COM double hop test

a.cpp

#include <windows.h>

static const GUID CLSID_B =
{ 0x51addb47, 0xd26a, 0x4f69, { 0xb9, 0x7b, 0x7e, 0x30, 0x62, 0x8d, 0xcf, 0x5b } };


int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    CoInitializeEx(0, COINIT_APARTMENTTHREADED);

    HRESULT hr;
    IDispatch *pb, *pc;
    if (FAILED(hr = CoCreateInstance(CLSID_B, 0, CLSCTX_ALL, IID_IDispatch, (void**)&pb)))
    {
        MessageBox(0, L"Cannot connect to B", 0, MB_OK);
        return 0;
    }

    VARIANT v = { VT_EMPTY };
    DISPPARAMS DispParams = { 0, 0, 0, 0 };
    if (FAILED(hr = pb->Invoke(0, IID_NULL, 0, 0, &DispParams, &v, 0, 0)))
    {
        MessageBox(0, L"Cannot connect to C", 0, MB_OK);
        return 0;
    }
    if (V_VT(&v) != VT_DISPATCH || !V_DISPATCH(&v))
    {
        MessageBox(0, L"Result from B not an object", 0, MB_OK);
        return 0;
    }
    pc = V_DISPATCH(&v);
    pc->AddRef();
    VariantClear(&v);

    while(MessageBox(0, L"Cancel to quit, OK to test", 0, MB_OKCANCEL) != IDCANCEL)
    {
        hr = pc->Invoke(0, IID_NULL, 0, 0, &DispParams, 0, 0, 0);
        if(FAILED(hr))
            MessageBox(0, L"Fail", 0, MB_OK);
    }

    CoUninitialize();
 
    return 0;
}

b.cpp

#include <windows.h>

// {51ADDB47-D26A-4F69-B97B-7E30628DCF5B}
static const GUID CLSID_B =
{ 0x51addb47, 0xd26a, 0x4f69, { 0xb9, 0x7b, 0x7e, 0x30, 0x62, 0x8d, 0xcf, 0x5b } };

static const GUID CLSID_C =
{ 0x7d8df6fe, 0xbf5, 0x408a, { 0xba, 0x3, 0xf5, 0xae, 0x4b, 0xeb, 0x35, 0x2 } };

class CServer : IDispatch
{
    STDMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        if (InlineIsEqualGUID(riid, IID_IDispatch) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = this;
            return S_OK;
        }
        else
            return E_NOINTERFACE;
    }

    STDMETHODIMP_(ULONG) AddRef() { return 1; }
    STDMETHODIMP_(ULONG) Release() { return 1; }

    STDMETHODIMP GetTypeInfoCount(UINT* pctinfo) { return E_NOTIMPL; }
    STDMETHODIMP GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo** ppTInfo) { return E_NOTIMPL; }
    STDMETHODIMP GetIDsOfNames(REFIID, LPOLESTR*, UINT, LCID, DISPID*) { return E_NOTIMPL; }

    STDMETHODIMP Invoke(
        _In_  DISPID dispIdMember,
        _In_  REFIID riid,
        _In_  LCID lcid,
        _In_  WORD wFlags,
        _In_  DISPPARAMS* pDispParams,
        _Out_opt_  VARIANT* pVarResult,
        _Out_opt_  EXCEPINFO* pExcepInfo,
        _Out_opt_  UINT* puArgErr)
    {
        if (dispIdMember == 0)
        {
            if (!pVarResult)
                return E_POINTER;

            IDispatch* pd;
            HRESULT hr;
            if (SUCCEEDED(hr = CoCreateInstance(CLSID_C, 0, CLSCTX_ALL, IID_IDispatch, (void**)&pd)))
            {
                V_VT(pVarResult) = VT_DISPATCH;
                V_DISPATCH(pVarResult) = pd;
                return S_OK;
            }
            else
                return hr;
        }
        else
            return E_NOTIMPL;
    }
} g_Server;

class CFactory :public IClassFactory
{
    STDMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        if (InlineIsEqualGUID(riid, IID_IClassFactory) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = this;
            return S_OK;
        }
        else
            return E_NOINTERFACE;
    }

