Codebeispiel zum Lesen von Geräteeigenschaften

Im folgenden Codebeispiel wird gezeigt, wie Geräteeigenschaften aus einem Eigenschaftenspeicher gelesen werden. Ein Beispiel zum Öffnen eines Eigenschaftenspeichers für das aktuelle Funktionsinstanzobjekt finden Sie unter Codebeispiel zum Öffnen eines Eigenschaftenspeichers.

/**************************************************************************\
* CWSDDevice::ReadDeviceProperty
*
* Reads a LPWTSR device property from the device metadata and returns 
* a copy of its value as a BSTR character string. 
*
*
* Arguments:
*
*    pPropertyStore        - IPropertyStore interface, optional
*                            (if not specified a new property store
*                            Is opened and closed for each call)
*    pPropertyKey          - property key (for example, PKEY_PNPX_FirmwareVersion)
*    pbstrPropertyValue    - Returns the property value; this value must be 
*                            freed by caller using SysFreeString
*
* Return Value:
*
*     S_OK if operation is successful, an error HRESULT otherwise
*
\**************************************************************************/

HRESULT
CWSDDevice::ReadDeviceProperty(
    __in_opt IPropertyStore *pPropertyStore,
    __in const PROPERTYKEY  *pPropertyKey,
    __out BSTR              *pbstrPropertyValue)
{
    HRESULT hr = S_OK;
    IPropertyStore *pThisPropertyStore = NULL;
    BOOL bClosePropertyStore = FALSE;
    PROPVARIANT propvar = {0};

    if ((!pbstrPropertyValue) || (!pPropertyKey))
    {
        hr = E_INVALIDARG;
        WIAS_ERROR((g_hInst, "Invalid argument, hr = 0x%08X", hr));
    }

    if ((!m_pFunctionInstance) || (!m_pScanProxy))
    {
        hr = E_UNEXPECTED;
        WIAS_ERROR((g_hInst, "ScanProxy not initialized, hr = 0x%08X", hr));
    }

    if (SUCCEEDED(hr))
    {
        pThisPropertyStore = pPropertyStore;
 
        if (!pThisPropertyStore)
        {
            hr = m_pFunctionInstance->OpenPropertyStore(STGM_READ, &pThisPropertyStore);
            if ((SUCCEEDED(hr)) && (!pThisPropertyStore))
            {
                hr = E_POINTER;
                WIAS_ERROR((g_hInst, 
                    "IFunctionInstance::OpenPropertyStore returned hr = 0x%08X with a NULL property store interface, hr = 0x%08X", hr));
            }
            if (FAILED(hr))
            {
                WIAS_ERROR((g_hInst, "IFunctionInstance::OpenPropertyStore failed, hr = 0x%08X", hr));
            }

            if (pThisPropertyStore)
            {
                bClosePropertyStore = TRUE;
            }
        }    
    }
 
    PropVariantInit(&propvar);

    if (SUCCEEDED (hr))
    {
        hr = pThisPropertyStore->GetValue(*pPropertyKey, &propvar);
        if (FAILED(hr))
        {
            WIAS_ERROR((g_hInst, "IPropertyStore::GetValue failed, hr = 0x%08X", hr));
        }
    }

    if ((SUCCEEDED(hr)) && (VT_EMPTY == propvar.vt))
    {
        hr = E_FAIL;
        WIAS_ERROR((g_hInst, "IPropertyStore::GetValue returned no value (VT_EMPTY), hr = 0x%08X", hr));
    }

    if ((SUCCEEDED(hr)) && (VT_LPWSTR != propvar.vt))
    {
        hr = E_UNEXPECTED;
        WIAS_ERROR((g_hInst, "IPropertyStore::GetValue returned an unexpected PROPVARIANT type, hr = 0x%08X", hr));
    }

    if (SUCCEEDED (hr))
    {
        *pbstrPropertyValue = SysAllocString(propvar.pwszVal);
        if (!(*pbstrPropertyValue))
        {
            hr = E_OUTOFMEMORY;
            WIAS_ERROR((g_hInst, 
                "Cannot make a copy of the value returned by IPropertyStore::GetValue, %ws, hr = 0x%08X", propvar.pwszVal, hr));
        }
    }

    PropVariantClear(&propvar);

    if ((bClosePropertyStore) && (pThisPropertyStore))
    {
        pThisPropertyStore->Release();
        pThisPropertyStore = NULL;
    }

    return hr;
}