PRB: Using Extension DLL, Database/OLE/Sockets in Regular DLL

• The Microsoft Foundation Classes (MFC) included with: - Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2,

       4.2b, 5.0
  

SYMPTOMS

When using an Extension DLL from a Regular DLL, you may run into one or more of a set of related problems. Note that because the MFC Database, OLE, and Sockets support DLLs are implemented as Extension DLLs, you might see similar problems if you are using these MFC features, even if you're not explicitly using any of your own Extension DLLs. Some symptoms are:

• When attempting to de-serialize an object of a type of class defined in the Extension DLL, the message appears in the TRACE debug window and the object fails to serialize:

        Warning: Cannot load CYourClass from archive. Class not defined.
  

  An exception indicating 'bad class' may be thrown.

• Resources stored in the 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 Extension DLL.
• AfxDoForAllClasses doesn't do for any classes in the 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 DLL is properly using the MFC Database classes.

CAUSE

The Extension DLL is not wired into the CDynLinkLibrary object chain of the Regular DLL.

RESOLUTION

Create and export an initialization function in the Extension DLL that creates a CDynLinkLibrary object. Call this initialization function exactly once from each Regular DLL that uses the Extension DLL. For an example of this, see the Sample Code section below.

If you are using any MFC OLE, MFC Database (or DAO), or MFC Sockets support in your Regular DLL, you need to call a predefined initialization function for each MFCO4xD.DLL, MFCD4xD.DLL, and MFCN4xD.DLL you are using. These are the DLLs that are linked to automatically when using MFC OLE, MFC Database, or MFC Sockets support, respectively. For example, if you are using MFC OLE in an application or DLL, that app or DLL uses the MFCO4xD.DLL. See the References section below for more information.

For database support, add a call to AfxDbInitModule() to your Regular DLL's CWinApp::InitInstance function. Make sure this call occurs before any base- class call or any added code that accesses the MFCD4xD.DLL. This function takes no parameters and returns void.

For OLE support, add a call to AfxOleInitModule() to your Regular DLL's CWinApp::InitInstance. Note that the COleControlModule InitInstance() function calls AfxOleInitModule() already, so if you are building an OLE control and are using COleControlModule, you should not add this call to AfxOleInitModule().

For Sockets support, add a call to AfxNetInitModule() to your Regular DLL's CWinApp::InitInstance.

Note that release builds of MFC DLLs and applications do not use separate DLLs for database, sockets, or OLE support. It is safe to call these initialization functions in release mode, however.

STATUS

This behavior is by design.

MORE INFORMATION

During each of the operations mentioned in the Symptoms section above, MFC needs to search for a desired value or object. For example, during de- serialization, MFC needs to search through all of the currently available Runtime Classes to match objects in the archive with their proper Runtime Class.

As a part of these searches, MFC scans through all of the Extension DLLs in use by walking a chain of CDynLinkLibrary objects. CDynLinkLibrary objects attach automatically to a chain during their constructor and are created by each Extension DLL in turn during initialization. In addition, every module (EXE or Regular DLL) has its own chain of CDynLinkLibrary objects. In other words, the chain of Extension DLLs used by an Application is different from the chain of Extension DLLs used by one Regular DLL which is different from the chain of Extension DLLs used by another Regular DLL.

This last point is one of the most common causes of the aforementioned problems mentioned in the Symptoms section. In order for an Extension DLL to get wired into a CDynLinkLibrary chain, it must create a CDynLinkLibrary object in the context of every module that will be using the Extension DLL. For example, if an Extension DLL is going to be used from Regular DLLs, it must provide an exported initialization function that creates a CDynLinkLibrary object. Every Regular DLL that uses the Extension DLL must call the exported initialization function.

If an Extension DLL is only going to be used from an MFC application (EXE) and never from a Regular DLL, then it is sufficient to create the CDynLinkLibrary object in the Extension DLL's DllMain. This is what the AppWizard Extension DLL code does. When loading an 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. This prevents any need to call the initialization function from an application. Nevertheless, it is safer and more recommended that all applications use the initialization function and that the Extension DLL not create a CDynLinkLibrary object in DllMain. Note that it is not a good idea to have multiple CDynLinkLibrary objects from one Extension DLL in any one chain, especially if the Extension DLL will be dynamically unloaded from memory. Don't call the initialization function more than once from any one module.

The implementations of the AfxDbInitModule, AfxOleInitModule, and AfxNetInitModule functions can be found in the DLLDB.CPP, DLLOLE.CPP, and DLLNET.CPP files in the \MsDev\Mfc\Src directory, respectively.

Sample Code

This sample code assumes that the Regular DLL is implicitly linking to the Extension DLL. This is accomplished by linking to the Import Library (.LIB) of the Extension DLL when building the Regular DLL.

These lines should be in the source of the Extension DLL:

   /////////////////////
   // YourExtDLL.cpp:

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

   static AFX_EXTENSION_MODULE NEAR extensionDLL = { NULL, NULL };

   extern "C" int APIENTRY
   DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
   {
       if (dwReason == DLL_PROCESS_ATTACH)
       {
           // 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 DLL.
   extern "C" void WINAPI InitYourExtDLL()
   {
       // Create a new CDynLinkLibrary for this app.
       new CDynLinkLibrary(extensionDLL);

       // Add other initialization here.
   }

   /////////////////////
   // YourExtDLL.Def:

   LIBRARY      YOUREXTDLL

   CODE         PRELOAD MOVEABLE DISCARDABLE
   DATA         PRELOAD SINGLE

   EXPORTS
       InitYourExtDLL

   /////////////////////////
   // 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 DLL initializing\n");

       // Wire any extension DLLs into CDynLinkLibrary chain.
       InitYourExtDLL();

       return TRUE;
   }

REFERENCES

For information on Regular DLLs, see MFC Technical Note 11.

For information on Extension DLLs, see MFC Technical Note 33.

For an example of an Extension DLL with an exported initialization function, see the DLLHUSK Sample on the Visual C++ CD-ROM in

        Samples
            MFC Samples
                Advanced MFC Samples

    DLLINIT.CPP     Extension DLL code
    DLLMODUL.CPP    Regular DLL code
    DLLDB.CPP       MFC Database Debug DLL (MFCD) code
    DLLOLE.CPP      MFC OLE Debug DLL (MFCO) code
    DLLNET.CPP      MFC Sockets Debug DLL (MFCN) code

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Keywords          : MfcDAO MfcDatabase MfcDLL MfcOLE kbcode kbprg
Technology        : kbMfc
Version           : 4.0 4.1 4.2 4.2b 5.0
Platform          : NT WINDOWS