WPD property retrieval in C#

Article
12/11/2006

WPD property values are retrieved as PROPVARIANTs through the WPD API. Unfortunately there is no native support for PROPVARIANTs in C#. To correctly retrieve the property values (or anything that is in a PROPVARIANT) through the WPD API in C#, we must use marshalling.

PortableDeviceApiLib.tag_inner_PROPVARIANT is the data-type exposed through the WPD Interop layer. This data-type exposes the .vt member, but it does not expose the anonymous union which is the raison d'être for the PROPVARIANT structure.

To get to the union, we need to re-define the PROPVARIANT data-type in C#. We can get by with the following definition:

 using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit, Size = 16)]
public struct PropVariant
{
    [FieldOffset(0)]
    public short variantType;
    [FieldOffset(8)]
    public IntPtr pointerValue;
    [FieldOffset(8)]
    public byte byteValue;
    [FieldOffset(8)]
    public long longValue;
}

This follows exactly the struct layout of a PROPVARIANT. We haven't included all the PROPVARIANT fields, feel free to add them as and when you need them. For our example, we'll manage with the ones above.

Now that we have a C# specific PROPVARIANT definition, we should be able to convert back and forth to the interop version. This isn't as easy as a simple cast - we'll have to use marshalling to do this.

To convert from the interop tag_inner_PROPVARIANT to our C# PropVariant:

 PortableDeviceApiLib.tag_inner_PROPVARIANT ipValue = 
                     new PortableDeviceApiLib.tag_inner_PROPVARIANT();

IntPtr ptrValue = Marshal.AllocHGlobal(Marshal.SizeOf(ipValue));
Marshal.StructureToPtr(ipValue, ptrValue, false);

PropVariant pv = (PropVariant)Marshal.PtrToStructure(p, typeof(PropVariant));

To convert from our C# PropVariant to the interop tag_inner_PROPVARIANT:

 PropVariant pvValue = new PropVariant();

IntPtr ptrValue = Marshal.AllocHGlobal(Marshal.SizeOf(pvValue));
Marshal.StructureToPtr(pvValue, ptrValue, false);

PortableDeviceApiLib.tag_inner_PROPVARIANT ipValue =
    (PortableDeviceApiLib.tag_inner_PROPVARIANT)
    Marshal.PtrToStructure(ptrValue, typeof(PortableDeviceApiLib.tag_inner_PROPVARIANT));

Essentially what we are doing for both cases is getting a pointer to the original structure and then casting it back (via marshalling) to the second structure.

Now let's see this in action - we'll attempt to retrieve all the properties for the DEVICE object and display the value for all string properties.

 static void DisplayProperties(
    PortableDeviceApiLib.PortableDeviceClass pPortableDevice, 
    string objectID)
{
    //
    // Retrieve IPortableDeviceContent interface required to
    // obtain the IPortableDeviceProperties interface
    //
    PortableDeviceApiLib.IPortableDeviceContent pContent;
    pPortableDevice.Content(out pContent);

    //
    // Retrieve IPortableDeviceProperties interface required
    // to get all the properties
    //
    PortableDeviceApiLib.IPortableDeviceProperties pProperties;
    pContent.Properties(out pProperties);

    //
    // Call the GetValues API, we specify null to indicate we
    // want to retrieve all properties
    //
    PortableDeviceApiLib.IPortableDeviceValues pPropValues;
    pProperties.GetValues(objectID, null, out pPropValues);

    //
    // Get count of properties
    //
    uint cPropValues = 0;
    pPropValues.GetCount(ref cPropValues);
    Console.WriteLine("Received " + cPropValues.ToString() + " properties");

    for (uint i = 0; i < cPropValues; i++)
    {
        //
        // Retrieve the property at index 'i'
        //
        PortableDeviceApiLib._tagpropertykey propKey = 
                        new PortableDeviceApiLib._tagpropertykey();
        PortableDeviceApiLib.tag_inner_PROPVARIANT ipValue = 
                        new PortableDeviceApiLib.tag_inner_PROPVARIANT();
        pPropValues.GetAt(i, ref propKey, ref ipValue);

        //
        // Allocate memory for the intermediate marshalled object
        // and marshal it as a pointer
        //
        IntPtr ptrValue = Marshal.AllocHGlobal(Marshal.SizeOf(ipValue));
        Marshal.StructureToPtr(ipValue, ptrValue, false);

        //
        // Marshal the pointer into our C# object
        //
        PropVariant pvValue = 
            (PropVariant)Marshal.PtrToStructure(ptrValue, typeof(PropVariant));

        //
        // Display the property if it a string (VT_LPWSTR is decimal 31)
        //
        if (pvValue.variantType == 31 /*VT_LPWSTR*/)
        {
            Console.WriteLine("{0}: Value is \"{1}\"", 
                (i + 1).ToString(), Marshal.PtrToStringUni(pvValue.pointerValue));
        }
        else
        {
            Console.WriteLine("{0}: Vartype is {1}", 
                (i + 1).ToString(), pvValue.variantType.ToString());
        }
        //PropVariant pv = new PropVariant(ip);
    }
}

The example assumes that a device connection has already been opened. It also assumes that the PropVariant definition (from above) is present somewhere in the class definition.

We need to start with obtaining the IPortableDeviceProperties interface. Once we have the interface, we call the GetValues API to retrieve the properties. This is in a ValuesCollection object, so we use GetAt to retrieve each property. The retrieved property is of type tag_inner_PROPVARIANT which we convert to our C# PropVariant definition using marshalling. Once we have the data in our PropVariant structure, we can reference the string by marshalling the pointer value in the PropVariant to a string object.

As an exercise, try extending this example to display the value for properties of different vartypes. In our next post, we'll see how we can set property values.

WPD property retrieval in C#

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