近來在論壇上很有多帖子問到如何使用DLL中的資源(包括對話框，圖標等)的問題，現在筆者就來就此問題談談，包含在DLL內部使用資源，DLL中使用其它DLL中的資源和在應用程序中使用資源。
我們先以圖標為例說起(其它的資源與此圖標的加載原理大致相同)，我們要加載圖標，一般是調用AfxGetApp()->LoadIcon(…)；下面是CWinApp::LoadIcon的實現(afxwin2.inl)：
_AFXWIN_INLINE HICON CWinApp::LoadIcon(LPCTSTR lpszResourceName) const
{ return ::LoadIcon(AfxFindResourceHandle(lpszResourceName,
RT_GROUP_ICON), lpszResourceName); }
_AFXWIN_INLINE HICON CWinApp::LoadIcon(UINT nIDResource) const
{ return ::LoadIcon(AfxFindResourceHandle(MAKEINTRESOURCE(nIDResource),
RT_GROUP_ICON), MAKEINTRESOURCE(nIDResource)); }
可以看到CWinApp::LoadIcon實際上調用了API .LoadIcon,下面是API LoadIcon的原型：
HICON LoadIcon( HINSTANCE hInstance,
LPCTSTR lpIconName
);
hInstance
[in] Handle to an instance of the module whose executable file contains the icon to be loaded. This parameter must be NULL when a standard icon is being loaded.
hInstance是我們要加載ICON的模塊實例，這個實例從何來，當然我們可以直接傳入DLL的實例，對於通過LoadLibrary動態加載的DLL我們可以很容易的得到其句柄，但對於我們直接鏈接的DLL得到其句柄則要費一番周折。可以看到CWinApp::LoadIcon是通過AfxFindResouceHandle找到此句柄的。下面是AfxFindResourceHandle的定義(afxwin.h)：
#ifndef _AFXDLL
#define AfxFindResourceHandle(lpszResource, lpszType) AfxGetResourceHandle()
#else
HINSTANCE AFXAPI AfxFindResourceHandle(LPCTSTR lpszName, LPCTSTR lpszType);
#endif
我們先討論在靜態鏈接庫中使用MFC DLL的情況。可以看到，我們如果在靜態庫中使用MFC DLL的話(#ifndef _AFXDLL),實際上就是調用的AfxGetResourceHandle，MSDN中的說明是
AfxGetResourceHandle
This function accesses the application’s resources directly by using the HINSTANCE handle returned, for example, in calls to the Windows function FindResource.
HINSTANCE AfxGetResourceHandle( );
Return Value
An HINSTANCE handle where the default resources of the application are loaded.
函數返回的應用程序加載的缺省資源的HINSTANCE句柄,HINSTANCE相當於HMODULE，也就是資源所在的模塊句柄。顯然在此，我們使用的是DLL中的資源，那么我們就應該返回此DLL中的HINSTANCE了，如果讓AfxGetResourceHandle返回DLL的HINSTANCE呢？答案是通過AfxSetResourceHandle設置。MSDN中AfxSetResouceHandle的說明如下：
AfxSetResourceHandle
This function sets the HINSTANCE handle that determines where the default resources of the application are loaded.
void AfxSetResourceHandle(
HINSTANCE hInstResource );
Parameters
hInstResource
Specifies the instance or module handle to an .EXE or DLL file from which the application’s resources are loaded.
