Question

I have a DLL made in C++ that is then used in a C# program. the C# program references the DLL and i call the classes and can use the functions defined in the DLL.

Now i want to add other functions to the DLL so i can run C++ code in C#. I Follow the example of how the other functions in the DLL are defined; .h and .cpp and i create a new function that executes code. I put it in the same places as the previously define functions, but when i put the DLL in my C# code my function is not there. when i create an instance of the DLL class my function is not a part of it.

I put it in the .h as a public function of the class and gave it code to run in the .cpp but its not found.

Why can’t my C# program see the function i made even though its following the same suite as the other functions?

UPDATE

code.h:

#pragma once

#include <windows.h>
#include <PvDisplayWnd.h>
#include <vfw.h>
#include "NPvResult.h"
#include "NPvDeviceInfo.h"
#include "NPvBuffer.h"
#include <string>

using namespace System;
using namespace System::Runtime::InteropServices;

public ref class NPvDisplayWnd
{
public :
    NPvDisplayWnd();
    bool Handle();
    NPvResult^ ShowModeless(long locx, long locy, long x, long y);
    NPvResult^ ShowModal();
    NPvResult^ NPvDisplayWnd::Create();
    int^ Display( NPvBuffer^ aBuffer, int^ x);
    NPvResult^ Work( const std::string afilename, unsigned short asizex, unsigned short asizey, unsigned short aBPP, double aFPS);
    NPvResult^ Close();
    NPvResult^ DoEvents();
}

code.cpp:

#include "NPvDisplayWnd.h"

NPvDisplayWnd::NPvDisplayWnd(){code}
NPvResult^ NPvDisplayWnd::Create(){code}
bool NPvDisplayWnd::Handle()
NPvResult^ NPvDisplayWnd::ShowModeless(long locx, long locy, long x, long y){code}
NPvResult^ NPvDisplayWnd::ShowModal(){code}
int^ NPvDisplayWnd::Display( NPvBuffer^ aBuffer, int^ x){code}
NPvResult^ NPvDisplayWnd::DoEvents(){code}
NPvResult^ NPvDisplayWnd::Work( const std::string aFileName, unsigned short aSizeX, unsigned short aSizeY, unsigned short aBPP, double aFPS){code}
NPvResult^ NPvDisplayWnd::Close(){code}

The function that i added was the Work() function. if i build this and place it in my C# code, it sees all the functions except work. To make sure i was using the right .dll, i changed the name of ShowModeless() to ShowModeless__F() and rebuilt it and added it to my C# and the change carried over, but i still didn’t see my Work() function.

Answer 1

On Windows, you must explicitly export symbols in order for users to be able to access them via DLLs. This is done using the __declspec(export) syntax. This is needed during compilation and linking of the DLL.

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Now that we know you’re using C++/CLI instead of regular C++, we can see a problem. Work takes a std::string. But Work is intended to be a managed function, and managed code cannot take a std::string as a parameter. You need to take a managed string type instead.