private static string get_unique_id()
{
     object uniqueId;
     var hexString = string.Empty;
     if (DeviceExtendedProperties.TryGetValue("DeviceUniqueId", out uniqueId))
     hexString = BitConverter.ToString((byte[])uniqueId).Replace("-", string.Empty);
     return hexString;
}

don't  forgot to add capability ID_CAP_IDENTIFY_DEVICE

get windows phone unique id

phone, windows

C#