Monday, August 08, 2005

Find 'hidden' resources in extension DLL's

While creating your new, nifty grid control for your MFC extension DLL, you put your resources into your MFC extension control. Then when you compile and run your test application, or worse, your production application, it can’t find the controls resources. A worse problem is that it silently uses the wrong resources.

Identifying the problem

The problem is that MFC assumes that all of the applications' resources reside in the main executable. There are two ways to solve this problem. The first method is to let MFC hunt for the correct resource using AfxFindResourceHandle. The second method revolves around treating the extension DLL like a separate resource DLL.

However, there are significant disadvantages to both methods. The main disadvantage with the first method, which uses AfxFindResourceHandle, is that there can’t be any duplicate resource IDs in any of your applications DLL’s. Also, this method is slower.Both occur because AfxFindResourceHandle searches the entire link chain in the application and returns the first module with a matching resource.

The primary disadvantage with the second method is that you have to explicitly get a handle to the target module; and since loading each kind of resource uses different functions, you end up with code that is almost identical for each kind of resource.

Ask MFC politely for the resource

To have MFC do all the heavy lifting, call AfxFindResourceHandle, like this:


This will work fine--until someone adds an icon with a duplicate ID to the code base. There is one advantage to this method of loading a resource: You don’t have to know which module the resource resides in.

When something can go wrong, it will.

Being a firm believer in Murphy’s Law; I like to be as explicit as possible. So I prefer the second method of loading resources in my extension DLL’s. Although it requires a little more investment upfront, it pays off when your code goes into maintenance mode. Basically, for each kind of resource you use in your extension DLL, you’ll write a function like this:

CString LoadResString(UINT ResID, CString strModuleName)
TCHAR szBuffer[256];
//The module handle can be obtained in a variety of ways the
//following is the most
//straight forward
HMODULE mod = ::GetModuleHandle(strModuleName);
if (!mod){
AfxMessageBox(_T("GetModuleHandle failed in LoadResString."));
RaiseException(1814, 0, 0, NULL);
//This is where the resource actually gets loaded and will be
//different in each function
int count = LoadString(mod, ResID, szBuffer, 255);
if (!count){
AfxMessageBox(_T("LoadString failed in LoadResString."));
RaiseException(1814, 0, 0, NULL);
CString rslt(szBuffer);
return rslt;

MFC extension DLL’s are a nice feature but because of all the legacy code that Microsoft has to support we are stuck with some “interesting” features like this problem with MFC not looking in the extension DLL for resources first. It’s an easy problem to overcome but it can give you a headache the first time you encounter it. It doesn’t help that you have to know what the problem is before you can find the answer in the documentation.

Note: Technically, there is a third method for specifying which module to load resources from. I chose not to examine it in this article because it's an even less explicit than the AfxFindResourceHandle method.

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