Error 424 with C++ ATL OleAutomation

Question

I am unexpectedly receiving an Error 424 from an object that works.

I have a number of OleAutomation classes written in C++ using ATL.

I can instantiate them with either early or late binding. I can call methods and properties on them and obtain expected results.

However I receive Error 424 if I test whether they are null.

Are there any suggestions as to the problem ?

Excel VBA code that triggers error:

Sub ObjectTest()
    Dim o As About

    Set o = New About

    ' this works - prints 1.4
    Debug.Print "Version ", o.Version

    ' generates Error 424 
    Debug.Print "About ", o Is Null
End Sub

The IDL

[
     object,
     oleautomation,
     uuid(...),
     dual,
     nonextensible,
     pointer_default(unique),
     public
]
interface IAbout : IDispatch
{
     ...
     [propget, id(3), helpstring("Library version string")]
     HRESULT Version([out, retval] BSTR* pResult);
     ...
};


[
    uuid(...)       
]
coclass About
{
    [default] interface IAbout;
};

The registration

HKCR
{
    MyLibrary.About.1 = s 'About MyLibrary'
    {
        CLSID = s '{...}'
    }
    MyLibrary.About = s 'About MyLibrary'
    {
        CLSID = s '{...}'
        CurVer = s 'MyLibrary.About.1'
    }
    NoRemove CLSID
    {
        ForceRemove {...} = s 'Class About'
        {
            ProgID = s 'MyLibrary.About.1'
            VersionIndependentProgID = s 'MyLibrary.About'
            ForceRemove Programmable
            InprocServer32 = s '%MODULE%'
            {
                val ThreadingModel = s 'Apartment'
            }
            TypeLib = s '{...}'
            Version = s '1.0'
        }
    }
}

And the class

class ATL_NO_VTABLE CAbout :
    public ATL::CComObjectRootEx<ATL::CComSingleThreadModel>,
    public ATL::CComCoClass<CAbout, &CLSID_About>,
    public ATL::ISupportErrorInfoImpl<&CLSID_About>,
    public ATL::IDispatchImpl<IAbout, &IID_IAbout, &LIBID_MyLibrary, /*wMajor =*/ 1, /*wMinor =*/ 0>
{
public:
    CAbout()
    {
    }

DECLARE_REGISTRY_RESOURCEID(IDR_ABOUT)

DECLARE_NOT_AGGREGATABLE(CAbout)

BEGIN_COM_MAP(CAbout)
    COM_INTERFACE_ENTRY(IAbout)
    COM_INTERFACE_ENTRY(IDispatch)
    COM_INTERFACE_ENTRY(ISupportErrorInfo)
END_COM_MAP()



    DECLARE_PROTECT_FINAL_CONSTRUCT()

    HRESULT FinalConstruct()
    {
        return S_OK;
    }

    void FinalRelease()
    {
    }

public:

    ...
    STDMETHOD(get_Version)( /* [out, retval] */ BSTR* pResult );
    ...

};

OBJECT_ENTRY_AUTO(__uuidof(About), CAbout)

Any help with why it generates error 424 for test "is null"?

Answer 1

The error is "Object Required" and is solely related to VBA side (unrelated to C++ and ATL). You should not test for Nullness this way, since this is what is generating the error.

See answer on Error checking for NULL in VBScript for Null testing options you actually have.