    STDMETHODIMP_(ULONG) AddRef() { return 1; }
    STDMETHODIMP_(ULONG) Release() { return 1; }

    STDMETHODIMP CreateInstance(IUnknown* pOuter, REFIID riid, void** ppv)
    {
        if (pOuter)
            return CLASS_E_NOAGGREGATION;
        if (InlineIsEqualGUID(riid, IID_IDispatch) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = &g_Server;
            return S_OK;
        }
        return E_NOINTERFACE;
    }

    STDMETHODIMP LockServer(BOOL) { return S_OK; }

} g_Factory;

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
    _In_opt_ HINSTANCE hPrevInstance,
    _In_ LPWSTR    lpCmdLine,
    _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    CoInitializeEx(0, COINIT_APARTMENTTHREADED);

    DWORD dw;
    CoRegisterClassObject(CLSID_B, &g_Factory, CLSCTX_LOCAL_SERVER, REGCLS_MULTIPLEUSE, &dw);

    MSG msg;
    while (GetMessage(&msg, nullptr, 0, 0))
        DispatchMessage(&msg);

    CoUninitialize();

    return (int)msg.wParam;
}

c.cpp

#include <windows.h>

static const GUID CLSID_C =
{ 0x7d8df6fe, 0xbf5, 0x408a, { 0xba, 0x3, 0xf5, 0xae, 0x4b, 0xeb, 0x35, 0x2 } };

class CServer: IDispatch
{
    STDMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        if (InlineIsEqualGUID(riid, IID_IDispatch) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = this;
            return S_OK;
        }
        else
            return E_NOINTERFACE;
    }

    STDMETHODIMP_(ULONG) AddRef() { return 1; }
    STDMETHODIMP_(ULONG) Release() { return 1; }

    STDMETHODIMP GetTypeInfoCount(UINT* pctinfo) { return E_NOTIMPL; }
    STDMETHODIMP GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo** ppTInfo) { return E_NOTIMPL; }
    STDMETHODIMP GetIDsOfNames(REFIID, LPOLESTR*, UINT, LCID, DISPID*) { return E_NOTIMPL; }

    STDMETHODIMP Invoke(
        _In_  DISPID dispIdMember,
        _In_  REFIID riid,
        _In_  LCID lcid,
        _In_  WORD wFlags,
        _In_  DISPPARAMS* pDispParams,
        _Out_opt_  VARIANT* pVarResult,
        _Out_opt_  EXCEPINFO* pExcepInfo,
        _Out_opt_  UINT* puArgErr)
    {
        return S_OK;
    }
} g_Server;

class CFactory :public IClassFactory
{
    STDMETHODIMP QueryInterface(REFIID riid, void** ppv)
    {
        if (InlineIsEqualGUID(riid, IID_IClassFactory) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = this;
            return S_OK;
        }
        else
            return E_NOINTERFACE;
    }

    STDMETHODIMP_(ULONG) AddRef() { return 1; }
    STDMETHODIMP_(ULONG) Release() { return 1; }

    STDMETHODIMP CreateInstance(IUnknown* pOuter, REFIID riid, void** ppv)
    {
        if (pOuter)
            return CLASS_E_NOAGGREGATION;
        if (InlineIsEqualGUID(riid, IID_IDispatch) || InlineIsEqualGUID(riid, IID_IUnknown))
        {
            *ppv = &g_Server;
            return S_OK;
        }
        return E_NOINTERFACE;
    }

    STDMETHODIMP LockServer(BOOL) { return S_OK; }

} g_Factory;

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
    _In_opt_ HINSTANCE hPrevInstance,
    _In_ LPWSTR    lpCmdLine,
    _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    CoInitializeEx(0, COINIT_APARTMENTTHREADED);

    DWORD dw;
    CoRegisterClassObject(CLSID_C, &g_Factory, CLSCTX_LOCAL_SERVER, REGCLS_MULTIPLEUSE, &dw);

    MSG msg;
    while (GetMessage(&msg, nullptr, 0, 0))
        DispatchMessage(&msg);

    CoUninitialize();

    return (int)msg.wParam;
}

This is a companion gist to this StackOverflow answer. Demonstrates the double hop marshaling in COM. Build each file as a separate project, using the Windows Desktop app template in Visual Studio.