Re: CoInitialize returns FALSE?
On Tue, 26 Feb 2008 17:02:07 +0100, "Giovanni Dicanio"
<giovanni.dicanio@invalid.com> wrote:
"Guido Franzke" <guidof73@yahoo.de> ha scritto nel messaggio
news:%23JzBgXGeIHA.4260@TK2MSFTNGP05.phx.gbl...
HRESULT hRes = CoInitialize(NULL);
in my OnCreate function of my dialog, the function returned S_OK. In
OnDestroy I call CoUninitialize().
Although I have not changed my functions, the CoInitialize now always
returns S_FALSE, i.e. the COM library is already initialized. Nevertheless
my programm functions normally.
Note that S_FALSE is a *success* code (not an error code).
In fact, the leading "S_" means "success".
In addition, CoInitialize calls nest, and you must call CoUninitialize as
many times as you called CoInitialize to properly uninitialize COM.
In fact, I think that it is not good practice to check the return value of
type HRESULT using equalities with specific values, like S_OK.
I think that correct tests are done using proper macros, like SUCCEEDED and
FAILED, e.g.:
HRESULT hr;
...
hr = pComObject->DoSomething(...);
if ( FAILED( hr ) )
... error
// or if ( SUCCEEDED(hr) ) --> OK
The thing is, some functions require you to distinguish between S_OK and
S_FALSE, and SUCCEEDED returns true for both. Examples include:
GetActiveObject
http://msdn2.microsoft.com/en-us/library/ms221467(VS.80).aspx
<q>
S_OK
Success.
Other return codes
Failure.
</q>
The only safe thing for a non-guru at COM like me is to assume "Other
return codes" includes S_FALSE. Another example is:
LocateCatalogs
http://msdn2.microsoft.com/en-us/library/ms690986(VS.85).aspx
<q>
S_OK
Successful completion.
S_FALSE
No machine and catalog can be found that index the scope, or iBmk is beyond
the count of machines and catalogs that index the scope.
E_INVALID_ARG
The function received an invalid pointer.
QUERY_E_INVALID_DIRECTORY
The function received an invalid scope.
</q>
--
Doug Harrison
Visual C++ MVP