PRB: A _USRDLL with No CWinApp Object Fails to Load

WINDOWS   | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

  - Microsoft Visual C++ for Windows, versions 1.0, 1.5,

       1.51 and 1.52
      - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, and 4.0
  

SYMPTOMS

If you build an MFC _USRDLL using Visual C++, version 1.0, without defining a CWinApp object, when its LibMain() function is called you will get the following message in the debugger Output Window:

   <unknown application>: File appcore.cpp, Line 602, Assertion Failed!
   AfxAbort called

   <unknown application>: File appcore.cpp, Line 559, Assertion Failed!

   <unknown application>: File afxwin1.inl, Line 25, Assertion Failed!
   AfxAbort called

In order to support resource-only DLLs, the assertions described above were removed in Microsoft Visual C++ 32-bit Edition, versions 2.0, 2.1, 2.2, and 4.0. Using those versions, calling a CWinApp member function through the pointer returned by AfxGetApp() will result in an access violation if there is no CWinApp object defined in the DLL.

CAUSE

When an attempt is made to load a DLL for the first time, the DLL’s entry point (LibMain() for Windows, DllMain() for Windows NT) is called to initialize its instance. For a _USRDLL, the entry point calls AfxWinInit(), which calls AfxGetApp(). If no CWinApp object has been instantiated in the DLL, AfxGetApp() returns "NULL". AfxGetApp() cannot access the calling application's CWinApp (if the calling application is an MFC application). This causes the assertion.

This behavior is by design, because most of the classes provided by MFC rely on the instantiation of a CWinApp object. Even some simple objects such as an AfxMessageBox make calls to AfxGetApp(). If no CWinApp object exists [and your DLL uses any GUI objects that rely on calls to AfxWinInit() and AfxWinTerm()], assertions and program failures will occur.

RESOLUTION

Be sure to instantiate an object of type CWinApp or a class derived from CWinApp. If it is important that you not have a CWinApp object in your _USRDLL, see the "MORE INFORMATION" section of this article, below, for suggestions.

NOTE: Visual C++ 4.0 automatically generates the code containing the declaration, definition, and instantiation of the CWinApp derived class for the DLL, so the following sections do not apply.

MORE INFORMATION

It is not necessary for your DLL to call AfxWinInit() and AfxWinTerm() for it to use the MFC collection classes and the simple class types such as CString. If you wish to use only these MFC classes in your DLL, you can omit the instantiation of a CWinApp object, but you also must write your own LibMain() or DllMain() function for the DLL.

For an example of the code that you may want to include in your entry point, you can look at what MFC normally does in AfxWinInit().

NOTE: SetCurrentHandles() is called, so afxCurrentInstanceHandle and afxCurrentResourceHandle get set equal to m_hInstance. You may need to set these values yourself.

Sample Code for Windows

The code below shows one possible implementation of LibMain(). For the sake of pointing out the initialization portion, LibMain() calls MyInit(), which consists of a very simplified simulation of AfxWinInit().

   // Compile options needed: /D "_USRDLL"

   #include <afxwin.h>

   BOOL AFXAPI MyInit(HINSTANCE);

   extern "C" int FAR PASCAL LibMain(HINSTANCE hInstance,
      WORD wDataSegment, WORD wHeapSize, LPSTR lpszCmdLine)
   {
      // Initialize DLL's instance(/module) not the app's
      if (!MyInit(hInstance))
      {
         return 0;    // Init Failed
      }

   // nothing to run
      return 1;   // ok
   }

   BOOL AFXAPI MyInit(HINSTANCE hInstance)
   {
      afxCurrentInstanceHandle = hInstance;
      afxCurrentResourceHandle = hInstance;

   // one instance initialization stuff goes here

   // Handle critical errors
      ::SetErrorMode(SEM_FAILCRITICALERRORS);

      return TRUE;
   }

Sample Code for Windows NT

The code below shows one possible implementation of DllMain(). For the sake of pointing out the initialization portion, DllMain() calls MyInit(), which consists of a very simplified simulation of AfxWinInit().

   // Compile options needed: /D "_USRDLL"

   #include <afxwin.h>

   BOOL AFXAPI MyInit(HINSTANCE);

   extern "C" int APIENTRY
   DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
   {
           if (dwReason == DLL_PROCESS_ATTACH)
           {
                   // Initialize DLL's instance(/module) not the app's
                   if (!MyInit(hInstance))
                   {
                           return 0;       // Init Failed
                   }

           }
           return 1;   // ok
   }

   BOOL AFXAPI MyInit(HINSTANCE hInstance)
   {
           afxCurrentInstanceHandle = hInstance;
           afxCurrentResourceHandle = hInstance;

           // one instance initialization stuff goes here

           // Handle critical errors
           ::SetErrorMode(SEM_FAILCRITICALERRORS);

           return TRUE;
   }