Emulating CoCreateInstance()

Introduction

A while ago, I wrote a small utility that converts DOT syntax into an image using the WinGraphViz.DLL COM component.

However, it is not very usual to have the user register this component before running the tool, thus I started looking around for a way to use COM components as if they were normal DLLs. The research yielded that there are many techniques to accomplish that:

1. Registration-Free COM (for XP and above)
2. Emulating the CoCreateInstance()

To learn more about Registration-Free COM, please check the references at the end of this article.

Emulating CoCreateInstance()

Normally, to create an instance, you would do something like:

hr = CoCreateInstance(CLSID_DOT, NULL, CLSCTX_ALL,
                      IID_IDOT, (LPVOID *)&pIDOT);

This will cause OLE to fetch the associated DLL from the registry and call its DllGetClassObject() method to get a class factory, then from the class factory an instance of your required IID will be created.

For that reason, we may emulate the CoCreateInstance() by the following code:

HRESULT __stdcall MyCoCreateInstance(
  LPCTSTR szDllName,
  IN REFCLSID rclsid,
  IUnknown* pUnkOuter,
  IN REFIID riid,
  OUT LPVOID FAR* ppv)
{
  HRESULT hr = REGDB_E_KEYMISSING;

  HMODULE hDll = ::LoadLibrary(szDllName);
  if (hDll == 0)
    return hr;

  typedef HRESULT (__stdcall *pDllGetClassObject)(IN REFCLSID rclsid, 
                   IN REFIID riid, OUT LPVOID FAR* ppv);

  pDllGetClassObject GetClassObject = 
     (pDllGetClassObject)::GetProcAddress(hDll, "DllGetClassObject");
  if (GetClassObject == 0)
  {
    ::FreeLibrary(hDll);
    return hr;
  }

  IClassFactory *pIFactory;

  hr = GetClassObject(rclsid, IID_IClassFactory, (LPVOID *)&pIFactory);

  if (!SUCCEEDED(hr))
    return hr;

  hr = pIFactory->CreateInstance(pUnkOuter, riid, ppv);
  pIFactory->Release();

  return hr;
}

Notice how this function takes a parameter holding the DLL's name.

Using the code

You need your application to run first if the COM is registered, if not you would resort to emulating the CoCreateInstance(). Your code could look like this:

hr = CoCreateInstance(CLSID_DOT, NULL, CLSCTX_ALL,
     IID_IDOT, (LPVOID *)&pIDOT);

if (hr == REGDB_E_CLASSNOTREG)
{
  hr = MyCoCreateInstance(_T("WinGraphViz.dll"), CLSID_DOT, 
       NULL, IID_IDOT, (LPVOID *)&pIDOT);
}

if (FAILED(hr))
{
  cout << "CoCreateInstance Failed: " << hr 
       << "nn";
  return -1;
}

Reference

The following links were helpful during the building of this simple snippet:

• Mikhail Arkhipov WebLog: Registration-free COM
• Dave Templin: Simplify App Deployment with ClickOnce and Registration-Free COM
• Jeff Glatt: COM in Plain C
• Registration-Free Activation of COM Components: A Walkthrough

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