How to fail gracefully Outlook COM

Thanks, I'll give it a go.  Faint of heart I'm not, but neither am I a C++ programmer!  Variants are the only option for the COM add-in due to interfacing with VB (or more importantly, due to the object types).

Unfortunately that isn't working.  I don't know if the below code helps - it shows what getMapiobjectProperty does.  It seems that even if I pass Nothing (from VB) as the MapiObject parameter, it is still passed as VT_DISPATCH, so I need to determine whether that is a valid object.

LPUNKNOWN CPMapiWrapper::getMapiobjectProperty(LPDISPATCH disp)
{
	DISPID		rgDispId;
	DISPPARAMS	dispparams = {NULL, NULL, 0, 0};
	VARIANT		vaResult;	
	LPOLESTR	pName = L"MAPIOBJECT";
	VariantInit(&vaResult);
 
	if (SUCCEEDED(disp->GetIDsOfNames(IID_NULL,&pName,1,LOCALE_SYSTEM_DEFAULT,&rgDispId)))
		if (SUCCEEDED(disp->Invoke(rgDispId, IID_NULL, 0, DISPATCH_PROPERTYGET, &dispparams, &vaResult, NULL, NULL)))
			if (vaResult.vt == VT_UNKNOWN)
			{
				return vaResult.punkVal;
			}
		
	return NULL;
}

                                              

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I'm sorry but I do not understand your mail. What did you pass to this function? LPDISPATH surely is a valid IDispatch interface or not?
What should the MapiObject parameter be?
The code you show looks quite ok to me, the only code which  seems not to be ok is return NULL, This does not look ok to me but I may be wrong.

I do also not understand what you mean with this from your initial question
rameter (e.g. the folder object is VB Nothing, or unset)

should that be the first,second or third parameter.


I've the impression you use some wrapper around the MAPI stuff.

Common tasks are show here:
http://msdn.microsoft.com/en-us/library/ms528097(EXCHG.10).aspx

if you however do IDispatch programming with C/C++ you probably want to use the disphelper.c file also
http://disphelper.sourceforge.net/readme.htm

In your case you just have on parameter while accessing the getter but if you do have more parameter you also have to be sure that you pass in the the parmater in reverse order.

But that's another story. Without understanding where the problem is I can not do more than trying to give you hints. I'm still absolutly unsure what you like to achieve.

Regards
Friedrich

The null return is fine - if null is returned from the above function then the calling code returns an E_NOINTERFACE code.  I don't mind null returns, I can check for them - but what is happening is if I pass the VB value Nothing in as a parameter instead of the expected MAPIObject, then I get a GPF rather than an error (which is what I want).

There is no wrapper beyond the above code.  From VB, I would call EnumerateFolders passing a folder object as pFolderItem.  However, if I pass Nothing, I don't get an error, I get a crash, which is not what I want (obviously!).  The problem being that sometimes if there is an error elsewhere in the VB code, a parameter may end up being Nothing, so I just need an error returned.

I don't know how to explain it any further... EnumerateFolders is the function presented via COM to VB.

I presume the:
if (!unk) return E_NOINTERFACE;
Is in fact checking whether unk is null, and if so returning E_NOINTERFACE?!  

Nothing in VB is a value, or in the case of an object (which this is) it means that it doesn't in fact refer to ANY object.
http://msdn.microsoft.com/en-us/library/0x9tb07z%28VS.80%29.aspx

I test for VT_EMPTY (but this test doesn't catch it).  When I try testing for NULL (with either pFolderItem==NULL, or !pFolderItem syntax) I receive a compile error:

error C2678: binary '==' : no operator defined which takes a left-hand operand of type 'struct tagVARIANT' (or there is no a
cceptable conversion)

Thing is, as I only catch VARIANTS that are either VT_DISPATCH or VT_DISPATCH | VT_BYREF, anything that isn't that type will result in an error.  Which means that NOTHING is still being passed as VT_DISPATCH.