Null when accessing the Access property of a Win32_Volume instance - windows

Question

As a way to resolve GetVolumeInformation() not reporting FILE_READ_ONLY_VOLUME for a locked SD Card without any file-system, I found that the Win32_Volume class has an Access uint16 property that:
Describes whether the media is readable. This property is inherited from CIM_StorageExtent. This can be one of the following values.
I'm interacting with WMI from a C++ program, so when reading the Access property I get a VARIANT structure with the result.
According to the docs, a VARIANT contains an union, so I should first check the vt property to determine its type. vt is a VARTYPE, which is an int according to the docs. The vt value I get after accessing the Access property is 1, which again according to the docs, is null. I can confirm this result by trying to access most of the union members, which are not set at all.
Here's a complete runnable example (run with cl /EHsc test.cpp):
#include <iostream>
#include <comdef.h>
#include <Wbemidl.h>
#pragma comment(lib, "wbemuuid.lib")
int main(int argc, char **argv) {
IWbemLocator *locator = NULL;
IWbemServices *services = NULL;
IEnumWbemClassObject* enumerator = NULL;
IWbemClassObject *classObject = NULL;
if (FAILED(CoInitializeEx(NULL, COINIT_MULTITHREADED))) return 1;
if (FAILED(CoInitializeSecurity(NULL, -1, NULL, NULL, RPC_C_AUTHN_LEVEL_DEFAULT, RPC_C_IMP_LEVEL_IMPERSONATE, NULL, EOAC_NONE, NULL))) return 1;
if (FAILED(CoCreateInstance(CLSID_WbemLocator, NULL, CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPVOID *)&locator))) return 1;
if (FAILED(locator->ConnectServer(_bstr_t(L"ROOT\\CIMV2"), NULL, NULL, 0, NULL, 0, 0, &services))) return 1;
if (FAILED(CoSetProxyBlanket(services, RPC_C_AUTHN_WINNT, RPC_C_AUTHZ_NONE, NULL, RPC_C_AUTHN_LEVEL_CALL, RPC_C_IMP_LEVEL_IMPERSONATE, NULL, EOAC_NONE))) return 1;
if (FAILED(services->ExecQuery(bstr_t("WQL"), bstr_t("SELECT * FROM Win32_Volume"), WBEM_FLAG_FORWARD_ONLY | WBEM_FLAG_RETURN_IMMEDIATELY, NULL, &enumerator))) return 1;
while (enumerator) {
ULONG code = 0;
if (FAILED(enumerator->Next(WBEM_INFINITE, 1, &classObject, &code))) return 1;
if (code == 0) break;
VARIANT variant;
if (FAILED(classObject->Get(L"DriveLetter", 0, &variant, NULL, NULL))) return 1;
if (variant.bstrVal == NULL) {
VariantClear(&variant);
classObject->Release();
continue;
};
std::wcout << " DriveLetter : " << variant.bstrVal[0] << std::endl;
VariantClear(&variant);
if (FAILED(classObject->Get(L"Access", 0, &variant, NULL, NULL))) return 1;
std::wcout << "Access VARTYPE -> " << variant.vt << std::endl;
VariantClear(&variant);
classObject->Release();
}
enumerator->Release();
services->Release();
locator->Release();
CoUninitialize();
return 0;
}
This is an example output from my system (Windows 10 Pro x86_64):
DriveLetter : C
Access VARTYPE -> 1
DriveLetter : F
Access VARTYPE -> 1
DriveLetter : E
Access VARTYPE -> 1
I can access other string properties, like DriveLetter, just fine, which makes me think I'm doing something wrong with this particular property.
UPDATE 1: Looks like I get the same results with any uint16 property, but not with uint32, nor uint64, which seem to work just fine.