how to call function from DLL in C#/Python

Question

I have next C++ code for create DLL file

// MathFuncsDll.h

#ifdef MATHFUNCSDLL_EXPORTS
#define MATHFUNCSDLL_API __declspec(dllexport) 
#else
#define MATHFUNCSDLL_API __declspec(dllimport) 
#endif

namespace MathFuncs
{
    // This class is exported from the MathFuncsDll.dll
    class MyMathFuncs
    {
    public: 
        // Returns a + b
        static MATHFUNCSDLL_API double Add(double a, double b); 

        // Returns a - b
        static MATHFUNCSDLL_API double Subtract(double a, double b); 

        // Returns a * b
        static MATHFUNCSDLL_API double Multiply(double a, double b); 

        // Returns a / b
        // Throws const std::invalid_argument& if b is 0
        static MATHFUNCSDLL_API double Divide(double a, double b); 
    };
}

// MathFuncsDll.cpp : Defines the exported functions for the DLL application.
//

#include "stdafx.h"
#include "MathFuncsDll.h"
#include <stdexcept>

using namespace std;

namespace MathFuncs
{
    double MyMathFuncs::Add(double a, double b)
    {
        return a + b;
    }

    double MyMathFuncs::Subtract(double a, double b)
    {
        return a - b;
    }

    double MyMathFuncs::Multiply(double a, double b)
    {
        return a * b;
    }

    double MyMathFuncs::Divide(double a, double b)
    {
        return a / b;
    }
}

after compile I have dll file and i want to call for example ADD function

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace call_func
{
    class Program
    {
        [DllImport("MathFuncsDll.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern double  MyMathFuncs::Add(double a, double b);

        static void Main(string[] args)
        {
            Console.Write(Add(1, 2));
        }
    }
}

but got this message error img

or in python code

Traceback (most recent call last):
  File "C:/Users/PycharmProjects/RFC/testDLL.py", line 6, in <module>
    result1 = mydll.Add(10, 1)
  File "C:\Python27\lib\ctypes\__init__.py", line 378, in __getattr__
    func = self.__getitem__(name)
  File "C:\Python27\lib\ctypes\__init__.py", line 383, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'Add' not found

please help how I can fix this code, and call for example ADD function.

Thank you

Answer 1

Since it is C++ you are compiling, the exported symbol name will be mangled .

You can confirm this by looking at your DLL's exports list, using a tool like DLL export viewer .

It's best to provide a plain C export from DLLs when you intend to call them via an FFI . You can do this using extern "C" to write a wrapper around your C++ methods.

See also:

• Developing C wrapper API for Object-Oriented C++ code