C++ dll export undefined

I am trying to create a C++ DLL that I can import in c#, but my code won't compile.

Here is the code:

//main.cpp
#include "C:\Users\Mihai\Desktop\body.cpp"
#include "C:\Users\Mihai\Desktop\header.h"

extern "C"_declspec(dllexport) int sumTwo(int var_x, int var_y)
{
    myClass MC(var_x, var_y);
    return MC.sumX_Y();

}

//body.cpp
#pragma once
#include "Header.h"

myClass::myClass(int var_x, int var_y)
{
    x = var_x;
    y = var_y;
}

int myClass::sumX_Y()
{
    return x + y;
}

//Header.h
#pragma once

class myClass
{
public:
    myClass(int var_x, int var_y);
    int sumX_Y();
private:
    int x;
    int y;

};

/*

Here are the errors:

Severity    Code    Description Project File    Line    Suppression State
Error (active)  E0130   expected a '{'  SumadllptCsharp e:\work\Info\SumadllptCsharp\SumadllptCsharp\main.cpp   4  
Error (active)  E0040   expected an identifier  SumadllptCsharp e:\work\Info\SumadllptCsharp\SumadllptCsharp\main.cpp   4  
Error (active)  E0020   identifier "dllexport" is undefined SumadllptCsharp e:\work\Info\SumadllptCsharp\SumadllptCsharp\main.cpp   4  
Error (active)  E0020   identifier "var_x" is undefined SumadllptCsharp e:\work\Info\SumadllptCsharp\SumadllptCsharp\main.cpp   6  
Error (active)  E0020   identifier "var_y" is undefined SumadllptCsharp e:\work\Info\SumadllptCsharp\SumadllptCsharp\main.cpp   6  
Error   C3690   expected a string literal, but found a user-defined string literal instead  SumadllptCsharp e:\work\info\sumadllptcsharp\sumadllptcsharp\main.cpp   4  
Error   C2144   syntax error: 'int' should be preceded by ';'   SumadllptCsharp e:\work\info\sumadllptcsharp\sumadllptcsharp\main.cpp   4  
Error   C4430   missing type specifier - int assumed. Note: C++ does not support default-int    SumadllptCsharp e:\work\info\sumadllptcsharp\sumadllptcsharp\main.cpp   4

Solution

Here's your updated code:

DummyClass.h

#pragma once


class DummyClass {
  public:
    DummyClass(int var_x, int var_y);
    int sumX_Y();

  private:
    int x;
    int y;
};

DummyClass.cpp

#include "DummyClass.h"


DummyClass::DummyClass(int var_x, int var_y)
{
    x = var_x;
    y = var_y;
}


int DummyClass::sumX_Y()
{
    return x + y;
}

header.h:

#pragma once

#if defined(_WIN32)
#  if defined(Q047496315_STATIC)
#    define Q047496315_EXPORT_API
#  else
#    if defined(Q047496315_EXPORTS)
#      define Q047496315_EXPORT_API __declspec(dllexport)
#    else
#      define Q047496315_EXPORT_API __declspec(dllimport)
#    endif
#  endif
#else
#  define Q047496315_EXPORT_API
#endif

#if defined(__cplusplus)
extern "C" {
#endif

Q047496315_EXPORT_API int sumTwo(int var_x, int var_y);

#if defined(__cplusplus)
}
#endif

main.cpp

#define Q047496315_EXPORTS
#include "header.h"
#include "DummyClass.h"


int sumTwo(int var_x, int var_y)
{
    DummyClass dc(var_x, var_y);
    return dc.sumX_Y();
}

Notes:

There were 2 major problems with your code:
- The class definition wasn't complete: missing }; at the end of header.h (don't know whether this isn't a copy/paste issue)
- The extern "C"_declspec(dllexport), as I specified in my comment
Other than that, I did some other non critical corrections. Please take a look at the differences between your code and mine
- Rename the class: personally I hate stuff names starting with My, it tells me that their purpose is not clear in creators' mind
- Reorganize the files:
  - The class is in its own 2 files now
  - split main.cpp in 2 (and use header.h as one of them) in order for the project to be usable from C / C++ (although you said you only need it from C#)

Output (build from VStudio 2015 Community):

1>------ Build started: Project: q47496315, Configuration: Debug Win32 ------
1>  main.cpp
1>  DummyClass.cpp
1>  Generating Code...
1>  q47496315.vcxproj -> C:\Work\Dev\StackOverflow\q47496315\Win32-Debug\q47496315.dll
1>  q47496315.vcxproj -> C:\Work\Dev\StackOverflow\q47496315\Win32-Debug\q47496315.pdb (Full PDB)
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========

Here's also a (partial) image of the .dll loaded in Dependency Walker, showing that the function is being exported:

References (that might be useful):

Noroc!