Windows Universal App Serial Ports won't open, SerialDevice.FromIdAsync always null
So Microsoft do not mention that you need to add something to the app manifest for serial communication nor is there a tick box in the app manifest GUI for serial communication.
The following needs to be added to your app manifest (create the <Capabilities> section if it doesn't exist):
<Capabilities> <DeviceCapability Name="serialcommunication"> <Device Id="any"> <Function Type="name:serialPort" /> </Device> </DeviceCapability></Capabilities>UPDATE 10/2016
Microsoft have updated their documentation to reflect this requirement.
https://msdn.microsoft.com/en-us/library/windows/apps/windows.devices.serialcommunication.aspx