QueryInterface fails with E_ACCESSDENIED

Question

The following method is a DCOM server method. The COM client and server is running in different WinXP machines. The COM client calls RegisterClient method to register callback interface. The problem is QueryInterface method fails with error code E_ACCESSDENIED. What could be the reason for the issue?

STDMETHODIMP CGEMExtension::RegisterClient(IUnknown** ppGEMExtensionEvents, int* nClientId, int* nResult)
{
    HRESULT  hRes = (*ppGEMExtensionEvents)->QueryInterface(IID_IGEMExtension,(void**)&pUnknown);
    return hRes;
}

Answer 1

When you get an E_ACCESSDENIED, it means you have a permissions problem (don't waist your time on firewalls or registrations - the former will raise errors telling you the service is not available, and the latter will tell you the class is not registered or so). COM relies on Windows permissions, so this is what you should focus on.

In your case, if I understand the case correctly, the server actually calls the client, in order to get the right interface. For that, the user running the server should have the right permissions on the client side. A few suggestions:

1. As daramarak suggests, have the server and the client use the same domain user, or the same local user with the same password.
2. On the client, set this setting to "classic".
3. Give the server's user, if known to the client, additional permissions using DCOMCNFG.