If the path is a legacy DOS device such as CON, COM1, or LPT1, it is converted into a device path by prepending \\.\ and > returned.
A path that begins with a legacy device name is always interpreted as a legacy device by the Path.GetFullPath(String) method. For > example, the DOS device path for CON.TXT is \\.\CON, and the DOS device path for COM1.TXT\file1.txt is \\.\COM1.
From https://docs.microsoft.com/en-us/dotnet/standard/io/file-path-formats#handle-legacy-devices: