Shell Extension Class Factory

Every shell extension DLL is loaded by Explorer through the same two entry points shown here: DllGetClassObject (which must return an IClassFactory) and DllCanUnloadNow (which tells the shell when it is safe to unload). This file is that whole layer for "Project Epsilon", a context-menu/icon/property-sheet/copy-hook extension. The pattern: a global reference count (g_cRefThisDll) incremented in every object's constructor and decremented in its destructor; a class factory whose CreateInstance refuses aggregation and hands back the main CShellExt object; and one QueryInterface that serves six interfaces from the same object - IShellExtInit, IContextMenu, IExtractIcon, IPersistFile, IShellPropSheetExt, and IShellCopyHook. Use this as the skeleton when you are writing your own shell extension; the individual interface implementations live in their own files.

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <windows.h>
#include <windowsx.h>
#include <shlobj.h>

// Initialize GUIDs (should be done only and at-least once per DLL/EXE)
#pragma data_seg(".text")
#define INITGUID
#include <initguid.h>
#include <shlguid.h>
#pragma data_seg()

UINT g_cRefThisDll = 0;	// Reference count of this DLL.
HINSTANCE g_hmodThisDll = NULL; // Handle to this DLL itself.

extern "C" int APIENTRY DllMain(HINSTANCE hInstance, DWORD dwReason, LPVOID lpReserved)
{
	if (dwReason == DLL_PROCESS_ATTACH)
	{
		// Extension DLL one-time initialization
		g_hmodThisDll = hInstance;
	}
	else if (dwReason == DLL_PROCESS_DETACH)
	{
	}

	return 1;	// ok
}

STDAPI DllCanUnloadNow(void)
{
	return (g_cRefThisDll == 0 ? S_OK : S_FALSE);
}

STDAPI DllGetClassObject(REFCLSID rclsid, REFIID riid, LPVOID *ppvOut)
{
	*ppvOut = NULL;

	if (IsEqualIID(rclsid, CLSID_ShellExtension))
	{
		CShellExtClassFactory *pcf = new CShellExtClassFactory;

		return pcf->QueryInterface(riid, ppvOut);
	}

	return CLASS_E_CLASSNOTAVAILABLE;
}

CShellExtClassFactory::CShellExtClassFactory()
{
	m_cRef = 0L;

	g_cRefThisDll++;	
}
																				
CShellExtClassFactory::~CShellExtClassFactory()
{
	g_cRefThisDll--;
}

STDMETHODIMP CShellExtClassFactory::QueryInterface(REFIID riid, LPVOID FAR *ppv)
{
	*ppv = NULL;

	// Any interface on this object is the object pointer

	if (IsEqualIID(riid, IID_IUnknown) || IsEqualIID(riid, IID_IClassFactory))
	{
		*ppv = (LPCLASSFACTORY)this;

		AddRef();

		return NOERROR;
	}

	return E_NOINTERFACE;
}	

STDMETHODIMP_(ULONG) CShellExtClassFactory::AddRef()
{
	return ++m_cRef;
}

STDMETHODIMP_(ULONG) CShellExtClassFactory::Release()
{
	if (--m_cRef)
		return m_cRef;

	delete this;

	return 0L;
}

STDMETHODIMP CShellExtClassFactory::CreateInstance(LPUNKNOWN pUnkOuter, REFIID riid, LPVOID *ppvObj)
{
	*ppvObj = NULL;

	// Shell extensions typically don't support aggregation (inheritance)

	if (pUnkOuter)
		return CLASS_E_NOAGGREGATION;

	// Create the main shell extension object. The shell will then call
	// QueryInterface with IID_IShellExtInit--this is how shell extensions are
	// initialized.

	LPCSHELLEXT pShellExt = new CShellExt();  //Create the CShellExt object

	if (NULL == pShellExt)
		return E_OUTOFMEMORY;

	return pShellExt->QueryInterface(riid, ppvObj);
}

STDMETHODIMP CShellExtClassFactory::LockServer(BOOL fLock)
{
	return NOERROR;
}

And the main object, whose single QueryInterface fans out to all six interfaces:

CShellExt::CShellExt()
{
	m_cRef = 0L;
	m_pDataObj = NULL;

	g_cRefThisDll++;
}

CShellExt::~CShellExt()
{
	if (m_pDataObj)
		m_pDataObj->Release();

	g_cRefThisDll--;
}

STDMETHODIMP CShellExt::QueryInterface(REFIID riid, LPVOID FAR *ppv)
{
	*ppv = NULL;

	if (IsEqualIID(riid, IID_IShellExtInit) || IsEqualIID(riid, IID_IUnknown))
	{
		*ppv = (LPSHELLEXTINIT)this;
	}
	else if (IsEqualIID(riid, IID_IContextMenu))
	{
		*ppv = (LPCONTEXTMENU)this;
	}
	else if (IsEqualIID(riid, IID_IExtractIcon))
	{
		*ppv = (LPEXTRACTICON)this;
	}
	else if (IsEqualIID(riid, IID_IPersistFile))
	{
		*ppv = (LPPERSISTFILE)this;
	}
	else if (IsEqualIID(riid, IID_IShellPropSheetExt))
	{
		*ppv = (LPSHELLPROPSHEETEXT)this;
	}
	else if (IsEqualIID(riid, IID_IShellCopyHook))
	{
		*ppv = (LPCOPYHOOK)this;
	}

	if (*ppv)
	{
		AddRef();

		return NOERROR;
	}

	return E_NOINTERFACE;
}

STDMETHODIMP_(ULONG) CShellExt::AddRef()
{
	return ++m_cRef;
}

STDMETHODIMP_(ULONG) CShellExt::Release()
{
	if (--m_cRef)
		return m_cRef;

	delete this;

	return 0L;
}

Two details worth keeping: the #pragma data_seg(".text") around initguid.h keeps the GUID constants from bloating the read-only data section, and DllCanUnloadNow returning S_FALSE while any object is alive is what prevents Explorer from unloading the DLL mid-operation.