Path functions behaving differently on UNC device paths than implied from documentation

[s5bug]

Description

File path formats on Windows systems states:

For device UNCs, the server/share portion forms the volume. For example, in \\?\server1\utilities\\filecomparer\, the server/share portion is server1\utilities. This is significant when calling a method such as Path.GetFullPath(String, String) with relative directory segments; it is never possible to navigate past the volume.

The way this reads to me is that \\?\server1\utilities\\ should be considered the "root" of \\?\server1\utilities\\filecomparer\.

Specifically, the documentation states

• "the server/share portion forms the volume"
• "in [...] the server/share portion is server1\utilities"
• "...it is never possible to navigate past the volume"

This seems to not be respected:

Reproduction Steps

> dotnet fsi

Microsoft (R) F# Interactive version 12.4.0.0 for F# 7.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> open System.IO;;
> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

Expected behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\utilities\\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = <null>

Actual behavior

> Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");;
val it: string = "\\?\server1\"

> Path.GetDirectoryName("""\\?\server1\utilities\\""");;
val it: string = "\\?\server1\utilities"

Regression?

I don't know how to test old .NET versions, I'm not a long-time .NET user.

Known Workarounds

No response

Configuration

> dotnet --version
7.0.100

> Get-ComputerInfo -Property "Os*"

OsName                                     : Microsoft Windows 10 Home
OsType                                     : WINNT
OsOperatingSystemSKU                       : WindowsHome
OsVersion                                  : 10.0.19045
OsArchitecture                                          : 64-bit
OsLanguage                                              : en-US

I would be pretty sure it's Windows (Path)-specific.

Other information

No response

[danmoseley]

FYI a good way to do a test on .NET Framework (ie to check old behavior) is http://sharplab.io (pick it from the drop-down)

[s5bug]

It seems sharplab is not a fan of System.IO.Path:

Unbreakable.AssemblyGuardException: Type System.IO.Path is not allowed.

Example Code

using System;
using System.IO;

Console.WriteLine(
  Path.Combine("\\\\?\\server1\\utilities\\\\filecomparer\\", "..\\..")
);

[ghost]

Tagging subscribers to this area: @dotnet/area-system-io See info in area-owners.md if you want to be subscribed.

Issue Details

---

### Description [File path formats on Windows systems](https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats) states: > For device UNCs, the server/share portion forms the volume. For example, in `\\?\server1\utilities\\filecomparer\`, the server/share portion is `server1\utilities`. This is significant when calling a method such as [Path.GetFullPath(String, String)](https://learn.microsoft.com/en-us/dotnet/api/system.io.path.getfullpath#system-io-path-getfullpath(system-string-system-string)) with relative directory segments; it is never possible to navigate past the volume. The way this reads to me is that `\\?\server1\utilities\\` should be considered the "root" of `\\?\server1\utilities\\filecomparer\`. Specifically, the documentation states - "the server/share portion forms the volume" - "in [...] the server/share portion is `server1\utilities`" - "...it is never possible to navigate past the volume" This seems to not be respected: ### Reproduction Steps ```fs > dotnet fsi Microsoft (R) F# Interactive version 12.4.0.0 for F# 7.0 Copyright (c) Microsoft Corporation. All Rights Reserved. For help type #help;; > open System.IO;; > Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");; val it: string = "\\?\server1\" ``` ### Expected behavior ```fs > Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");; val it: string = "\\?\server1\utilities\\" > Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");; val it: string = "\\?\server1\utilities\\" > Path.GetDirectoryName("""\\?\server1\utilities\\""");; val it: string = ``` ### Actual behavior ```fs > Path.GetFullPath("""..\..""", """\\?\server1\utilities\\filecomparer\""");; val it: string = "\\?\server1\" > Path.GetPathRoot("""\\?\server1\utilities\\filecomparer\""");; val it: string = "\\?\server1\" > Path.GetDirectoryName("""\\?\server1\utilities\\""");; val it: string = "\\?\server1\utilities" ``` ### Regression? I don't know how to test old .NET versions, I'm not a long-time .NET user. ### Known Workarounds _No response_ ### Configuration ``` > dotnet --version 7.0.100 ``` ``` > Get-ComputerInfo -Property "Os*" OsName : Microsoft Windows 10 Home OsType : WINNT OsOperatingSystemSKU : WindowsHome OsVersion : 10.0.19045 OsArchitecture : 64-bit OsLanguage : en-US ``` I would be pretty sure it's Windows (Path)-specific. ### Other information _No response_

Author:	s5bug
Assignees:	-
Labels:	`area-System.IO`, `untriaged`
Milestone:	-

[huoyaoyuan]

Path.GetPathRoot(@"\\?\server1\utilities\\filecomparer\") returns empty string on .NET Framework. Path.GetFullPath(path, basePath) isn't supported on .NET Framework. So this is likely not a regression.

[s5bug]

What about Path.Combine for .NET Framework, as shown in the example code?

[jozkee]

What about Path.Combine

Path.Combine doesn't resolve relative segments, what you are referring to is being discussed in https://github.com/dotnet/runtime/issues/2162.

[ghost]

Added needs-breaking-change-doc-created label because this issue has the breaking-change label.

1. [ ] Create and link to this issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.

Tagging @dotnet/compat for awareness of the breaking change.