Enumerating WPD devices in C#
Let's take a look at how we can enumerate WPD devices in C#. This post assumes that you already have a project set up using these instructions. (Update: You will also need to follow the disassembly/reassembly instructions below for this to work correctly.)
static void Main(string[] args)
{
//
// Get an instance of the device manager
//
PortableDeviceApiLib.PortableDeviceManagerClass devMgr
= new PortableDeviceApiLib.PortableDeviceManagerClass();
//
// Probe for number of devices
//
uint cDevices = 1;
devMgr.GetDevices(null, ref cDevices);
//
// Re-allocate if needed
//
if (cDevices > 0)
{
string[] deviceIDs = new string[cDevices];
devMgr.GetDevices(deviceIDs, ref cDevices);
for (int ndxDevices = 0; ndxDevices < cDevices; ndxDevices++)
{
Console.WriteLine("Device[{0}]: {1}",
ndxDevices + 1, deviceIDs[ndxDevices]);
}
}
else
{
Console.WriteLine("No WPD devices are present!");
}
}
This code maps almost line-by-line to the C++ enumeration example. We manufacture an instance of the device manager using PortableDeviceApiLib.PortableDeviceManagerClass. We then use the instance to probe how many devices are there. If you remember, the GetDevices API will return back in cDevices the number of devices (actually the size of the string array that should be specified to retrieve all the device IDs).
Once we know the number of devices, we allocate an appropriately sized array and then call GetDevices again. This time, the array will contain all the device IDs. To display them, we simply iterate over the array and print them out.
Gotchas:
- We could use the string[] notation since the GetDevices API returned an array of CoTaskMemAlloc'd LPWSTR strings
- We can't use the string datatype for API calls that require a buffer allocated by the caller. GetDeviceFriendlyName is an example of this - such API calls require prior allocation of ushort[] and then on success, character-by-character conversion to a C# string.
Update
Mike R. brought to my notice that the above sample only enumerates one device even if more than one are connected. This is a marshalling restriction - we can work around it by manually fixing up the generated Interop assembly. Follow the steps below to edit the assembly:
- Disassemble the PortableDeviceApi interop using the command -
ildasm Interop.PortableDeviceApiLib.dll /out:pdapi.il
- Open the IL in Notepad and search for the following string
instance void GetDevices([in][out] string& marshal( lpwstr) pPnPDeviceIDs,
- Replace all instances of the string above with the following string
instance void GetDevices([in][out] string[] marshal([]) pPnPDeviceIDs,
- Save the IL and reassemble the interop using the command -
ilasm pdapi.il /dll /output=Interop.PortableDeviceApiLib.dll
You can now rebuild your project. You can now first call GetDevices with a NULL parameter to get the count of devices and then call it again with an array to get the device IDs.
