Might be a bug of PnpObject.FindAllAsync API

Question

Hi,

I try to use bellow code to get the "Manufacturer" "ItemNameDisplay" and "ModelName" of end-user's device.

But the problem is that in some device, this API will block the whole application for the second call and then crash.

string[] properties = { "System.Devices.Manufacturer", "System.ItemNameDisplay", "System.Devices.ModelName" };
var containers = await PnpObject.FindAllAsync(PnpObjectType.DeviceContainer, properties);

Answer 1

Hi Haibo,

If you debug the crash in Visual Studio, what is the exception and stack? And with what device(s) do you see the crash?

Regards,

lisa

Answer 2

Apology for my late reply and thanks a lot for kindly rely.

Actually, I found it may be my fault of usage.

I tracked it for a while, and found if I call this api as following way, it will throw a invalid exception:

string[] properties = { "System.ItemNameDisplay", "System.Devices.ModelName", "System.Devices.Connected" };
var containers = PnpObject.FindAllAsync(PnpObjectType.DeviceContainer, properties).GetResults();

So could you help to figure out why that? thanks in advance!

Br

Answer 3

Are you using the sample?

http://code.msdn.microsoft.com/windowsapps/Device-Enumeration-Sample-a6e45169/sourcecode?fileId=44728&pathId=1979427360

Here's the snippet of code showing the right usage (with await):

string[] properties = { "System.ItemNameDisplay", "System.Devices.ModelName", "System.Devices.Connected" }; 
var containers = await PnpObject.FindAllAsync(PnpObjectType.DeviceContainer, properties); 
 
DeviceContainersOutputList.Items.Clear(); 
rootPage.NotifyUser(containers.Count + " device container(s) found", NotifyType.StatusMessage); 
foreach (PnpObject container in containers) 
{ 
    DeviceContainersOutputList.Items.Add(new DisplayItem(container)); 
} 

Answer 4

Sure, I used this sample.

Actually it mean it must run as a async Task.

If wrapper the snippet as async func, and call it using Result for simple, it will cause another problem.  like this:

async void EnumerateDeviceContainers(object sender, RoutedEventArgs eventArgs)
{
    var list = modelName().Result; // this will freeze whole app
}

async Task<ICollection<DisplayItem>> modelName()
{
    string[] properties = { "System.ItemNameDisplay", "System.Devices.ModelName", "System.Devices.Connected" };
    var containers = await PnpObject.FindAllAsync(PnpObjectType.DeviceContainer, properties);

    List<DisplayItem> items = new List<DisplayItem>();

    foreach (PnpObject container in containers)
    {
        items.Add(new DisplayItem(container));
    }

    return items;
}

It would be an special case, but it indeed will cause freeze.

Answer 5

Thanks for the clarification.  This is not a bug.

Task<TResult>.Result() waits until the async operation is complete before returning. As you can tell, this is not recommended in the UI thread because it will cause the app to freeze.

http://msdn.microsoft.com/en-us/library/dd321468.aspx

If you are trying to return the items in a wrapper function, you should call await instead of .Result().

async void EnumerateDeviceContainers(...)

{

   var list = await modelName();  // this will assign the result to list asynchronously.

   // put code that uses list here 

}