In a recent project I needed to be able to dynamically load
CFormViews into the
MainFrame of my application. I searched the developer sites and found a couple articles that explained how to load document/views into MDI applications but unfortunately, I needed to use the SDI approach. I searched MSDN and found even more articles discussing the
technique under MDI's. I eventually decided that it shouldn't be too difficult and here is the result.
I started out by referencing an article on Codeguru.com entitled
"Replacing a view
in a doc-view application" by Jorge Lodos Vigil. That article gave me the basics for switching view in an SDI application. The next problem to solve was how to export the
CView derived class) from a DLL and what kind of DLL? In the MFC AppWizard(dll) you have three types of DLLs that you can create, they are:
- Regular DLL with MFC statically linked
- Regular DLL using shared MFC DLL
- MFC Extension DLL (using shared MFC DLL)
After some research and the usual trial and error, I discovered that the third choice MFC Extension DLL
was the proper DLL type for my requirements. An MFC Extension DLL will only work from an MFC application and in this case that's exactly what was needed. There are other distinctions between the types of DLLs and I urge you to discover them on your own.
The DLL implementation
After creating an MFC Extension DLL project go to the ResourceView in the Workspace and create a new Dialog Resource of type
IDD_FORMVIEW. Next select 'Insert|New Class... ' from the main menu and create a new
CFormView derived class using the new dialog as the Dialog ID.
Add the controls and functionality as you normaly would and then add this code under the
DllMain(), ensuring that you replace the variables as explained in the code.
extern "C" AFX_EXT_API UINT Init(CRuntimeClass** view)
*view = RUNTIME_CLASS(CYourClass);
That's pretty much all you need to do for the DLL side of things. Looks pretty simple, right? Let's move onto the main application or the 'host'.
The 'Host' application
There are endless ways that we can set up the ‘host’ application to load up the DLLs and import the views. In this example I’m not going to do anything fancy, as I want you to understand how to accomplish the task. I will explain the method I have chosen, based on my requirements, at the end of the article so stay tuned.
Create an MFC executable project using the Single Document (SDI) option, you can modify all the options in the Appwizard but on the last page select
CView or a
CView derived class as the Base class.
Select the ResourceView tab in the Workspace, open the Menu treeitem then double-click on the
IDR_MAINFRAME and add a menuitem under the View menu or
create a new menu - it makes no difference. Then select your new menuitem and bring up the Class Wizard and add command routing for it, here is the code that will load up the DLL, grab the exported function and retrieve the
typedef UINT ( * LPDLLFUNC)(CRuntimeClass**);
LPDLLFUNC lpfnDllFunc = NULL;
HINSTANCE hDLL = NULL;
hDLL = LoadLibrary("InitialContact.dll");
lpfnDllFunc = (LPDLLFUNC)::GetProcAddress(hDLL,"Init_");
AfxMessageBox("Function not found in DLL");
CSdiDllFramesDoc* pDoc = (CSdiDllFramesDoc*) GetActiveDocument();
AfxMessageBox("Dll not found!");
For clarity, I present the code from Jorge Lodos Vigil's article
referenced above (comments removed):
BOOL CYourDoc::SwitchToView(CRuntimeClass* pNewViewClass)
CFrameWnd* pMainWnd = (CFrameWnd*)AfxGetMainWnd();
CView* pOldActiveView = pMainWnd->GetActiveView();
::SetWindowLong(pOldActiveView->m_hWnd, GWL_ID, 0);
context.m_pNewViewClass = pNewViewClass;
context.m_pCurrentDoc = this;
CView* pNewView = STATIC_DOWNCAST(CView, pMainWnd->CreateView(&context));
if (pNewView != NULL)
There really isn't anything new here. We have
typedef'ed a function
pointer and created a variable of that type. We then loaded the library, retrieved the address of our exported function and loaded that into our function pointer. Then, using a
CRuntimeClass we passed that into our exported function via the function pointer. We then grab a pointer to our
CDocument class and call the member
SwitchToView() passing in the
CRuntimeClass that we retrieved from the DLL.
All in all it was much easier than I thought and I say that with about three failed attempts, having started with the first DLL type and working my way down the list.
An Advanced Example
The requirements in my implementation were that the menuitems were to be read
from a database. The table in the database held the following information:
- Menu Caption
- Menu ID
- DLL Name
- Function Name
With that I decided upon this class framework to hold the information read in from the database. I also provided the class the ability to
UnLoadPlugin() the DLLs. Next, I created a
CMap to hold each instance of the
HSMenuItem class and set the menu id up to be the key. This
seemed to be a logical choice and after you see how I handle the menu selections you'll see the ease of use and extensibility that the 'host' application provides. Here's the
HSMenuItem(CString _MenuCaption, DWORD _MenuID,
CString _LibraryName, CString _FuncName) :
m_lpfnDllFunc = NULL;
m_hDLL = NULL;
m_hDLL = LoadLibrary(m_LibraryName);
m_lpfnDllFunc = (LPDLLFUNC)::GetProcAddress(m_hDLL,m_FuncName);
m_bLoaded = TRUE;
m_lpfnDllFunc = NULL;
typedef UINT ( * LPDLLFUNC)(CRuntimeClass**);
CString m_MenuCaption; DWORD m_MenuID; CString m_LibraryName; CString m_FuncName;
typedef CMap <DWORD,DWORD,HSMenuItem*,HSMenuItem*> mapMenuItems;
Loading the Menus up from the database and dynamically creating them is an
exercise left to the reader. I will show you how easy it was to handle the menu
options. First I hooked up my menus to go through the message map using
ON_COMMAND_RANGE routing that to my
HandleMenu(UINT menuid) function that looks like this:
BOOL CMainFrame::HandleMenu(UINT menuid)
if (menuid != 0)
HSMenuItem* mi = NULL;
if(mi != NULL)
CAlarmAssistantDoc* pDoc = (CAlarmAssistantDoc*) GetActiveDocument();
As you can see this is very flexible and will load up any
derived class that you have exported from a DLL. I've found this approach very
useful in projects where you can deliver optional functionality in stages.
Even better, bug fixes do not require a full recompile of the application -
just the DLL.
My thanks go out to Mr. Vigil for his
SwitchToView() as well
as all the other contributors to CodeProject.com.
Dave has been programming for the past 20+ years first on a variety of platforms and operating systems using various languages. As a hobbyist Dave cut his teeth on the Commodore Pet and the 64 coding in basic and then moving to 6502 ASM. Dave moved to the Amiga using 68000 ASM and then C. His knowledge of the C language offered the stepping stone for him to make his hobby his profession taking a position coding C on an AIX Unix platform. Since then he has worked on many flavors of Unix, QNX, Windows (3.11 – present), and has been coding games for his Pocket PC in his spare time.
Dave lives in Indiana with his two teenage daughters and two cats.