CODEWITHSUNDEEP
c#c++dllmarshallingunmanaged
Sean Sen Wang
Sean Sen Wang

Reputation: 193

Using unmanaged C++ code in C# returns 0 for all type double

I am trying to write a simple calculator DLL in C++ and use the DLL in my C# GUI. However, I am always getting "0" as my return value for any use of the double type. This is the C++ side:

MathDLL.h

#ifndef MATH_DLL_H
#define MATH_DLL_H

#define MATHMANAGERDLL_API __declspec(dllexport)

extern "C" MATHMANAGERDLL_API double __stdcall Add(double x, double y);

#endif //MATH_DLL_H

MathDLL.cpp

#include "MathDLL.h"
#ifdef _MANAGED
#pragma managed(push, off)
#endif
#define NULL 0
MathManager* mathManager;

MATHMANAGERDLL_API double __stdcall Add(double x, double y)
{
    if (mathManager == NULL)
        return false;

    return mathManager->add(x, y);
}
#ifdef _MANAGED
#pragma managed(pop)
#endif

MathManager.h

#ifndef MATH_MANAGER_H
#define MATH_MANAGER_H
class MathManager
{
public:
    MathManager();
    ~MathManager();

    double __stdcall add(double x, double y);
};

#endif //MATH_MANAGER_H

MathManager.cpp

#include "MathManager.h"

MathManager::MathManager()
{

}

MathManager::~MathManager()
{

}

double __stdcall MathManager::add(double x, double y)
{
    return x+y;
}

I am importing the DLL functions like so in C#:

SomeWinFormApp.cs

...
// Import Math Calculation Functions (MathDLL.h)
    [DllImport("MATH_DLL.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "Add")]
    public static extern double Add(double x, double y);

When I call Add(), I am getting a return value of 0. I even edited the C++ side to just

double __stdcall MathManager::add(double x, double y)
{
    return 1.0;
}

But I still get 0. What could be wrong here? I was getting PInvoke errors earlier, which is why I changed to __stdcall. If I used __cdecl, I would still get 0's.

Any help is appreciated. Thanks!

Upvotes: 1

Views: 146

Answers (1)

Jean-François Fabre
Jean-François Fabre

Reputation: 140276

you declare that

MathManager* mathManager;

which is not defined. You're lucky it's actually NULL, thus your protection code works and returns false.

if (mathManager == NULL) return false;

you could do much simply without any pointers:

MathManager mathManager;

MATHMANAGERDLL_API double __stdcall Add(double x, double y)
{        
    return mathManager.add(x, y);
}

Upvotes: 3

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