A Journey 2 Eternity

When working in MFC/ATL and then using a resource dll can at times cause bugs because resource handle is not properly set. For e.g. if you have a dialog resource in a dll and you instantiate a CDialog which results in an error because resource cannot be located in this application or dll since it’s in the resource dll. So solution for above problem will be to set correct resource handle.

MFC provides two functions…

1. AfxGetResourceHandle

2. AfxSetResourceHandle

So we should first save our current resource handle and then set new resource handle before loading such a resource. Also don’t forget to set our old handle since its good practice.

Internally MFC calls FindResource and LoadResource using this handle, so if it’s not properly set this will cause failure in loading resources.

In ATL it’s quite similar except that we’ve got a new function called AddResourceInstance which adds our new resource handle to existing list of handles. So when a look up is done given resource handle is also used. Following functions are provided in ATL to work with resource handles…

1. AddResourceInstance

2. GetResourceInstance – Similar to AfxGetResourceHandle

3. SetResourceInstance – Similar to AfxGetSetResourceHandle

For newbies this is always a painful bug to resolve as they don’t know what went wrong since they expect this to be automagically done.

Sample code snippet:

class CEMCResourceHandle
{
public:
	CEMCResourceHandle() {
		m_hPrevInst = AfxGetResourceHandle();
		m_hDLLInstance = ::LoadLibrary(szESIMFCControlsDLLName);
		if (m_hDLLInstance) {
			AfxSetResourceHandle(m_hDLLInstance);
		}
	}

	virtual ~CEMCResourceHandle() {
		AfxSetResourceHandle(m_hPrevInst);
		if(m_hDLLInstance != NULL) {
			FreeLibrary(m_hDLLInstance);
		}
	}

	//! Retrieves the current resource instance
	//!
	//! \return Return the current resource instance
	HINSTANCE GetCurrentInstance() const { return m_hDLLInstance; }

	//! Retrieves the previous resource instance
	//!
	//! \return Return the previous resource instance
	HINSTANCE GetPrevInstance() const { return m_hPrevInst; }

private:
	//! Handle of previous resource instance
	HMODULE m_hPrevInst;

	//! Handle of loaded DLL resource instance
	HMODULE m_hDLLInstance;
};

// Just made an temporary instance of CEMCResourceHandle class
// Load resource from the DLL
CEMCResourceHandle resHandle;

// Set bitmap to the MoveUp button
m_btnUp.SetBitmap(::LoadBitmap(resHandle.GetCurrentInstance(), MAKEINTRESOURCE(IDB_BITMAP_UPARROW)));