Problem in optional and defaultvalue MIDL attributes

[Kumar.V.K@discussions.microsoft.com Vinoth Kumar.V.K]

Hi

I have a problem while using the optional and defaultvalue MIDL attributes
in a method declaration.

Here is my sample MIDL method declaration

HRESULT Add ( [in] int a, [in] int b, [in, optional, defaultvalue(10)] int c)

I have written a COM client by importing the typelibrary of the above IDL
defition

but while calling the Add method from the client it ask me to give all the
three parameters eventhough i declared the third parameter as optional value

but if i use the ".h" and "_i.c" files the compiler accepting two arguments.

May i know why this discrepencey?

can i get the optional attribute feature whil imporing the tlb file also?
please suggest me a way to use optional parameters by imporing the tlbs in
the client side code

this same code is working fin in VB.

Waiting for your response

with regards

vinoth .v.k

[Kim Gräsman]

Hi Vinoth,

> HRESULT Add ( [in] int a, [in] int b, [in, optional, defaultvalue(10)] int c)
>
> I have written a COM client by importing the typelibrary of the above IDL
> defition
>
> but while calling the Add method from the client it ask me to give all the
> three parameters eventhough i declared the third parameter as optional value

#import doesn't always carry over the optional and defaultvalue attributes. I believe it only does it for VARIANTs, though I haven't looked at it that carefully.

One option that might work for you is to make the last value a VARIANT instead, and then in your implementation check:

STDMETHODIMP CMyImpl::Add(..., VARIANT c)
{
HRESULT hr;

int actualC = 10;
if (c.vt != VT_ERROR)
{
// Specify better flags if you care about locale conversions, etc.
hr = VariantChangeType(&c, &c, 0, VT_I4);
if (FAILED(hr)) return hr;

actualC = c.lVal;
}

// Work with actualC from here on
}

That should work with both VB and C++ clients.

--
Best regards,
Kim Gräsman

[Alexander Nickolov]

[optional] and [defaultvalue()] don't make sense together.
The argument is either optional (no value at all), or has a
default value supplied by the caller (if the program source
didn't provide one). Default values can only be applied
to scalar arguments. [optional] can only be applied to
VARIANT arguments. Even though both look the same,
the mechanics underneath are completely different.

--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnic...@mvps.org
MVP VC FAQ: http://www.mvps.org/vcfaq
=====================================

"Vinoth Kumar.V.K" <Vinoth Kuma...@discussions.microsoft.com> wrote in
message news:C17E4C2B-CF46-4F06...@microsoft.com...

[kartiklunkad]

Alexander Nickolov wrote on 12/06/2004 13:34 ET :
> [optional] and [defaultvalue()] don't make sense together.
> The argument is either optional (no value at all), or has a
> default value supplied by the caller (if the program source
> didn't provide one). Default values can only be applied
> to scalar arguments. [optional] can only be applied to
> VARIANT arguments. Even though both look the same,
> the mechanics underneath are completely different.
>

> Alexander Nickolov
> Microsoft MVP [VC], MCSD
> email:

> MVP VC FAQ: http://www.mvps.org/vcfaq
>

> "Vinoth Kumar.V.K" <Vinoth
> wrote in
> message news:

>> Hi
>>
>> I have a problem while using the optional and defaultvalue MIDL attributes
>> in a method declaration.
>>
>> Here is my sample MIDL method declaration
>>
>> HRESULT Add ( [in] int a, [in] int b, [in, optional, defaultvalue(10)] int
>> c)
>>
>> I have written a COM client by importing the typelibrary of the above IDL
>> defition
>>
>> but while calling the Add method from the client it ask me to give all the
>> three parameters eventhough i declared the third parameter as optional
>> value
>>
>> but if i use the ".h" and "_i.c" files the compiler
>> accepting two
>> arguments.
>>
>> May i know why this discrepencey?
>>
>> can i get the optional attribute feature whil imporing the tlb file also?
>> please suggest me a way to use optional parameters by imporing the tlbs in
>> the client side code
>>
>> this same code is working fin in VB.
>>
>> Waiting for your response
>>
>> with regards
>>
>> vinoth .v.k
>>
>>
>>
>>
>>
>>
>

As you have said [optional] and [defaultvalue()] don't make sense together.
Can
you provide an example of how either are implemented in MIDL function
declaration and then the function implementation in C++. Or if you could lead
me
to a place where I could find this as I am unable to find it on Google .
Thanks
Alot.

Regards,
Kartik Lunkad