DllGetClassObject function (combaseapi.h)

Article
02/22/2024

Retrieves the class object from a DLL object handler or object application.

OLE does not provide this function. DLLs that support the OLE Component Object Model (COM) must implement DllGetClassObject in OLE object handlers or DLL applications.

Syntax

HRESULT DllGetClassObject(
  [in]  REFCLSID rclsid,
  [in]  REFIID   riid,
  [out] LPVOID   *ppv
);

Parameters

[in] rclsid

The CLSID that will associate the correct data and code.

[in] riid

A reference to the identifier of the interface that the caller is to use to communicate with the class object. Usually, this is IID_IClassFactory (defined in the OLE headers as the interface identifier for IClassFactory).

[out] ppv

The address of a pointer variable that receives the interface pointer requested in riid. Upon successful return, *ppv contains the requested interface pointer. If an error occurs, the interface pointer is NULL.

Return value

This function can return the standard return values E_INVALIDARG, E_OUTOFMEMORY, and E_UNEXPECTED, as well as the following values.

Return code	Description
S_OK	The object was retrieved successfully.
CLASS_E_CLASSNOTAVAILABLE	The DLL does not support the class (object definition).

Remarks

If a call to the CoGetClassObject function finds the class object that is to be loaded in a DLL, CoGetClassObject uses the DLL's exported DllGetClassObject function.

Notes to Callers

You should not call DllGetClassObject directly. When an object is defined in a DLL, CoGetClassObject calls the CoLoadLibrary function to load the DLL, which, in turn, calls DllGetClassObject.

Notes to Implementers

You need to implement DllGetClassObject in (and export it from) DLLs that support COM.

Examples

The following is an example (in C++) of an implementation of DllGetClassObject. In this example, DllGetClassObject creates a class object and calls its QueryInterface method to retrieve a pointer to the interface requested in riid. The implementation releases the reference it holds to the IClassFactory interface because it returns a reference-counted pointer to IClassFactory to the caller.

HRESULT_export CALLBACK DllGetClassObject 
    (REFCLSID rclsid, REFIID riid, LPVOID * ppvObj) 
{ 
    HRESULT hr = E_OUTOFMEMORY; 
    *ppvObj = NULL; 
 
    CClassFactory *pClassFactory = new CClassFactory(rclsid); 
    if (pClassFactory != NULL)   { 
        hr = pClassFactory->QueryInterface(riid, ppvObj); 
        pClassFactory->Release(); 
    } 
    return hr;
}

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	combaseapi.h (include Objbase.h)

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