我們只需將DLL的HINSTANCE傳入AfxSetResouceHanle就行了。如何得到DLL的HINSTANCE呢，我們可以通過DLL聲明一個接口HINSTANCE GetInstance獲得，也可以通過EnumProcessMoudules找到（詳細的過程見MSDN中EnumProcessModules的說明和示例）。
我們使用完DLL中的資源要使用EXE中的資源的資源怎么辦呢？我們需要在使用完成后用AfxSetResource重新將資源模塊的句柄設置為原來的值，如果來保證在資源使用完成后完成這一個工作呢，即使在使用過程中發生異常了，為此我們利C++類的構造和析構機制創建了這一類：
class CLocalResource
{
public:
CLocalResource(HINSTANCE hInstance)
{
m_hInstOld=AfxGetInstanceHandle();
AfxSetInstanceHandle(hInstance);
}
virtual ~CLocalResource()
{
AfxSetInstanceHandle(m_hInstOld);
}
protected:
HINSTANCE m_hInstOld;
};
我們只需在使用DLL的資源之前構造一個CLocalInstance就行了。
void CXXXX::LoadResouceFromDLL(HINSTANCE hInst,UINT nResID,…)
{
CLocalResouce localRes(hInst);
…
}
下面來討論在動態庫中使用MFC DLL的情況(也就是定義了_AFXDLL的情況)
來看看AfxGetInstanceHandle ,AfxGetResourceHandle和AfxSetResouceHandle的實現(afxwin1.inl中)：
_AFXWIN_INLINE HINSTANCE AFXAPI AfxGetInstanceHandle()
{ ASSERT(afxCurrentInstanceHandle != NULL);
return afxCurrentInstanceHandle; }
_AFXWIN_INLINE HINSTANCE AFXAPI AfxGetResourceHandle()
{ ASSERT(afxCurrentResourceHandle != NULL);
return afxCurrentResourceHandle; }
_AFXWIN_INLINE void AFXAPI AfxSetResourceHandle(HINSTANCE hInstResource)
{ ASSERT(hInstResource != NULL); afxCurrentResourceHandle = hInstResource; }
實際上訪問的就是afxCurrentInstanceHandle,afxCurrentResourceHandle。
/////////////////////////////////////////////////////////////////////////////
// Global functions for access to the one and only CWinApp
…
#define afxCurrentInstanceHandle AfxGetModuleState()->m_hCurrentInstanceHandle
#define afxCurrentResourceHandle AfxGetModuleState()->m_hCurrentResourceHandle
….
AFX_MODULE_STATE* AFXAPI AfxGetModuleState()
{
_AFX_THREAD_STATE* pState = _afxThreadState;
AFX_MODULE_STATE* pResult;
if (pState->m_pModuleState != NULL)
{
// thread state's module state serves as override
pResult = pState->m_pModuleState;
}
else
{
// otherwise, use global app state
pResult = _afxBaseModuleState.GetData();
}
ASSERT(pResult != NULL);
return pResult;
}
其中的_AFX_THREAD_STATE在此我們就不討論了，有興趣的讀者可以閱讀(afxstat_.h和afxstate.cpp)。
那AfxGetModuleState()->m_hCurrentResourceHandle又是在哪初始化的呢？
我們可以在appinit.cpp中的BOOL AFXAPI AfxWinInit(HINSTANCE hInstance, HINSTANCE hPrevInstance,LPTSTR lpCmdLine, int nCmdShow)看到
// set resource handles
AFX_MODULE_STATE* pModuleState = AfxGetModuleState();
pModuleState->m_hCurrentInstanceHandle = hInstance;
pModuleState->m_hCurrentResourceHandle = hInstance;
和appinit.cpp中的void CWinApp::SetCurrentHandles()中看到
AFX_MODULE_STATE* pModuleState = _AFX_CMDTARGET_GETSTATE();
pModuleState->m_hCurrentInstanceHandle = m_hInstance;
pModuleState->m_hCurrentResourceHandle = m_hInstance;
CWinApp::SetCurrentHandles()也是由AfxWinInit調用的，那AfxWinInit又由誰調用呢？
我們可以在DllMain(dllinit.cpp和dllmodul.cpp，分別對應於MFC擴展DLL和MFC規則DLL)看到
(dllinit.cpp,MFC擴展DLL)
static AFX_EXTENSION_MODULE coreDLL;
….
extern "C"
BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID)
{
if (dwReason == DLL_PROCESS_ATTACH)
{
…….
// initialize this DLL's extension module
VERIFY(AfxInitExtensionModule(coreDLL, hInstance));
#ifdef _AFX_OLE_IMPL
AfxWinInit(hInstance, NULL, _T(""), 0);
….
#endif
….
