[python-win32] win32com and extended error messages: IDL with [out, retval] long* error)

[Sandy Walsh]

Thanks for the prompt reply Mark.

That was it ... the C++ was returning the error code and not S_OK. 
Everything is cool now. Also the "9999" string for int was a problem as 
well ... good catch.

Keep up the great work and thanks again!

-Sandy


Mark Hammond wrote:
> Sandy Walsh wrote:
>> Hi,
>>
>> I have a C++ COM Server with an IDL entry of:
>>
>> [
>>    object,
>>    uuid(D989CB9A-0013-4E1A-BFB6-98A7946BD255),
>>    dual,
>>    helpstring("IFoo Interface"),
>>    pointer_default(unique)
>> ]
>> interface IFoo : IDispatch
>> {
>>    [ id(1), helpstring("method Connect")]
>>        HRESULT Connect([in] BSTR x, [in] short y, [in] BSTR z, [out, 
>> retval] long* error);
>> };
>>
>> A call from Python to the C++ object works fine unless I return 
>> something other than S_OK for "error".
>
> What does the C++ code look like?
>
>>            self.assertEqual(0, self.control.Connect("10.0.0.0", 
>> "9999", ""))
>
> That looks strange - your IDL says y is an integer, but you are 
> passing a string.
>
>> The call failed with code 80020009: Exception occurred.
>> The source of the error is None
>> The error message is None
>> More info can be found in None (id=0)
>> Win32 exception occurred releasing IUnknown at 0x00e84c58
>>
>> (where HR of 0x80020009 is DISP_E_EXCEPTION)
>
> So something is returning DISP_E_EXCEPTION as the HRESULT for the 
> method (which is quite different than having a dispatch "return value" 
> (which is passed as a param to the C++ implementation) named error.
>
>> I've traced the python side of the code and how it appears to me is 
>> the "error" out variable is not being created in the argument list.
>
> No - it is a return value: it comes back in the VARIANT holding the 
> result.  Note we are going via IDispatch, so something, presumably 
> ATL, is converting the IDispatch::Invoke call into a native vtable 
> based call into your implementation.
>
>> I'm not sending extended error messages on the C++ side, just setting 
>> this error code out variable.
>
> How are you setting it exactly?  And what do you mean by "sending 
> extended error messages" - how would they be sent?
>
> > I can only assume that the exception is
>> being generated in the MS Com libraries when it tries to marshal the 
>> error variable into a non-existent spot?
>
> I don't think the COM libraries would throw that kind of exception.
>
> Is it possible that your C++ implementation is returning the error 
> code instead of setting it?  Your C++ method should always return 
> S_OK, but set "error" via a pointer passed to your function.
>
> Cheers,
>
> Mark
>
>
>

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