pm

I suspect there is still some problem with your build process; check that you are correctly exporting your function names. As I asked above, what is the value of MYDLL_API in your build of the dll. You should also be able to check whether your functions are being exported correctly by using Dependency Walker.

01.mandar 15-Nov-11 1:59am

#ifdef MYDLL_EXPORTS
#define MYDLL_API __declspec(dllexport)
#else
#define MYDLL_API __declspec(dllimport)
#endif

extern "C" MYDLL_API char * Reader(); (i need to figure out this too)
extern "C" MTDLL_API int init();
extern "C" MTDLL_API int init_set(char*);
extern "C" MTDLL_API int init_base();

01.mandar 15-Nov-11 2:18am

well when i checked in dependency walker
i dont know but some function are still exported as c++

E
C | extern "C" MYDLL_API char * Reader();
C | extern "C" MTDLL_API int init();
C++| extern "C" MTDLL_API int init_set(char*);
C++| extern "C" MTDLL_API int init_base();

if decorated option is checked
E
C | Reader;
C | init
C++| ??init_set@@YAPADD@Z
C++| ??init_base@@YAHXZ

i cant figure out why this function are still decorated when i used extern c

01.mandar 15-Nov-11 3:16am

tried compiling with Gz option but still giving decorated name

Richard MacCutchan 15-Nov-11 4:25am

I don't think /Gz will help. See also my latest solution entry below.

2 solutions

Solution 1

You need to use Marshaling[^].

C++ takes a string as the raw ASCII (for char *) data, followed by a single byte with the value of 0. Not sure how C# stores it's string, but I bet there is a length stored in there somewhere.

So, a very simple example:
MYDLL.cpp:

C++

#include <Windows.h>
#include <stdio.h>

//MUST have extern "C" if in C++, otherwise the name will be mangled

extern "C" __declspec(dllexport) int init_set(char *sz) {
	printf("%s\n", sz);
	return 0;
}

BOOL APIENTRY DllMain(HMODULE hModule, DWORD  nReason, LPVOID lpReserved) {
	return TRUE;
}

and the C# file:

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices; //has the Marshal class

namespace CS_CLI
{

    class Program
    {
        [DllImport("MYDLL.dll")]
        public static extern int init_set(IntPtr s); //The type is a pointer (to characters) in C, so we need to say this, with IntPtr

        static void Main(string[] args)
        {
            String str = "I am a managed String"; //Some random string
            IntPtr ptrCString = (IntPtr)Marshal.StringToHGlobalAnsi(str); //Our actual marshal. This creates a copy of the string in unmanaged memory. This also converts the unicode string used in C# to ascii (char* is ASCII, wchar_t* is Unicode)
            init_set(ptrCString); //Call our function
            Marshal.FreeHGlobal(ptrCString); //Free the copy of the memory we made 2 lines above
        }
    }
}

Posted 12-Nov-11 18:44pm

Andrew Brock

Comments

01.mandar 15-Nov-11 1:56am

i have tried the code above and some more possibilities
http://msdn.microsoft.com/en-us/magazine/cc164193.aspx#S2

but still have error "An unhandled exception of type 'System.EntryPointNotFoundException' occurred in usedll.exe
Additional information: Unable to find an entry point named 'init_set' in DLL 'MYDLL.dll'."

==================================================
Function Name : int __cdecl inti_set(char *)
Address : 0x10026708
Relative Address : 0x00026708
Ordinal : 1 (0x1)
Filename : MYDLL.dll
Full Path : D:\GSM\usedll\bin\Debug\MYDLL.dll
Type : Exported Function
==================================================

==================================================
Function Name : int __cdecl inti_base(void)
Address : 0x1002609b
Relative Address : 0x0002609b
Ordinal : 2 (0x2)
Filename : MYDLL.dll
Full Path : D:\GSM\usedll\bin\Debug\MYDLL.dll
Type : Exported Function
==================================================

==================================================
Function Name : Reader
Address : 0x10026307
Relative Address : 0x00026307
Ordinal : 3 (0x3)
Filename : MYDLL.dll
Full Path : D:\GSM\usedll\bin\Debug\MYDLL.dll
Type : Exported Function
==================================================

==================================================
Function Name : init
Address : 0x1002704a
Relative Address : 0x0002704a
Ordinal : 4 (0x4)
Filename : MYDLL.dll
Full Path : D:\GSM\usedll\bin\Debug\MYDLL.dll
Type : Exported Function
==================================================

#ifdef MYDLL_EXPORTS
#define MYDLL_API __declspec(dllexport)
#else
#define MYDLL_API __declspec(dllimport)
#endif

extern "C" MYDLL_API char * Reader(); (i need to figure out this too)
extern "C" MTDLL_API int init();
extern "C" MTDLL_API int init_set(char*);
extern "C" MTDLL_API int init_base();

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Richard MacCutchan · Accepted Answer · 2011-11-14T22:26:00

I have just created a simple DLL as follows, which creates the correct linkage, and my C# test program can invoke the methods:
1. Header file (zzdll.h)

C++

#pragma once

#define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
// Windows Header Files:
#include <windows.h>

#ifdef MYDLL_EXPORTS
#define MYDLL_API __declspec(dllexport)
#else
#define MYDLL_API __declspec(dllimport)
#endif

extern "C" MYDLL_API int __stdcall init_set(char*);
extern "C" MYDLL_API int __stdcall init_valid();

2. Implementation file (zzdll.cpp)

C++

#include "zzdll.h"


BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
					 )
{
	switch (ul_reason_for_call)
	{
	case DLL_PROCESS_ATTACH:
	case DLL_THREAD_ATTACH:
	case DLL_THREAD_DETACH:
	case DLL_PROCESS_DETACH:
		break;
	}
	return TRUE;
}


MYDLL_API int __stdcall init_set(char*)
{
	return 1;
}

MYDLL_API int __stdcall init_valid()
{
	return 1;
}

The tag MYDLL_EXPORTS is defined to the compiler.

[edit]
Added __stdcall for correct function linkage.
[/edit]