// wire up this DLL into the resource chain
CDynLinkLibrary* pDLL = new CDynLinkLibrary(coreDLL, TRUE);
ASSERT(pDLL != NULL);
pDLL->m_factoryList.m_pHead = NULL;
….
}
else if (dwReason == DLL_PROCESS_DETACH)
{
….
// cleanup module state for this process
AfxTermExtensionModule(coreDLL);
….
// cleanup module state in OLE private module state
AfxTermExtensionModule(coreDLL, TRUE);
….
}
…
}
可以看到在提供自動化支持時，將調用AfxWinInit(但MFC的DLL向導，對於擴展DLL卻不允許添加自動化支持)。
(在dllmodul.cpp中，MFC常規DLL)
#ifdef _AFXDLL
static AFX_EXTENSION_MODULE controlDLL;
….
#endif
extern "C"
BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID /*lpReserved*/)
{
if (dwReason == DLL_PROCESS_ATTACH)
{
….
_AFX_THREAD_STATE* pState = AfxGetThreadState();
AFX_MODULE_STATE* pPrevModState = pState->m_pPrevModuleState;
// Initialize DLL's instance(/module) not the app's
if (!AfxWinInit(hInstance, NULL, _T(""), 0))
{
AfxWinTerm();
goto Cleanup; // Init Failed
}
….
#ifdef _AFXDLL
// wire up this DLL into the resource chain
VERIFY(AfxInitExtensionModule(controlDLL, hInstance));
CDynLinkLibrary* pDLL; pDLL = new CDynLinkLibrary(controlDLL);
ASSERT(pDLL != NULL);
#else
AfxInitLocalData(hInstance);
#endif
…
}
else if (dwReason == DLL_PROCESS_DETACH)
{
….
#ifdef _AFXDLL
AfxTermExtensionModule(controlDLL, TRUE);
#else
AfxTermLocalData(hInstance, TRUE);
#endif
}
…
}
看到上面的代碼，其實與MFC擴展DLL的代碼大同小異，只過是MFC常規DLL可以支持在靜態鏈接庫全用MFC DLL，相應的初始化/反初始化函數就變為了AfxInitLocalData/ AfxTermLocalData,在此我們不對在靜態鏈接庫中使用MFC DLL的情況作更多討論。
以及在winmain.cpp中的int AFXAPI AfxWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,LPTSTR lpCmdLine, int nCmdShow),在appmodul.cpp中我們可以看到入口函數的實現：
extern "C" int WINAPI
_tWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPTSTR lpCmdLine, int nCmdShow)
{
// call shared/exported WinMain
return AfxWinMain(hInstance, hPrevInstance, lpCmdLine, nCmdShow);
}
看看AfxInitExtensionModule , AfxTermExtensionModule CDynLinkLibrary的定義(afxdll_.h)和實現(在dllinit.cpp中)就明白了DllMain做了什么：
BOOL AFXAPI AfxInitExtensionModule(AFX_EXTENSION_MODULE& state, HMODULE hModule)
{
// only initialize once
if (state.bInitialized)
{
AfxInitLocalData(hModule);
return TRUE;
}
state.bInitialized = TRUE;
// save the current HMODULE information for resource loading
ASSERT(hModule != NULL);
state.hModule = hModule;
state.hResource = hModule;
// save the start of the runtime class list
AFX_MODULE_STATE* pModuleState = AfxGetModuleState();
state.pFirstSharedClass = pModuleState->m_classList.GetHead();
pModuleState->m_classList.m_pHead = pModuleState->m_pClassInit;
#ifndef _AFX_NO_OLE_SUPPORT
// save the start of the class factory list
state.pFirstSharedFactory = pModuleState->m_factoryList.GetHead();
