Thanks for your reply, maybe there is something I did not express very clear. Here is the thing, I was asked to develop a third-party app by using the API of "TCS" which is a app developed by Nokia, it is short for Tetra Connectivity Server and basically a programm used in telecommunication. By adopting this "#import XXX/tcsapi.dll" in my code, I can freely used the api of it(e.g getVoiceCall()). The __uuidof(TCSUser) has value and can be checked in the registry, but the error still be there.

COSERVERINFO si;
si.dwReserved1=0;
si.pwszName=szIP;
si.pAuthInfo=NULL;
si.dwReserved2=0;
IID iid=__uuidof(ITCSUser);
MULTI_QI qi={&iid,0,0};
HRESULT hRes=CoCreateInstanceEx(__uuidof(TCSUser),NULL,CLSCTX_REMOTE_SERVER,&si,1,&qi);
if (FAILED(hRes))
{
Log(_T("Can't create user object, error 0x%.8X"),hRes);
return false;
}

I am developing a third party app by using this so-called TCS Api, the GUID of both class and interface(ITCSUser,CTCSUser) can be found in registry. while the CLASSNOTREG error will be generated after executing CoCreateInstanceEx. I have been stuck here so long and not yet figure it out. I am a beginner of COM/DCOM, could anyone help or give a little hint. By the way, this app is run on Windows server 2008 r2, someone told me the OS is not supported DCOM. Is this really mattered?