Windows Store App Unit Testing a USB device

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I'm writing a USB device API for Windows Store Apps that uses Windows.Devices.USB API from Windows 8.1 to connect and communicate with the custom USB device. I'm using the Visual Studio 2013 dev preview IDE. The following function in the library is used to connect to the USB device. (Simplified for clarity)

public static async Task<string> ConnectUSB()
    {
        string deviceId = string.Empty;
        string result = UsbDevice.GetDeviceSelector(new Guid("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"));
        var myDevices = await Windows.Devices.Enumeration.DeviceInformation.FindAllAsync(result, null);
        if (myDevices.Count > 0)
        {
            deviceId = myDevices[0].Id;
        }
        UsbDevice usbDevice = null;
        try
        {
            usbDevice = await UsbDevice.FromIdAsync(deviceId);
        }
        catch (Exception)
        {
            throw;
        }
        if (usbDevice != null)
            return "Connected";
        return string.Empty;
    }

When called from the Windows Store App project, this function connects to the device flawlessly. However, when called from the Unit Test Library for Windows Store Apps project, the statement in the try block throws an exception.

A method was called at an unexpected time. (Exception from HRESULT: 0x8000000E)

from what I've looked around, this happens when an Async function is called without the await keyword. But I'm using the await keyword alright!

Some more info, I am unable to use NUnit to write unit tests for Store Apps so am using the MSTest Framework.

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public async Task TestMethod1()
    {
        await ConnectToUSB.ConnectUSB();
    }
}

Also, I've included the following capability tags in the manifest files of both the App store projects too without which it's impossible for the Store Apps to connect to devices.

<m2:DeviceCapability Name="usb">      
  <m2:Device Id="vidpid:ZZZZ XXXX">
    <m2:Function Type="name:vendorSpecific" />
  </m2:Device>
</m2:DeviceCapability>

Is there something I'm missing or is this a bug in the MSTest Framework?

unit-testing
usb
windows-store-apps
mstest
windows-8.1
asked on Stack Overflow Sep 2, 2013 by Xearo

2 Answers

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I think the problem is that await UsbDevice.FromIdAsync(deviceId); must be called on the UI thread because the app has to ask the user for access.

You have to CoreDispatcher.RunAsync to ensure you're on the UI thread or actually be in the code behind for a page.

answered on Stack Overflow Jun 11, 2014 by Greg Gorman
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I had the same problem with Unit Test App (Universal Windows) in VS 2017. I verify answer of my predecessor Greg Gorman(see below). And I found this is true. If you uses inside method body this construct:

Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(
    Windows.UI.Core.CoreDispatcherPriority.Normal,
   async () =>
   {
     ...
     UsbDevice usbDevice = await UsbDevice.FromIdAsync(deviceId);
     ...
   }).AsTask().Wait();

the FromIDAsync will work as you expect.

For your example change the test method to this:

[TestClass]
public class UnitTest1
{
  [TestMethod]
  public async Task TestMethod1()
  {
    Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(
      Windows.UI.Core.CoreDispatcherPriority.Normal,
     async () =>
     {
       await ConnectToUSB.ConnectUSB();
     }).AsTask().Wait();
  }
}
answered on Stack Overflow Sep 2, 2017 by Stefan Babos • edited Sep 2, 2017 by Stefan Babos

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