pModuleState->m_factoryList.m_pHead = pModuleState->m_pFactoryInit;
#endif
return TRUE;
}
…
void AFXAPI AfxTermExtensionModule(AFX_EXTENSION_MODULE& state, BOOL bAll)
{
// make sure initialized
if (!state.bInitialized)
return;
// search for CDynLinkLibrary matching state.hModule and delete it
ASSERT(state.hModule != NULL);
AFX_MODULE_STATE* pModuleState = AfxGetModuleState();
AfxLockGlobals(CRIT_DYNLINKLIST);
for (CDynLinkLibrary* pDLL = pModuleState->m_libraryList; pDLL != NULL; )
{
CDynLinkLibrary* pNextDLL = pDLL->m_pNextDLL;
if (bAll || pDLL->m_hModule == state.hModule)
delete pDLL; // will unwire itself
pDLL = pNextDLL;
}
AfxUnlockGlobals(CRIT_DYNLINKLIST);
// delete any local storage attached to this module
AfxTermLocalData(state.hModule, TRUE);
// remove any entries from the CWnd message map cache
AfxResetMsgCache();
}
class CDynLinkLibrary : public CCmdTarget
{
DECLARE_DYNAMIC(CDynLinkLibrary)
public:
// Constructor
explicit CDynLinkLibrary(AFX_EXTENSION_MODULE& state, BOOL bSystem = FALSE);
CDynLinkLibrary(HINSTANCE hModule, HINSTANCE hResource);
// Attributes
HMODULE m_hModule;
HMODULE m_hResource; // for shared resources
….
BOOL m_bSystem; // TRUE only for MFC DLLs
// Implementation
public:
CDynLinkLibrary* m_pNextDLL; // simple singly linked list
virtual ~CDynLinkLibrary();
….
};
CDynLinkLibrary::CDynLinkLibrary(AFX_EXTENSION_MODULE& state, BOOL bSystem)
{
…
// copy info from AFX_EXTENSION_MODULE struct
ASSERT(state.hModule != NULL);
m_hModule = state.hModule;
m_hResource = state.hResource;
m_classList.m_pHead = state.pFirstSharedClass;
….
m_bSystem = bSystem;
// insert at the head of the list (extensions will go in front of core DLL)
DEBUG_ONLY(m_pNextDLL = NULL);
AfxLockGlobals(CRIT_DYNLINKLIST);
m_pModuleState->m_libraryList.AddHead(this);
AfxUnlockGlobals(CRIT_DYNLINKLIST);
}
CDynLinkLibrary::CDynLinkLibrary(HINSTANCE hModule, HINSTANCE hResource)
{
…
m_hModule = hModule;
m_hResource = hResource;
m_classList.m_pHead = NULL;
…
m_bSystem = FALSE;
// insert at the head of the list (extensions will go in front of core DLL)
DEBUG_ONLY(m_pNextDLL = NULL);
AfxLockGlobals(CRIT_DYNLINKLIST);
m_pModuleState->m_libraryList.AddHead(this);
AfxUnlockGlobals(CRIT_DYNLINKLIST);
}
…
CDynLinkLibrary::~CDynLinkLibrary()
{
// remove this frame window from the list of frame windows
AfxLockGlobals(CRIT_DYNLINKLIST);
m_pModuleState->m_libraryList.Remove(this);
AfxUnlockGlobals(CRIT_DYNLINKLIST);
}
由此我們可以看出DLL初始化時先調AfxInitExtensionModule是為了構造一個AFX_EXTENSION_MODULE,然后將它插入插入一個AFX_MODULE_STATE鏈表中,同時將當前DLL的HINSTANCE插入此AFX_MODULE_STATE的CDynLinkLibrary鏈表中，以便稍后講到的AfxFindResourceHandle找到。DLL從內存移出時就從此鏈表中刪除自己。
