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Using Database, OLE, and Sockets MFC extension DLLs in regular MFC DLLs

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When using an MFC extension DLL from a regular MFC DLL, if the MFC extension DLL isn't wired into the CDynLinkLibrary object chain of the regular MFC DLL, you might run into one or more related problems. Because the debug versions of the MFC Database, OLE, and Sockets support DLLs are implemented as MFC extension DLLs, you might see similar problems if you're using these MFC features, even if you're not explicitly using any of your own MFC extension DLLs. Some symptoms are:

  • When attempting to deserialize an object of a type of class defined in the MFC extension DLL, the message "Warning: Cannot load CYourClass from archive. Class not defined." appears in the TRACE debug window and the object fails to serialize.

  • An exception indicating bad class might be thrown.

  • Resources stored in the MFC extension DLL fail to load because AfxFindResourceHandle returns NULL or an incorrect resource handle.

  • DllGetClassObject, DllCanUnloadNow, and the UpdateRegistry, Revoke, RevokeAll, and RegisterAll member functions of COleObjectFactory fail to locate a class factory defined in the MFC extension DLL.

  • AfxDoForAllClasses doesn't work for any classes in the MFC extension DLL.

  • Standard MFC database, sockets, or OLE resources fail to load. For example, AfxLoadString(AFX_IDP_SQL_CONNECT_FAIL) returns an empty string, even when the regular MFC DLL is properly using the MFC Database classes.

The solution to these problems is to create and export an initialization function in the MFC extension DLL that creates a CDynLinkLibrary object. Call this initialization function exactly once from each regular MFC DLL that uses the MFC extension DLL.

MFC OLE, MFC Database (or DAO), or MFC Sockets Support

If you're using any MFC OLE, MFC Database (or DAO), or MFC Sockets support in your regular MFC DLL, respectively, the MFC debug MFC extension DLLs MFCOxxD.dll, MFCDxxD.dll, and MFCNxxD.dll (where xx is the version number) are linked automatically. Call a predefined initialization function for each of the DLLs that you're using:

  • For database support, add a call to AfxDbInitModule to your regular MFC DLL in its CWinApp::InitInstance function. Make sure this call occurs before any base-class call or any added code that accesses the MFCDxxD.dll. This function takes no parameters and returns void.

  • For OLE support, add a call to AfxOleInitModule to your regular MFC DLL it its CWinApp::InitInstance function. The COleControlModule::InitInstance function calls AfxOleInitModule already, so if you're building an OLE control and use COleControlModule, you shouldn't add this call to AfxOleInitModule.

  • For Sockets support, add a call to AfxNetInitModule to your regular MFC DLL in CWinApp::InitInstance.

Release builds of MFC DLLs and applications don't use separate DLLs for database, sockets, or OLE support. However, it's safe to call these initialization functions in release mode.

CDynLinkLibrary Objects

During each operation mentioned at the beginning of this article, MFC needs to search for a particular value or object. For example, during deserialization, MFC needs to search through all the currently available run-time classes to match objects in the archive with their proper run-time class.

As a part of these searches, MFC scans through all the MFC extension DLLs in use by walking a chain of CDynLinkLibrary objects. CDynLinkLibrary objects attach automatically to a chain during their construction and are created by each MFC extension DLL in turn during initialization. Every module (application or regular MFC DLL) has its own chain of CDynLinkLibrary objects.

For an MFC extension DLL to get wired into a CDynLinkLibrary chain, it must create a CDynLinkLibrary object in the context of every module that uses the MFC extension DLL. To use an MFC extension DLL in regular MFC DLLs, the extension DLL must provide an exported initialization function that creates a CDynLinkLibrary object. Every regular MFC DLL that uses the MFC extension DLL must call the exported initialization function.

If you'll only use an MFC extension DLL from an MFC application, and never from a regular MFC DLL, then it's sufficient to create the CDynLinkLibrary object in the MFC extension DLL DllMain function. It's what the MFC DLL Wizard MFC extension DLL code does. When loading an MFC extension DLL implicitly, DllMain loads and executes before the application ever starts. Any CDynLinkLibrary creations are wired into a default chain that the MFC DLL reserves for an MFC application.

It's a bad idea to have multiple CDynLinkLibrary objects from one MFC extension DLL in any one chain. It's especially true if the MFC extension DLL may be dynamically unloaded from memory. Don't call the initialization function more than once from any one module.

Sample Code

This sample code assumes that the regular MFC DLL implicitly links to the MFC extension DLL. To link implicitly, link to the import library (LIB file) of the MFC extension DLL when you build the regular MFC DLL.

The following lines should be in the source of the MFC extension DLL:

// YourExtDLL.cpp:

// standard MFC extension DLL routines
#include "afxdllx.h"

static AFX_EXTENSION_MODULE extensionDLL;

extern "C" int APIENTRY
DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
    if (dwReason == DLL_PROCESS_ATTACH)
    {
        // MFC extension DLL one-time initialization
        if (!AfxInitExtensionModule(extensionDLL, hInstance))
           return 0;
    }
    return 1;   // ok
}

// Exported DLL initialization is run in context of
// application or regular MFC DLL
extern "C" void WINAPI InitYourExtDLL()
{
    // create a new CDynLinkLibrary for this app
    new CDynLinkLibrary(extensionDLL);

    // add other initialization here
}

Be sure to export the InitYourExtDLL function. You can use __declspec(dllexport), or export it in the DEF file for your DLL, as shown here:

// YourExtDLL.Def:
LIBRARY      YOUREXTDLL
CODE         PRELOAD MOVEABLE DISCARDABLE
DATA         PRELOAD SINGLE
EXPORTS
    InitYourExtDLL

Add a call to the InitInstance member of the CWinApp-derived object in each regular MFC DLL using the MFC extension DLL:

// YourRegularDLL.cpp:

class CYourRegularDLL : public CWinApp
{
public:
    virtual BOOL InitInstance(); // Initialization
    virtual int ExitInstance();  // Termination

    // nothing special for the constructor
    CYourRegularDLL(LPCTSTR pszAppName) : CWinApp(pszAppName) { }
};

BOOL CYourRegularDLL::InitInstance()
{
    // any DLL initialization goes here
    TRACE0("YOUR regular MFC DLL initializing\n");

    // wire any MFC extension DLLs into CDynLinkLibrary chain
    InitYourExtDLL();

    return TRUE;
}

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See also

MFC extension DLLs