Tips for using Dynamic Link Libraries (DLLs) with MFC

This documents intends to give a beginner some knowledge that can be helpfull when designing a project to the Windows plataform. When I start a new project I try to let all the code that seams to be the most changeable in a DLL. This give me the power to change the project having to compile and distribute the minimum piece of code.

But to do this work properly there are some tips that must be followed. Many of you might be asking asking Why not simply use COM ? The answer is that, of course, COM is a great choice in certain situations. However, DLLs are still a very viable alternative as well. Therefore, in this article. I hope to illustrate just when you should use DLLs and exactly how to use them within the framework of an MFC application.

A big problem of DLLs (specially those that use MFC) are the debug and release version. These versions are incompatible. You probably had the problem of running the debug application version with the DLL release version. The whole world gets crazy!! The best way, in fact the way Microsoft does, is to give diferent names to the DLLs. So the release DLL stays with the Visual C++ project name and the debug version would look like [Project Name]D.DLL. Using this approach you can send the two DLLs to the system directory and be happy. Those are the steps needed to achieve this (suposing the project name is AAA):

  1. Copy the AAA.def to AAAD.def and change all the AAA to AAAD;
  2. In the Project/Settings, select Win32 Debug
  3. Under the tab Link change the Output file name to AAAD.DLL;
  4. Below in this property page you can see something like:
    /def:".\AAA.def" /out:"Debug/AAAD.DLL"
    Change to:
    /def:".\AAAD.def" /out:"Debug/AAAD.DLL"

Now the the debug version will create AAAD.lib and AAAD.DLL files. When I create a DLL I create an include header to it (I think everybody does), which I named DLL header. This header has all the exported classes definitions. And to be more efficient I include the linking stuff in it, so to use the DLL you doesnt have to add the lib file to the Project Settings. My header looks like:

#ifndef DEF_MUDASDASDASDASDAS
#define DEF_MUDASDASDASDASDAS

#ifdef _DEBUG
#pragma comment(lib, AAAD.lib)
#else
#pragma comment(lib, AAA.lib)
#endif

//... the classes definitions goes here

#endif //MUDASDASDASDASDAS

Programming for Changes

The prefered kind of DLL used to export classes are the MFC extension DLLs. By using this you can easily instanciate a classe that is within a DLL. To do this you just declare the class like this:

class AFX_EXT_CLASS CFoo
{
 //...
}

In the application that uses this class you just include the DLL header and everything is cool. The problem is: everytime you need to include a member variable or a method to an exported class you have to change the DLL header, which means recompile all those who use the DLL. To new methods I dont know a way to overcome this recompilation, but for new variables theres a way.

Instead of declaring the member variables directly to the class body, you create a kind of implementation class, like the sample code:

class CFooImpl;
class CFoo
{
protected:
 CFooImpl* m_pThis;
};

So the CFooImpl class doesnt need to be declare to those how use this DLL. The implementation of CFoo would look like:

class CFooImpl
{
public:
 CString m_sName;
};

CFoo::CFoo()
{
 m_pThis = new CFooImpl;

 m_pThis->m_sName = _T("Unknown");
}

CFoo::~CFoo()
{
 delete m_pThis;
}

Another method to be prepared for changes is to use inteligents structs the way the Windows API does. So you declare a method that has an LPVOID as in and out parameter. Those pointers are address of structs instances. The trick is to define as the first struct member a DWORD regardings its size. This way you know which data is expected.

 
typedef struct tagCHANGEABLE
{
 DWORD dwSize;
 long lBytes;
}CHANGEABLE, *LPCHANGEABLE;

BOOL CFoo::Method(LPVOID lpIn)
{
 LPCHANGEABLE lpChangeable = (LPCHANGEABLE)lpIn;

 if (lpChangeable->dwSize == sizeof(CHANGEABLE))
 {
  //...
  return TRUE;
 }

 return FALSE;
}

Using it:

CFoo myFoo;

CHANGEABLE changeable;
memset(&changeable, 0, sizeof(changeable));

changeable.dwSize = sizeof(changeable);

myFoo.Method(&changeable);

DLL Loaded When Needed

Sometimes you have uncommon situations that you need to call a dialog or create a class instance. So you decide to put those in a DLL, but you dont want it to be loaded when the application gets executed. You want to load the DLL when needed (COM). This kind of DLL I call Dynamic DLL (stupid name I know Dynamic Dynamic link libraries). So you declare the exported function as:

__declspec( DLLexport )
void MyExportedFunc(DWORD dw)
{
 //...
}

We need to include this function in the defs files (debug and release). The debug def file would look like this:

; AAAD.def : Declares the module parameters for the DLL.

LIBRARY      "AAAD"

DESCRIPTION  'AAAD Windows Dynamic Link Library'

EXPORTS
 MyExportedFunc @1
 ; Explicit exports can go here

Now to use this function we need to load the library, find the function entry point and call it.

typedef void (*MYFUNC)(DWORD);  

#ifdef _DEBUG
 HINSTANCE hDLL = AfxLoadLibrary("AAADLLD");
#else
 HINSTANCE hDLL = AfxLoadLibrary("AAADLL");
#endif

if (hDLL)
{
 FARPROC pnProc = GetProcAddress(hDLL, "MyExportedFunc");
 MYFUNC pnMyfunc = (MYFUNC)pnProc;

 pnMyfunc(0);

 FreeLibrary(hDLL);
}

Remember that to use show a dialog you must take care of the resource stuffs (AfxSetResource..). You can use this approach to create class instances. The class definition must use pure virtual functions (to avoid unresolved external symbol). It is just like COM.

The class definition should look like this:

class CFoo
{
public:
 virtual void Initialize (CString sName) = 0;
};

You implement this "interface" with another class that is not visible through the DLL header file.

class CFooImp  : public CFoo
{
public:
 CFooImp();
 virtual ~CFooImp();

 void Initialize (CString sName)
 {
  m_sName  = sName;
 }

protected:
 CString m_sName;
};

To create an instance of this class (interface) you create an exported function.

__declspec(DLLexport)

CFoo* CreateFoo(DWORD dwVersion)
{
 if (dwVersion == CURRENT_VERSION)
  return new CFooImp;

 return NULL;
}

The application creates the class instance like this:

typedef CFoo* (*MYFUNC)(DWORD);  

#ifdef _DEBUG
 HINSTANCE hDLL = AfxLoadLibrary("AAADLLD");
#else
 HINSTANCE hDLL = AfxLoadLibrary("AAADLL");
#endif

if (hDLL)
{
 FARPROC pnProc = GetProcAddress(hDLL, " CreateFoo");
 MYFUNC pnMyfunc = (MYFUNC)pnProc;

 CFoo* pFoo = pnMyfunc(0);

 pFoo->Initialize(_T("Hi"));

 delete pFoo;

 FreeLibrary(hDLL);
}

Remember that you cannot free the library until you deleted the CFoo instance.

Conclusion

These examples explained the powers of well designed DLLs. But if the whole project has a bad design no miracle will make your applications easy to change and update. The good design is the first and most important step to the successfull project.



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