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Emulating CoCreateInstance()

, 16 Apr 2007
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Describes how to use COM objects without registration.

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:

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Elias Bachaalany
Web Developer
United States United States
Elias (aka lallousx86, @0xeb) has always been interested in the making of things and their inner workings.
 
His computer interests include system programming, reverse engineering, writing libraries, tutorials and articles.
 
In his free time, and apart from researching, his favorite reading topics include: dreams, metaphysics, philosophy, psychology and any other human/mystical science.
 
Former employee of Hex-Rays (the creators of IDA Pro), was responsible about many debugger plugins, IDAPython project ownership and what not.
 
Elias currently works at Microsoft as a software security engineer.
 
More articles and blog posts can be found here:
 
- http://lallousx86.wordpress.com/
- http://0xeb.wordpress.com/
- http://www.hexblog.com/?author=3

Comments and Discussions

 
QuestionWhat about IDispatch methods? Pinmembermertart12-Jun-10 2:50 
QuestionSingle Instance of COM Servers from different clients PinmemberGurmeetSingh8718-Feb-10 6:00 
GeneralCool, Great, Successfully used on Linux!!! PinmemberSharjith20-Jul-09 5:46 
QuestionHow do I get clsid/iid unless the COM dll is not registered Pinmemberehaerim23-Dec-07 18:50 
AnswerRe: How do I get clsid/iid unless the COM dll is not registered PinmemberSharjith21-Jul-09 6:50 
Questionun freed addtional dlls Pinmemberirol9-Nov-07 4:46 
AnswerRe: un freed addtional dlls Pinmemberlallous11-Nov-07 22:19 
GeneralRe: un freed addtional dlls Pinmemberirol12-Nov-07 23:57 
Hello,
 
lallous wrote:
Please check this comment http://www.codeproject.com/com/Emul_CoCreateInstance.asp?select=2226337&df=100&forumid=408558#xx2226337xx[^]

 
I checked all comments before posting my question. I tried every posible mix of load/free functions - without positive results.
I think, that problem is connected with different entry point in COM dll's. But I'm not able to resolve it myself.
 
Regards,
IrOl
QuestionOCX? Pinmembersammytheblowfish10-Jul-07 20:28 
QuestionRe: OCX? Pinmembersammytheblowfish14-Jul-07 12:18 
AnswerRe: OCX? Pinmembercharlieqin8-Nov-07 6:54 
QuestionFreeLibrary? Pinmemberpestking17-Apr-07 15:03 
AnswerRe: FreeLibrary? Pinmemberlallous17-Apr-07 18:49 
GeneralRe: FreeLibrary? PinmemberPhish Bulb13-Sep-07 5:27 
GeneralRe: FreeLibrary? Pinmemberlallous20-Sep-07 21:01 
GeneralGood stuff Pinmembersprymak16-Apr-07 23:13 

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