Determining Which Interface an Object Supports

[The feature associated with this page, Custom File and Stream Handlers, is a legacy feature. It has been superseded by MediaStreamSource class. MediaStreamSource class has been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use MediaStreamSource class instead of Custom File and Stream Handlers, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The QueryInterface method lets an application query an object to determine which interfaces it supports. The sample application sets the ppv pointer to the current interface.

STDMETHODIMP CAVIFileCF::QueryInterface( 
    const IID FAR& iid, 
    void FAR* FAR* ppv) 
{ 
    if (iid == IID_IUnknown) 
        *ppv = this;                     // set the interface pointer 
                                         // to this instance 
    else if (iid == IID_IClassFactory) 
        *ppv = this;                     // second chance to set the 
                                         // interface pointer to this 
                                         // instance 
    else 
        return ResultFromScode(E_NOINTERFACE); 
    AddRef();  //Increment the reference count 
    return NULL; 
}