讓我們來看看AfxFindResourceHandle的實現(dllinit.cpp)：
HINSTANCE AFXAPI AfxFindResourceHandle(LPCTSTR lpszName, LPCTSTR lpszType)
{
ASSERT(lpszName != NULL);
ASSERT(lpszType != NULL);
HINSTANCE hInst;
// first check the main module state
AFX_MODULE_STATE* pModuleState = AfxGetModuleState();
if (!pModuleState->m_bSystem)
{
hInst = AfxGetResourceHandle();
if (::FindResource(hInst, lpszName, lpszType) != NULL)
return hInst;
}
// check for non-system DLLs in proper order
AfxLockGlobals(CRIT_DYNLINKLIST);
CDynLinkLibrary* pDLL;
for (pDLL = pModuleState->m_libraryList; pDLL != NULL;
pDLL = pDLL->m_pNextDLL)
{
if (!pDLL->m_bSystem && pDLL->m_hResource != NULL &&
::FindResource(pDLL->m_hResource, lpszName, lpszType) != NULL)
{
// found it in a DLL
AfxUnlockGlobals(CRIT_DYNLINKLIST);
return pDLL->m_hResource;
}
}
AfxUnlockGlobals(CRIT_DYNLINKLIST);
// check language specific resource next
hInst = pModuleState->m_appLangDLL;
if (hInst != NULL && ::FindResource(hInst, lpszName, lpszType) != NULL)
return hInst;
// check the main system module state
if (pModuleState->m_bSystem)
{
hInst = AfxGetResourceHandle();
if (::FindResource(hInst, lpszName, lpszType) != NULL)
return hInst;
}
// check for system DLLs in proper order
AfxLockGlobals(CRIT_DYNLINKLIST);
for (pDLL = pModuleState->m_libraryList; pDLL != NULL; pDLL = pDLL->m_pNextDLL)
{
if (pDLL->m_bSystem && pDLL->m_hResource != NULL &&
::FindResource(pDLL->m_hResource, lpszName, lpszType) != NULL)
{
// found it in a DLL
AfxUnlockGlobals(CRIT_DYNLINKLIST);
return pDLL->m_hResource;
}
}
AfxUnlockGlobals(CRIT_DYNLINKLIST);
// if failed to find resource, return application resource
return AfxGetResourceHandle();
}
上面的代碼首先檢查主模塊的狀態,通過返回的是個AFX_MODULE_STATE的類對象指針. AFX_MODULE_STATE又是個什么呢？下面看看AFX_MODULE_STATE的定義(afxstat_.h中,它的實現在afxstate.cpp中，有興趣的讀者可以讀讀)：
// AFX_MODULE_STATE (global data for a module)
class AFX_MODULE_STATE : public CNoTrackObject
{
public:
#ifdef _AFXDLL
AFX_MODULE_STATE(BOOL bDLL, WNDPROC pfnAfxWndProc, DWORD dwVersion,
BOOL bSystem = FALSE);
#else
explicit AFX_MODULE_STATE(BOOL bDLL);
#endif
~AFX_MODULE_STATE();
CWinApp* m_pCurrentWinApp;
HINSTANCE m_hCurrentInstanceHandle;
HINSTANCE m_hCurrentResourceHandle;
LPCTSTR m_lpszCurrentAppName;
BYTE m_bDLL; // TRUE if module is a DLL, FALSE if it is an EXE
BYTE m_bSystem; // TRUE if module is a "system" module, FALSE if not
…
#ifdef _AFXDLL
// CDynLinkLibrary objects (for resource chain)
CTypedSimpleList<CDynLinkLibrary*> m_libraryList;
// special case for MFC71XXX.DLL (localized MFC resources)
HINSTANCE m_appLangDLL;
#endif
….
