Question

In COM when I have a well-known interface that I can’t change:

interface IWellKnownInterface {
     HRESULT DoStuff( IUnknown* );
};

and my implementation of IWellKnownInterface::DoStuff() can only work when the passed object implements some specific interface how do I handle this situation?

HRESULT CWellKnownInterfaceImpl::DoStuff( IUnknown* param )
{
    //this will QI for the specific interface        
    ATL::CComQIPtr<ISpecificInterface> object( param );
    if( object == 0 )
        //clearly the specifil interface is not supported
        return E_INVALIDARG;
    }
    // proceed with implementation
}

In case the specific interface is not supported which error code should I return? Is returning E_INVALIDARG appropriate?

Answer 1

E_INVALIDARG is a fine choice, but the most important thing is to ensure that the precise conditions for each return code that you use are well documented.

Additionally, you could consider implementing ISupportErrorInfo and then returning rich error information via CreateErrorInfo and SetErrorInfo. This is especially useful in cases where you think callers may benefit from having a custom error message generated at the point of failure, with all of the relevant context contained therein. In your case, this might be to identify specifically which argument is invalid and which interface was unimplemented for it to be so. Even though such a message is unlikely to be of value to an end user, it could be invaluable to a developer if it shows up in a log file or the event viewer.