USB device not valid in MonoLibUsb

Question

I'm trying to open a USB device handle in MonoLibUsb (on Linux), but every time I open it I get IsInvalid == true on the device handle.

The USB device is definitely compatible with LibUsb as I've connected it to my Windows PC and can successfully use LibUsbDotNet to talk to it. If I try to use LibUsbDotNet in Mono the application hangs when trying to open it, so I figured LibUsbDotNet is for Windows and MonoLibUsb is for Mono (the name kind of gives it away). However, even MonoLibUsb fails to properly use the device.

So why is the device handle returned invalid?

Code

private void UsbInit() {
    var sessionHandle = new MonoUsbSessionHandle();
    var profileList = new MonoUsbProfileList();
    profileList.Refresh(sessionHandle);

    List<MonoUsbProfile> usbList = profileList.GetList().FindAll(MyVidPidPredicate);

    foreach(MonoUsbProfile profile in usbList) {
        var deviceHandle = profile.OpenDeviceHandle();

        if (deviceHandle.IsInvalid) {
            Console.WriteLine(string.Format("IsInvalid: {0} - {1}", MonoUsbSessionHandle.LastErrorCode, MonoUsbSessionHandle.LastErrorString));
        }
    }
}

private bool MyVidPidPredicate(MonoUsbProfile profile) {
    if (profile.DeviceDescriptor.VendorID == 0xabcd && profile.DeviceDescriptor.ProductID == 0x1234)
        return true;
    return false;
}

Output

IsInvalid: Success -

Answer 1

This line in the documentation is very easy to overlook:

The user must have appropriate access permissions to the usb device before it can be used with linux.

If I'm starting the application as root (or through sudo) the device handle becomes valid.