Re: Multithreading issues related to an ActiveX control
Your code is trying to compensate for a faulty design. The problem with your approach is that you can't control which thread the
client code is going to use to call in to your DLL. This is disastrous since your COM object has been instantiated in a
single-threaded apartment.
You might imagine that switching to an MTA by calling CoInitializeEx (0, COINIT_MULTITHREADED) in your Initialize() function might
be the answer. This is also faulty. The thread that initializes the MTA is required to outlive the apartment, and again your DLL
code has no control over this. Moreover, I suspect the call to CoInitializeEx (0, COINIT_MULTITHREADED) will fail anyway, since it
is highly likely the thread has already called CoInitialize() by the parent code. (I note in your code you don't check the return
code of CoInitialize(). You should be.)
The best solution is for your DLL to spawn a worker thread. The worker thread can freely establish either an MTA or an STA,
whichever is more appropriate for your application, and will have full control over the object lifetime. The only obstacle is there
may be some trickiness to get the worker thread to shut down without the cooperation of the parent code. I'm guessing this won't be
a problem for you if the parent code is already calling Initialize() and Uninitialize(). That's where you will spawn and shut down
your thread.
Just a few comments about the code you posted:
=====================================
#import "c:/applicationpath/interface.dll"
using namespace XXX;
class CVerifyUser
{
public:
long VerifyUser(CString csUserID, CString csPassword)
{
BSTR bstrUserID = csUserID.AllocSysString();
BSTR bstrPassword = csPassword.AllocSysString();
try
{
HRESULT hr;
hr = m_pVerifyUser->raw_VerifyUser( &bstrUserID, &bstrPassword, &lRet);
}
//...
Do you call SysFreeString() on the bstrUserID and bstrPassword variables?
}
long Uninitialize()
{
if (m_pVerifyUser != NULL) {
m_pVerifyUser.Release();
}
CoUninitialize();
}
long Initialize()
{
CoInitialize(NULL);
You should be checking the return code here. It's quite possible CoInitialize(NULL) will fail. The STA may already be established by
the parent code. In this case, you don't want to call CoUninitialize() in you Uninitialize() routine.
//...
hr = m_pVerifyUser.CreateInstance("XXX.UserVerification");
}
CVerifyUser();
virtual ~CVerifyUser();
private:
_UserVerificationPtr m_pVerifyUser;
};
HTH
Brian