PRB: MFC Loads Wrong Resource in Extension DLL

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In .exe or .dll files that link to MFC dynamically, MFC resource-loading functions call AfxFindResourceHandle() to obtain the handle of the module where a resource is located. AfxFindResourceHandle() searches for resources by type and symbol value in:

• The module returned by AfxGetResourceHandle(). This is usually the application.
• The extension DLLs through the chain of CDynLinkLibrary objects.
• Any language-specific resource DLLs.
• Any attached MFC system DLLs (MFCxx.dll, for example).

NOTE: Some 16-bit MFC resource loading functions do not call AfxFindResourceHandle(), but instead use the value returned from AfxCurrentResourceHandle().

Each extension DLL creates, initializes, and then passes a CDynLinkLibrary object to AfxInitExtensionModule() that places the DLL in the resource chain. AfxTermExtensionModule() removes the DLL from the chain when the DLL is detached from the application.

A benefit of this design is that MFC automatically locates a resource for an application or extension DLL, even if that resource is located in a distant extension DLL or the application itself. All resources in the process are chained, so ID values are passed between DLLs and the application and the proper resources are loaded. A disadvantage is that there are no duplicate ID values between any of the extension DLLs or the application that uses them.

To set the default location where AfxFindResourceHandle() first checks for a resource, use AfxSetResourceHandle(). Because AfxFindResourceHandle() first checks the handle set by AfxSetResourceHandle(), it can be used to circumvent the chain and load a resource from one particular DLL or application. The resource handle is restored to its original value immediately after loading the resources. The current default resource handle is found with AfxGetResourceHandle().

The DLLHUSK sample included with Visual C++ also illustrates this technique. In Testdll2.cpp, CListOutputFrame::Create() sets the resource handle to the module handle stored in the AFX_EXTENSION_MODULE structure extension DLL. This structure is initialized with the module handle when it is passed to the CDynLinkLibrary constructor in InitTestDLL2().