Exported DLL Function Entry Points
For exported functions of a DLL, use the AFX_MANAGE_STATE macro to maintain the proper global state when switching from the DLL module to the calling application's DLL.
When called, this macro sets pModuleState, a pointer to an AFX_MODULE_STATE structure containing global data for the module, as the effective module state for the remainder of the containing scope of the function. Upon leaving the scope containing the macro, the previous effective module state is automatically restored.
This switching is achieved by constructing an instance of an AFX_MODULE_STATE class on the stack. In its constructor, this class obtains a pointer to the current module state and stores it in a member variable, and then sets pModuleState as the new effective module state. In its destructor, this class restores the pointer stored in its member variable as the effective module state.
If you have an exported function, such as one that launches a dialog box in your DLL, you need to add the following code to the beginning of the function:
AFX_MANAGE_STATE(AfxGetStaticModuleState( ))
This swaps the current module state with the state returned from AfxGetStaticModuleState until the end of the current scope.
Problems with resources in DLLs will occur if the AFX_MANAGE_STATE macro is not used. By default, MFC uses the resource handle of the main application to load the resource template. This template is actually stored in the DLL. The root cause is that MFC's module state information has not been switched by the AFX_MANAGE_STATE macro. The resource handle is recovered from MFC's module state. Not switching the module state causes the wrong resource handle to be used.
AFX_MANAGE_STATE does not need to be put into every function in the DLL. For example, InitInstance can be called by the MFC code in the application without AFX_MANAGE_STATE because MFC automatically shifts the module state before InitInstance and then switches it back after InitInstance returns. The same is true for all message-map handlers. Regular DLLs actually have a special master window procedure that automatically switches the module state before routing any message.