How to retrieve the Interface ID of a COM class so that it can be passed to CoCreateInstance?

Question

I want to programaticly retrieve the Interface ID for any class so that I can pass it to CoCreateInstance. Any help is very much appreciated!!

See "How Do I Get This" below:

HRESULT hResult;
CLSID ClassID;
void *pInterface;

if(!(hResult = SUCCEEDED(CoInitialize(NULL))))
{
    return 1;
}

if(S_OK == CLSIDFromProgID(OLESTR("Scripting.FileSystemObject"), &ClassID))
{
    hResult = CoCreateInstance(ClassID, NULL, CLSCTX_INPROC_SERVER,
        <<How Do I Get This?>>, (LPVOID *)&pInterface);
}

CoUninitialize();

EDIT: Thanks for all of the help, seems to work perfectly now! :

HRESULT hResult;
CLSID ClassID;
IClassFactory *pClf;
void *pVdb;

if(!(hResult = SUCCEEDED(CoInitialize(NULL))))
{
    return 1;
}

if(SUCCEEDED(CLSIDFromProgID(OLESTR("Scripting.FileSystemObject"), &ClassID))
{
    IDispatch *pDispatch;

    if(SUCCEEDED(CoCreateInstance(ClassID, NULL, CLSCTX_INPROC_SERVER,
            IID_IDispatch, (void **)&pDispatch))
    {
        OLECHAR *sMember = L"FileExists";

        DISPID idFileExists;

        if(SUCCEEDED(pDispatch->GetIDsOfNames(
                IID_NULL, &sMember, 1, LOCALE_SYSTEM_DEFAULT, &idFileExists))
        {
            unsigned int puArgErr = 0;

            VARIANT VarResult;
            EXCEPINFO pExcepInfo;

            VariantInit(&VarResult); 
            VariantInit(&pExcepInfo); 

            DISPPARAMS pParams;
            memset(&pParams, 0, sizeof(DISPPARAMS)); 
            pParams.cArgs = 1; 

            VARIANT Arguments[1];
            VariantInit(&Arguments[0]); 

            pParams.rgvarg = Arguments; 
            pParams.cNamedArgs = 0;
            pParams.rgvarg[0].vt = VT_BSTR;
            pParams.rgvarg[0].bstrVal = SysAllocString(L"C:\\Test.txt");

            hResult = pDispatch->Invoke(  
                idFileExists,
                IID_NULL,
                LOCALE_SYSTEM_DEFAULT,
                DISPATCH_METHOD,
                &pParams,
                &VarResult,
                &pExcepInfo,
                &puArgErr
            );

            SysFreeString(pParams.rgvarg[0].bstrVal);

            printf("File Exists? %d\n", abs(VarResult.boolVal));
        }

        pDispatch->Release();
    }
}

CoUninitialize();

Answer 1

You need to know upfront what interface you ask for. This you get from the product specifications, from SDK header files, or you can import the TLB of the COM object into your project.

the easisest way is to use #import