Why am I getting "unresolved external symbol"?

Question

C++ / C#

My DLL class needs to have static variables. However, when I try to use them, I get the LNK2001 error.

Here is part of my code:

.h

extern "C" GRAPHICLIBRARY_API int SizeX;

extern "C" GRAPHICLIBRARY_API int SizeY;

extern "C" GRAPHICLIBRARY_API vector<string> Screen;

.cpp

static class ScreenClass {
public:
    __declspec(dllexport) static int SizeX;
public:
    __declspec(dllexport) static int SizeY;

public:
    __declspec(dllexport) static vector<string> Screen;

public:
    __declspec(dllexport) static bool InitializeScreen(int sizeX,int sizeY) {

//Ошибка ссылается на строки ниже:

        ScreenClass::SizeX = sizeX; 
        ScreenClass::SizeY = sizeY; 

        ScreenClass::Screen.clear(); 
//.......
       }
}

Thank you in advance.

Answer 1

extern "C" GRAPHICLIBRARY_API int SizeX;
extern "C" GRAPHICLIBRARY_API int SizeY;
//Error refers to the lines below:
ScreenClass::SizeX = sizeX;
ScreenClass::SizeY = sizeY;

Do you see the register difference?