};
我們來看看我們關心的屬性
CWinApp* m_pCurrentWinApp;
HINSTANCE m_hCurrentInstanceHandle;
HINSTANCE m_hCurrentResourceHandle;
LPCTSTR m_lpszCurrentAppName;
BYTE m_bDLL; // TRUE if module is a DLL, FALSE if it is an EXE
BYTE m_bSystem; // TRUE if module is a "system" module, FALSE if not
…
// CDynLinkLibrary objects (for resource chain)
CTypedSimpleList<CDynLinkLibrary*> m_libraryList;
// special case for MFC71XXX.DLL (localized MFC resources)
HINSTANCE m_appLangDLL;
看了相關注釋，也就明白什么意思了，在此就不多解釋了。
如果當前主模塊是系統模塊(通過m_bSystem標識)，並且從當前模塊中找到了相應的資源就返回此模塊的HINSTANCE。接下來當前模塊所載的DLL鏈表中的所有用戶模塊中查找，本地化的MFC資源DLL中查找，系統DLL中查找。如果都找不到則從EXE(或者通過AfxSetResouceHandle中設置的HINSTANCE)中查找。
如果不考慮DLL鏈表，則需要我們每次使用DLL中的資源前調用AfxSetResouceHandle，使用后再調用AfxSetResourceHandle，就同前面講的在靜態鏈接庫中使用MFC DLL一樣。麻不麻煩？
怎么解決呢？解決方法是用資源DLL的HINSTANCE構造一CDynLinkLibrary對象插入主模塊(EXE)的AFX_MODULE_STATE的m_libraryList中。怎么做？
看了上面DllMain的實現代碼，我們可以照着做。
static AFX_EXTENSION_MODULE ResouceDLL = { NULL, NULL }
BOOL CXApp::InitInstance()
{
// Get the resource DLL instance
if (!AfxInitExtensionModule(ResouceDLL, hResourceModule))
return 0;
new CDynLinkLibrary(ResouceDLL);
…
}
int CGenericMFCApp::ExitInstance()
{
AfxTermExtensionModule(ResouceDLL);
return CWinApp::ExitInstance();
}
這樣我們就可以在應用程序中安全的使用DLL中的資源了，就好比EXE中的一樣。對於我們自己編寫入口函數的DLL，如果其中涉及到資源的話，我們可以參考上述的MFC常規DLL的實現。
對於我們在ActiveX和一些DLL中的AFX_MANAGE_STATE(AfxGetStaticModuleState());我們又怎么理解呢？看看afxstat_.h：
#ifdef _AFXDLL
…
class _AFX_THREAD_STATE;
struct AFX_MAINTAIN_STATE2
{
explicit AFX_MAINTAIN_STATE2(AFX_MODULE_STATE* pModuleState);
~AFX_MAINTAIN_STATE2();
protected:
AFX_MODULE_STATE* m_pPrevModuleState;
_AFX_THREAD_STATE* m_pThreadState;
};
#define AFX_MANAGE_STATE(p) AFX_MAINTAIN_STATE2 _ctlState(p);
#else // _AFXDLL
#define AFX_MANAGE_STATE(p)
#endif //!_AFXDLL
對於在靜態庫中使用MFC DLL時它就相當於不存在，反之，它構造一個AFX_MAINTAIN_STATE2對象,在afxstate.cpp中
#ifdef _AFXDLL
…
AFX_MAINTAIN_STATE2::AFX_MAINTAIN_STATE2(AFX_MODULE_STATE* pNewState)
{
m_pThreadState = _afxThreadState;
m_pPrevModuleState = m_pThreadState->m_pModuleState;
m_pThreadState->m_pModuleState = pNewState;
}
#endif //_AFXDLL
,
再看dllmodul.cpp中有關AfxGetStaticModuleState的實現：
static _AFX_DLL_MODULE_STATE afxModuleState;
…
AFX_MODULE_STATE* AFXAPI AfxGetStaticModuleState()
{
AFX_MODULE_STATE* pModuleState = &afxModuleState;
return pModuleState;
}
(MFC中太多的全局變量的確不雅！)
可以看到通過構造一個AFX_MAINTAIN_STATE2對象(再結合AfxGetResouceHandle的實現)，就使AfxFindResourceHandle返回的是ActiveX控件（實際上也是一個常規DLL）或DLL的HINSTANE了，這樣AfxFindResourceHandle就不必須在DLL鏈表中查找了，但缺點是每次使用資源（或響應消息）時都要構造一個AFX_MAINTAIN_STATE2對象,並且，而且使用這種方法在此DLL中無法使用其它DLL中的資源或在EXE中使用DLL中的資源(在不使用AfxSetResourceHandle/AfxGetResourceHandle的前提下)。當然，我們這兒關心的只是資源的使用，如果涉及到其它，包括事件響應或多個UI線程時，AFX_MODULE_STATE還是不能少的。