LNK2001 unresolved external when importing functions from MFC DLL - visual-c++

Question

I have created an MFC DLL and have exported the functions for example in the file SerialPort.h:
class CSerialPortApp : public CWinApp
{
public:
CSerialPortApp();
__declspec(dllexport) int SWrite(unsigned char* toSend, int len);
};
and in my MFC application I want to call the function in SerialInterface.h I have included the "SerialPort.h" from the DLL and called:
__declspec(dllimport) int SWrite(unsigned char* toSend, int len);
class SerialInterface
{
public:
};
for example.
I have added the SerialPort.lib file to my linker includes but when I try to compile I get
error LNK2001: unresolved external symbol "__declspec(dllimport) int __cdecl SWrite(unsigned char*, int)" (__imp_?SWrite##YAHPAEH#Z)
I am stuck as to the cause of this, I have tried rebuilding everything but nothing seems to help?
Thank you for any help!

Answer 1

You are using __declspec(dllexport) inside a class?
You either export global functions from the dll or whole class which may contain any functions. You don't have to export selected member functions from a class, I don't even know how that works.
It is a little strange that you are not properly exporting the SerialPort class from dll (as per your code) yet you can use it in your application and call its member function!? I am a little confused.

Answer 2

Well I found an alternative that works, I believe I was implementing it incorrectly.
I added a new class to my DLL that was not a CWinApp class:
class SerialPort
{
public:
__declspec(dllexport) SerialPort(void);
__declspec(dllexport) virtual ~SerialPort(void);
__declspec(dllexport) int SWrite(unsigned char* toSend, int len);
};
then included the header for this in my application and the lib and dll etc.
I then placed the included header file in the main CDialog header but importantly didn't need to import any of the functions:
#include "SerialPort.h"
class CPPUDlg : public CDialog
{
public:
CPPUDlg(CWnd* pParent = NULL); // standard constructor
SerialPort objSerialPort;
and then in my code I simply call
objSerialPort.SWrite(toSend, len);
I have not used dllimport to import the functions which I assumed I would need to but it now works!
Hope this helps anyone who may have a similar problem.