Diamond inheritance using Windows V2 Credential Providers

Question

Following situation:

I have a working V1 CP that implements the IConnectableCredentialProviderCredential interface. This dependency is necessary to have access to the IQueryContinueWithStatus interface, which I need.

class CCredential : public IConnectableCredentialProviderCredential
{
    [...]

IConnectableCredentialProviderCredential implements ICredentialProviderCredential .

The COM IUknown query method is done using this snippet:

IFACEMETHODIMP QueryInterface(__in REFIID riid, __deref_out void** ppv)
{
    static const QITAB qitOnlyCredUI[] =
    {
        QITABENT(CCredential, ICredentialProviderCredential),
        { 0 },
    };

    static const QITAB qitFull[] =
    {
        QITABENT(CCredential, ICredentialProviderCredential), // IID_ICredentialProviderCredential
        QITABENT(CCredential, IConnectableCredentialProviderCredential), // IID_IConnectableCredentialProviderCredential            
        { 0 },
    };

[...]

---

Now I want to migrate the provider to be a V2 CP and implement the ICredentialProviderCredential2 interface, but stay compatible to the IQueryContinueWithStatus interface. Also, dropping QITABENT(CCredential, ICredentialProviderCredential) out is no option, because qitOnlyCredUI is necessary in case a CredUI is requested, which does not support IQueryContinueWithStatus .

class CCredential : public IConnectableCredentialProviderCredential,
                    public ICredentialProviderCredential2,
                    public ICredentialProviderCredentialWithFieldOptions
{
    [...]

ICredentialProviderCredential2 implements ICredentialProviderCredential .

Implementing the COM QueryInterface method like:

IFACEMETHODIMP QueryInterface(__in REFIID riid, __deref_out void** ppv)
{
    static const QITAB qitOnlyCredUI[] =
    {
        QITABENT(CCredential, ICredentialProviderCredential),
        { 0 },
    };

    static const QITAB qitFull[] =
    {
        QITABENT(CCredential, ICredentialProviderCredential), // IID_ICredentialProviderCredential
        QITABENT(CCredential, ICredentialProviderCredential2), // IID_ICredentialProviderCredential2
        QITABENT(CCredential, IConnectableCredentialProviderCredential), // IID_IConnectableCredentialProviderCredential            
        QITABENT(CCredential, ICredentialProviderCredentialWithFieldOptions), //IID_ICredentialProviderCredentialWithFieldOptions
        { 0 },
    };

This is where the Diamond Problem starts, as ICredentialProviderCredential2 and IConnectableCredentialProviderCredential both implement ICredentialProviderCredential .

Trying to compile certainly leads to the compiler complaining about ambiguous definitions for the ICredentialProviderCredential base class.

Any ideas how this can be accomplished?

Answer 1

You will need to use the QITABENTMULTI() macro for your case. It's documented in shlwapi.h . I have no idea why it isn't on MSDN.