client.cpp

#include <Unknwn.h>
#include "winrt/Windows.Foundation.h"

namespace winrt {
    using namespace Windows::Foundation;
}

class __declspec(uuid("58d39529-4975-4440-b7a6-6d71bfb59559")) CLSID_SAMPLE;

int main() {
    CO_MTA_USAGE_COOKIE cookie{};
    CoIncrementMTAUsage(&cookie);

    while (true) {
        printf(".");

        auto stringable =
            winrt::create_instance<winrt::IStringable>(__uuidof(CLSID_SAMPLE), CLSCTX_LOCAL_SERVER);

        assert(stringable.ToString() == L"Stringable");

        winrt::com_ptr<IStream> stream;

        winrt::check_hresult(CoMarshalInterThreadInterfaceInStream(
            winrt::guid_of<winrt::IStringable>(),
            winrt::get_unknown(stringable),
            stream.put()));

        winrt::IStringable copy;

        winrt::check_hresult(CoUnmarshalInterface(
            stream.get(),
            winrt::guid_of<winrt::IStringable>(),
            winrt::put_abi(copy)));
    }
}

server.cpp

#include <Unknwn.h>
#include "winrt/Windows.Foundation.h"

namespace winrt {
    using namespace Windows::Foundation;
}

class __declspec(uuid("58d39529-4975-4440-b7a6-6d71bfb59559")) CLSID_SAMPLE;

struct Stringable : winrt::implements<Stringable, winrt::IStringable> {
    winrt::hstring ToString() {
        return L"Stringable";
    }
};

struct Factory : winrt::implements<Factory, IClassFactory> {
    HRESULT __stdcall CreateInstance(IUnknown*, GUID const& iid, void** object) {
        auto stringable = winrt::make<Stringable>();
        return stringable.as(iid, object);
    }

    HRESULT __stdcall LockServer(BOOL) {
        return S_OK;
    }
};

int main() {
    CO_MTA_USAGE_COOKIE cookie{};
    winrt::check_hresult(CoIncrementMTAUsage(&cookie));

    auto factory = winrt::make<Factory>();
    DWORD revoke = 0;

    winrt::check_hresult(CoRegisterClassObject(
        __uuidof(CLSID_SAMPLE),
        factory.get(),
        CLSCTX_LOCAL_SERVER,
        REGCLS_AGILE | REGCLS_MULTIPLEUSE,
        &revoke
    ));

    printf("Running!\n");
    getchar();
}